I like to have a concept of an engine that spaceships can use to lift off (and land) from/on a planet (regardless of it having an atmosphere or not) with the following constraints:

  1. It should be simple to "understand" for the audience
  2. It should not consume more than 10% of the ships mass in fuel
  3. It should not destroy the place of liftoff or landing
  4. It should not violate fundamental laws of physiscs conspicuous even to a 12 year old with some interest in physics.

To elaborate a little bit:

A classic reaction mass drive (thruster) needs either a lot of reaction mass (violating 2.) or a lot of energy (violating 3.) or both. A "gravitation absorber" that let us say blocks gravitation will in my opinion either violate 1. or 4. First of all, to just hold the ship in place floating free the absorber needs no energy at all (disturbing? Imagine how much energy a platform on a tower needs to hold you in place.) If he needs energy it must go somewhere (excess heat? Gravitational waves?) When you start to move up (with additional thrusters?) it becomes complicated (violating 1.) The faster you move up the more energy the absorber needs (violating 1.) to compensate for the potential energy your ship gets by gaining altitude. If it does not, 4. is violated. When it comes to landing there is the opposite problem. If you loose height, the absorber needs to convert the lost potential energy of your ship to somewhat. Somewhat harmless if you not want to violate 3. If you turn off the absorber, than you will gain speed and have to break somehow to avoid hitting the ground.

So is there any concept (beside assissted with external help like space elevator, space cable, ...) for a drive that could do this? Perhaps a reaction mass drive which uses particles (like neutrinos) that don't interact with conventional matter (how could they be produced in the needed amount and accelerated and transmit their impulse to the ship?) or something like that?

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    $\begingroup$ Space elevator? Seriously, it is literally a rocket science and will be that for your audience unless you handwave a lot. If such an engine was available, do you really think no one would try it yet? $\endgroup$ – Mołot Dec 12 '16 at 13:21
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    $\begingroup$ With the list of restrictions that you impose I could only think about a handwavium engine fueled by unobtanium... I mean, seriously... $\endgroup$ – SJuan76 Dec 12 '16 at 13:21
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    $\begingroup$ You're literally asking for the holy grail of space travel; good luck. $\endgroup$ – iAdjunct Dec 12 '16 at 13:41
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    $\begingroup$ Why #2? Sure, it'd be awfully nice to have, but is it really required for the story you are telling? The sheer size of a large booster is impressive in its own right; you can use that effect to your advantage in telling the story. $\endgroup$ – a CVn Dec 12 '16 at 14:08
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    $\begingroup$ I agree with @MichaelKjörling. What you want does not exist and cannot exist with our current scientific knowledge (if you want to reach escape velocity - 11km/s - with only 10% of the mass, you need to throw that mass back at 110 km/s, and that is before computing energy losses due to friction). Anything at 110 km/s is pretty destructive. You are writting an story, not designing an engine. $\endgroup$ – SJuan76 Dec 12 '16 at 14:31

20 Answers 20


I'm going to take the comment you posted on the question, because it is important.

The story should not be driven or build around the technology. But I have the ambition to also not just handwave it, because I'm myself interested in physics and does not like stories that doesn't care about the most fundamental laws. – Hothie 2016-12-12 14:26:57Z

Sorry for being blunt, but it is not possible to meet all four of your criteria with anything which we know how to build.

The go-to for rocket engines is the Tsiolkovsky rocket equation:

$$ \Delta v=v_e \ln\left(\frac{m_0}{m_f}\right) $$

Stated another way:

$$ \frac{\Delta v}{v_e}=\ln\left(\frac{m_0}{m_f}\right)=-\ln\left(\frac{m_f}{m_0}\right) $$

where $m_0$ is the initial mass, $m_f$ is the final mass (also known as dry mass), $v_e$ is the exhaust velocity, and $\Delta v$ is the resultant change in velocity. $\frac{m_f}{m_0}$ is referred to as the "mass ratio". Note that this applies only to single-stage rockets; that's why practical launch vehicles tend to use two or three stages. Look at how the mass ratio grows with the total delta-v divided by exhaust velocity (image by uhoh from this answer on Space Exploration SE):

For a mass ratio of 10% fuel to initial mass, we have $$ \Delta v=v_e \ln\left(\frac{1}{0.9}\right) \approx v_e \times 0.10536 $$

Ignoring drag and gravity losses (which only make this harder), a spacecraft launched from Earth must attain about 7 km/s of forward velocity to enter a reasonably stable orbit. You can get away with less forward velocity, but that costs you in terms of gravitational potential energy instead, so is not a solution. Rearranging the above, we get $$ 7\,000 \approx v_e \times 0.10536 \Rightarrow v_e \approx \frac{7\,000}{0.10536} \approx 66\,400~\text{m/s} $$

In other words, our rocket engine must have an exhaust velocity in excess of 66 km/s in order to attain the required delta-v within the desired mass ratio. The crux is that we also need sufficient thrust to get off the ground; if the engine does not have sufficient thrust, the rocket is almost literally sitting there spinning its wheels. (Compare How far would the STS get without the SRBs on Space Exploration.) Wikipedia has a decent table of methods of spacecraft propulsion, but none of the alternatives listed both:

  • Has at least been tested in a vacuum chamber on Earth
  • Provide a large amount of thrust
  • Provide sufficient exhaust velocity

We simply don't know how to get the necessary exhaust velocity to generate the required delta-v at the mass ratios you envision, with sufficient thrust to get off the ground.

Also, no matter what you are throwing out behind the engine at 65-70 km/s or more with sufficient mass to generate a reasonable amount of thrust, it's going to be dangerous. Even with our current puny 3-5 km/s exhaust velocity pea-shooters, the safety distances are considerable.

Rocket boosters for taking off from large bodies such as Earth will be large and mostly filled with fuel for the foreseeable future. Even the Apollo Lunar Module was on the order of 50% fuel, and the Moon's gravity (which was all it had to contend with) is far smaller than Earth's (to the tune of about 1/6th Earth gravity).

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    $\begingroup$ There is no reason at all to be throwing anything out the tail of your rocket at 65-70 KM/s at zero altitude. (Unless you want to use your rocket as a weapon). Virtually any sophisticated engine will have some way to throttle engine output! Even simple rockets can usually do that. You start off with just enough thrust to move the craft itself and slowly gain some altitude, then you throttle up once you have safely cleared your takeoff zone. Max thrust would occur at very high altitudes. $\endgroup$ – JBiggs Dec 12 '16 at 17:58
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    $\begingroup$ @JBiggs exhaust velocity is not the same thing as engine thrust, and reducing exhaust velocity (even temporarily) necessarily reduces total deltaV (see rocket equation in answer). Reducing exhaust velocity reduces exhaust momentum, so to get the same thrust, higher mass flow is required(more fuel per second), this means that to get a slow, safe exhaust near surface, we have to have an upper atmosphere exhaust velocity much higher than 66Km/s. We have no concepts for any practical thrusters with 2+ orders of magnitude variability in exhaust velocity and mass flow. $\endgroup$ – Leliel Dec 12 '16 at 19:01
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    $\begingroup$ In short, we're not doing it with any conceivable rocket. It's got to be something not based on rocket propulsion. But as far as we know, almost any such propulsion violates the conservation of momentum, which is a fairly fundamental rule in physics. $\endgroup$ – Leliel Dec 12 '16 at 19:03
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    $\begingroup$ @JBiggs When you start off is the exact moment when you need the greatest thrust. Not only are you carrying the greatest amount of fuel, you are also deepest in the gravity well. That's pretty much the reason why ion engines work great as thrusters for interplanetary probes, but are lousy for launch vehicles. $\endgroup$ – a CVn Dec 12 '16 at 19:59
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    $\begingroup$ And of course, when you are doing the math for real, since you won't be dumping all of the propellant in a single instant, you are integrating the rocket equation over ridiculously small values for $m_0-m_f$. $\endgroup$ – a CVn Dec 12 '16 at 20:17

There are various putative SSTO designs, such as the Skylon. This is a spaceplane: it uses the atmosphere as reaction mass to gain altitude and speed as far as it can. The concept has a 275 tonne vehicle reaching orbit and returning using 220 tonnes of (mostly) liguid hydrogen fuel.

For your 10% you will need — at the very least — a fuel with ten times the energy density of liquid hydrogen. So, a nuclear spaceplane? It is not physically impossible, and since the energy would not be released by combustion, it could work on thinner air than a ramjet.

As for not killing its crew or people in the vicinity of its departure with radiation, we have to rule out fission. So the power source has to be an aneutronic fusion design. D-He fusion is such, or there is a Boron-proton fusion reaction with a larger cross-section.

Making this work would involve technology way beyond anything that could be viewed as plausible today, but it's not physically impossible. If you are prepared to handwave a compact high efficiency muon generator, then muon-catalysed fusion might do the trick.

Alternatively, feeding raw energy along a wormhole is less implausible than having live human beings traverse a wormhole. In which case the power plant does not need to be on the spaceplane. The physics for this is extremely speculative, though.

  • $\begingroup$ You could also combine the spaceplane idea with ion thrusters, presuming you can deal with miniaturizing them by a few orders of magnitude. The latest toys on this front can hit an exhaust velocity of 210km/s... though with a mere 2.5N of force! $\endgroup$ – Cort Ammon Dec 12 '16 at 14:56
  • $\begingroup$ @CortAmmon Yeah, the huge amount of thrust you get from ion engines is a bit of an issue for anyone anywhere nearby... $\endgroup$ – a CVn Dec 12 '16 at 15:31
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    $\begingroup$ @MichaelKjörling they don't really generate that much thrust (Thrust is force, and force is mass times acceleration- you get to accelerate the particles to massive speeds, but the mass is so insignificant that you end up with very little force). They do allow for immense ISP (efficiency), though. $\endgroup$ – Delioth Dec 12 '16 at 17:35
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    $\begingroup$ Technically Skylon doesn't really use the atmosphere as "reaction mass". It uses air as supplementary fuel. The idea is that in a normal rocket, you have large tanks of hydrogen and large tanks of liquid oxygen that are all mixed in the rocket engine. The concept behind Skylon was: "Why not take that oxygen out of the air instead of relying on a tank?" It's an air-breathing rocket/scramjet hybrid. As far as I know, "reaction mass" implies something inert (like water) that is simply expelled to provide thrust in space. $\endgroup$ – JBiggs Dec 12 '16 at 17:52
  • $\begingroup$ Another point: it is entirely possible for a nuclear rocket to simply use the heat from a fission reaction without irradiating the reaction mass being used. So, we shouldn't rule out fission. Lots of design work was done in the area of nuclear powered rockets (and even airplanes!) once upon a time. $\endgroup$ – JBiggs Dec 12 '16 at 17:55

Introducing the VacuumJet!

Sinice we are on the ragged edge of physically possible here anyway, I thought I might take a crack at it.

How about a "VacuumJet" engine? This is a little thought experiment that I once worked out while trying to come up with plausible sounding explanations for some of the technology for a Robotech RPG game I was running.

We start with an advanced fusion reactor. This miniaturized fusion reactor is a super advanced stepchild of what we have now. It uses a torus of very strong magnetic field projectors to contain a doughnut shaped stream of plasma. We can use the fusion reactor to generate heat when we want it to, which is very useful in an atmosphere. This is because the reactor is housed in the thick, rear part of basically a scramjet with no moving parts. (Metal funnel that causes atmosphere to expand rapidly which forces new atmosphere in the front end and creates thrust. The term "Vacuumjet" comes from the fact that the heat actually creates a temporary vacuum inside the engine as air is forced to expand at a very high rate).

So, at altitudes right up to low orbit, our scramjet "Vacuumjet" uses heat from a reactor to provide enough thrust to get us nearly out of the atmosphere. (There is handwaving here at lower speeds, where scramjets are not very useful). As far as destroying the environment where the ship lands, it would cause some pretty heavy duty jet wash and a lot of hot air, but I wouldn't consider it highly destructive compared to a rocket.

Once you get up into low orbit, you introduce some reaction mass. Water works really well for this, and it has a very high expansion rate for a given density of water, plus, there is the advantage of having ice all over the solar system that can be mined pretty easily to reload your reaction mass when you run out. Water is basically just pumped into the scramjets the same way that air would be in an atmosphere, and the heat from the reactor would blow it out the back in the form of rapidly expanding steam. This all means that as long as you have a sufficient energy source to power the reactor, you only need a relatively limited amount of reaction mass once you get up to a very high altitude, just to get that last little push into high orbit.

In my RPG game, the energy source itself was a major handwave (among others). In your case, you may want to come up with some kind of unobtanium that fuels the fusion reactor (or go with a very efficient fission reactor, which is a very natural heat generator anyway). Your ship will have to keep track of the reactor fuel and periodically stock up on ice.

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    $\begingroup$ "Since we are on the ragged edge of physically possible here anyway, I thought I might take a crack at it." - I guess u are cheating here )) $\endgroup$ – MolbOrg Dec 12 '16 at 16:50
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    $\begingroup$ Not cheating at all. The OP asked that the answer not handwave physics SO OVERTLY that a scientifically aware child would be able to see through it. My handwaving is actually pretty sophisticated here and uses a lot of totally legitimate concepts. I am totally fulfilling the OP question's parameters. $\endgroup$ – JBiggs Dec 12 '16 at 16:51
  • $\begingroup$ Ok, I may agree on this one) As I see no handwavium here, except of over-complicating the reactor deal and atmosphere reentry. A bit of polishing and it will be fine. $\endgroup$ – MolbOrg Dec 12 '16 at 16:57
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    $\begingroup$ Does it require astrology or did you mean to write "torus" instead of "taurus"? Also, how are you making the expansion happen only in one direction or was the the handwave? Also note that the solution should not depend on an atmosphere. $\endgroup$ – Samuel Dec 12 '16 at 19:39
  • $\begingroup$ @Samuel, obviously astrology and witch-doctor finger bones are required... As to expansion; a scramjet or ramjet work by use of a specifically shaped chamber and the application of heat. The shape of the chamber forces air out one direction more than the other, and heat makes air expand. As to depending on atmosphere; Try reading an entire answer before commenting on it. $\endgroup$ – JBiggs Dec 13 '16 at 16:55

Laser rocket is the obvious choice for lift-off. There are many different designs, but ablative laser propulsion seems quite promising. Basically all you need is an array of high energy pulse lasers on the ground, blasting at ablative propellant target at the bottom of the rocket. Once high enough and with enough speed, remains of the ablative propellant would be dropped off, and actual space propulsion, what ever it is, would take over.

The ablative target should probably be something relatively chemically harmless, such as iron. Obvioisly there will be a safety zone required, because the energy pushing the rocket up will need to push other stuff down with equal momentum. Suitable strong ground-based magnetic field would allow directing the propellant plasma to some kind of blast pits, reducing size of the required safety zone.

To comment on need for external help, for launching off from any planet or location, the rocket could carry with it a "throw-away" laser launch system, with a sealed nuclear reactor with enough fuel to power desired number of launches, and lasers to match. The lasers for this use could also be direct nuclear pumped lasers, resulting in a very simple setup. Just choose your landing spot carefully so that you can set the launch system up! In this case I guess you have to count the weight of this launch system in your 10% "fuel weight" limit.

(Anybody is free to take this idea and write a better answer with actual maths on how powerful laser is needed, how much safety zone would be plausible, and if this can actually achive the 10% max on-board propellant weight requirement, though I think that is just a matter of total laser power.)

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    $\begingroup$ +1 Really, the only way to meet OP's requirements is with an energy-source based somewhere else. To meet the 10% requirement, the rocket could receive laser pulses from orbit or (in theory), from even further off. $\endgroup$ – Ryan Mills Dec 13 '16 at 1:20
  • $\begingroup$ Orbital lasers might be good for later parts of the flight, but I doubt enough power can be beamed from space for actual lift-off. There's just too much atmosphere in the way, the beam would disperse too much and fry the entire rocket. Maybe Geosynchronous laser and lift-off from the equator directly underneath could be made to plausibly work. Or other inclined orbit where the satellite would be momentarily almost geosynchronous for long enough to get the rocket out of the atmosphere. $\endgroup$ – hyde Dec 13 '16 at 7:27
  • $\begingroup$ @hyde That depends on the frequency of the laser. If you pick it correctly you can probably use space lasers. $\endgroup$ – Samuel Dec 13 '16 at 17:03
  • $\begingroup$ I prefer an electromagnetic railgun/mass-driver over laser propulsion for liftoff. However the laser may be better if the spacecraft needs to be manned or is otherwise transporting things that can be damaged by massive acceleration. $\endgroup$ – aroth Dec 14 '16 at 1:24
  • $\begingroup$ @aroth Note the "without external help" part in the question. Using deployable, discardable (or well, it could be picked up again if it is in the orbit instead of on the ground) laser is borderline for that, but I have hard time imagining deployment of a mass driver without external help. $\endgroup$ – hyde Dec 14 '16 at 5:16

What about carrying your space elevator with you? The majority of your ship's mass would be this tool, but maybe you could adapt it for your story.

Navigate into a stable orbit of the body you want to land on. Now, send your space elevator counterweight away from the planet, and a tether down to the planet, keeping your ship stationary. At the end of your tether would be something to grapple onto the planet. I sort've doubt that this would satisfy your third requirement about not destroying the landing spot, but maybe you could handwave that much if you're seriously considering this anyway. After you're grappled, you can now ride the tether down.

To leave, you ride back up, ungrapple and reel the rest of the equipment back in.

One drawback is that this method would have a limit based on the planetary body's size/stable orbit distance, as your elevator will only be so long.

  • $\begingroup$ carry an orbital ring with you instead of the elevator, because elevator heavily depend on gravity of the body, angular velocity of the body(how fast it rotates), nearby bodies and such. $\endgroup$ – MolbOrg Dec 12 '16 at 20:11
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    $\begingroup$ You can also have an orbital pinwheel. For the most fun exposition of this concept, read Charles Stross Saturn's Children. But in the context of this question, I think an orbital megastructure that periodically dips into the atmosphere of a planet is cheating. $\endgroup$ – nigel222 Dec 14 '16 at 11:00
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    $\begingroup$ I gotta say this would definitely make for the coolest villain entrance ever. Bonus points if you spend a whole chapter talking about the life of some farmer and then crush his house and farm in this scene. $\endgroup$ – Devsman Jan 9 '17 at 14:11
  • $\begingroup$ @MolbOrg In my eyes, the orbital ring is too constrained. The appeal of the mobile space elevator is that it can be adapted to be used on a large variety of different bodies you want to i̶n̶v̶a̶d̶e̶ land on. $\endgroup$ – agweber Jan 9 '17 at 15:28
  • $\begingroup$ I would say, exact opposite, as you have to have rotating planet to conquer, but with the ring it can be any planet, no matter how fast or slow it rotates. But sure at some fast rotating planets lift may have strategic advantage - so I suggest consensus - to have a tool which may fast roll out as ring or as lift whatever is more appropriate in a situation. $\endgroup$ – MolbOrg Jan 9 '17 at 22:27

I don't know about liftoff engine, but mass drivers could be used with some success to launch your spacecraft into space, using comparatively little energy in the process.

enter preformatted text here

By using what is essentially railgun technology, one could launch payloads into space with ease, causing little damage to the system in general.

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    $\begingroup$ There's a reason why that picture appears to be on the moon. Mass driver + atmosphere = burnup. $\endgroup$ – Joshua Dec 12 '16 at 19:44
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    $\begingroup$ I present thee Why don't we use catapults to get to space? on Space Exploration. $\endgroup$ – a CVn Dec 12 '16 at 20:12
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    $\begingroup$ @Joshua Burnup isn't the problem with proper ablation and materials. The issue is with the G forces that push on the passenger. To a 12 year old's knowledge of physics (as the OP asked), the mass driver could work, granted albeit is impossible in reality without as Star Wars would put it "inertial dampeners". I am under no delusions that this would actually work, but for OP's purposes, it might. $\endgroup$ – Anoplexian - Reinstate Monica Dec 12 '16 at 20:29
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    $\begingroup$ @Anoplexian It would work. You just have to make the gun silly long. $\endgroup$ – Aron Dec 13 '16 at 5:47
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    $\begingroup$ It's theoretically possible to give a rocket a useful head start by using a linear electric motor "sled" to accelerate it up the side of a hill. Engineering-wise, the problem is that you cannot abort the launch if the rocket motor fails to ignite properly near the end of the track! (Also 3-D changing G forces rather than all aligned on the Z axis). On the moon, with low G and no atmosphere, you could probably get all the way to orbital velocity using ground-based electrical propulsion. $\endgroup$ – nigel222 Dec 13 '16 at 10:17

A Stargate.

Put a stargate in orbit and have on on the surface. When you're ready to go, just push your spaceship through the stargate.

Granted, to use actual stargates from Stargate, you'll need to put it in orbit of a different world, but the idea still holds.

  • $\begingroup$ Good point. I'm thinking about some device which acts not like a portal but more like a star trek transporter. Then the energy consumption could make up for the changes in potential and kinetic energies. But I'm not sure where I let the energy vanish when "beaming" down. $\endgroup$ – Hothie Dec 12 '16 at 14:30
  • $\begingroup$ Violates rule 4. $\endgroup$ – TheBlackCat Dec 12 '16 at 18:36
  • $\begingroup$ @TheBlackCat - not necessarily. You don't know what it does to balance the energy equations (e.g. it could draw increased current to make up for the additional kinetic energy) $\endgroup$ – iAdjunct Dec 13 '16 at 4:40

Vertical launch; gas core water cooled matter-antimatter reaction drive

For deep space travel we use the much more efficient nuclear saltwater rocket; but to protect the environment of the planets upon which we land we spend the energy to convert matter to antimatter while still in deep space and use a matter/antimatter reaction drive instead for very high thrust without emitting long-term radiation.

The radiation released by a matter-antimatter drive consists mostly of muons, which have short half lives, and we direct them straight down during the early phases of launch.

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    $\begingroup$ Wouldn't the launch pad progressively become radioactive the more it's used? That's the usual effect of bombarding any matter with large doses of high-energy particles, muons or otherwise. (Also I thought a matter-antimatter drive would emit photons, but I could be wrong.) $\endgroup$ – zwol Dec 12 '16 at 18:14
  • $\begingroup$ Only certain sorts of radiation create long-lasting radioactive waste. Others don't. And what exactly results from matter-antimatter annihilation depends on the particular particles being annihiliated. Protons and antiprotons produce pions, which then decay into muons. The muons then decay into electrons or antielectrons and neutrinos. $\endgroup$ – TheBlackCat Dec 12 '16 at 18:43
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    $\begingroup$ @zwol: Thankfully, muons do not normally induce transmutations of the elements. $\endgroup$ – Joshua Dec 12 '16 at 18:50
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    $\begingroup$ Consider Michael’s note from his answer «no matter what you are throwing out behind the engine at 65-70 km/s or more, it's going to be dangerous. Even with our current puny 3-5 km/s exhaust velocity pea-shooters, the safety distances are considerable.» you postulate antimatter so you have sufficient power but even if you use air for reaction mass (e.g. a jet) you won’t fill #3 except at a prepared launch site. $\endgroup$ – JDługosz Dec 13 '16 at 5:23
  • $\begingroup$ "The radiation released by a matter-antimatter drive consists mostly of muons" Unfortunately, the remainder is often gamma rays, which are kind of problematic. $\endgroup$ – anaximander Dec 13 '16 at 9:19

invisible rocket

Others have pointed out that, even given some miracle power source, the energy and momentum of the rocket exhaust (as need to lift off) will be quite destructive.

My variation of that is: what if the rocket exhaust was not? Spray high mass, fast-moving particles about in great abundance, and have it not affect the surroundings.

I think the general idea as used in a Robert L Forward short story used neutrinos.

But you would do better to make a dark matter rocket since dark matter particles (specifically WIMPs) are heavy. This lets you use less energy to get the needed momentum (at the expense of consuming reaction mass).

So, what you need is a rocket that then changes the exhaust material into dark matter after you have manipulated it. That’s how neutrino beams are made: you form the beam with charged particles so you can manipulate them, and then those decay into neutrinos.

A dark matter rocket would seem to the outside to be a reactionless drive. It could be in the middle of the ship without an exhaust port.

speculative fiction mechanism

What kind of techobabble could do this? “supersymmetry”. And as far as we understand (cold dark matter being a thermal relic of the big bang) this is a real thing! Some as yet unknown mechanisms can “flip” matter into dark matter at very high temperatures.

Then, rather than just being hot in a thermal mess, you carefully tune the reaction by using “resonances” to set up the exact energy needed. This makes more efficient conversion without waste. This is the same thing you can bring up to make the fusion power plant work! So, explain that the same tech that allowed fusion rockets to be built also (once the details of the particles were known) to turn that into a dark matter rocket.

another mechanism

Looking up some links to explain how the “thermal relic” implies these things, I thought of another way. This also unifies with the power generation.

The ability to convert matter (as happened in the early universe) depends on multiple R-parity violations (that is, more than one process) that occurred at extraodinary temperatures. Now, to get it to happen at conditions possible to produce today, rather than cooking it at some ridiculously high temperature (so even rare things happen) you vary carefully tune the energies and reactants. Like a combination lock, multiple pathways must be promoted (by providing narrowly tuned conditions) at the same time. Note that careful tuning vs outrageous temperature is the same idea as before. But here, invoking “multiple channels at the same time” gives us the combination lock analogy.

Now here’s the kicker: the same stuff can induce proton decay. Make that part of the plot: not only do we have a power source from ordinary matter, that’s the very process that can produce dark matter beams.

more power

JBiggs’ answer, stripped to the essentials, is that given enough power (e.g. nuclear fusion) an airplane would be fine. Airplanes (VTOL, helicopters, and whatnot) fill the OP’s requirements on unobtrusive takeoffs.

What you need, then, is to accelerate to 18,000 miles per hour. With enough raw power, a jet can do that, using the atmosphere. However, look at the real-world reentry to see the problem: with thick air the ram pressure will bake the craft even if you had power to push through the air. With thinner air, you need a big scoop to gather enough air to attain sufficient thrust.

So suppose in airplane mode it simply flies as high and as fast as possible; that’s still a long way from orbital speed which must be achieved outside of the atmosphere. So you must switch to rocket mode. But, you don’t require the absolute thrust needed to lift vertically off the Earth! You could do with a low thrust, focusing on efficiency of reaction mass instead. Say you start at 2000 mph and angle your jet to gain altitude until the air is too thin to work; then fire the rocket at a fraction of a g thrust, building speed. A few minutes later your arc takes you back down to the air and you arrange to “skip” off the air! Repeat with successively longer hops as you gain speed and attain full orbit.

After writing that, I recall hearing the concept for real. Not for reaching orbit, but for an extended flight at mach 10 using air-breathing engines. I managed to find the reference to Hypersoar by Lawrence Livermore National Laboratory.

real-world designs?

Why not look into what real-world rocket scientists think of the idea of single stage to orbit like the Skylon or other spaceplanes. Look over near-future designs, and figure what you might do with sufficient power and how light you would need to build beyond current materials, and design your craft based on that.

  • $\begingroup$ FTR, nobody actually knows if dark matter particles are heavy (or indeed that it can be described as particles at all). The fact that dark matter interacts with visible matter through gravitation does suggest it, though. $\endgroup$ – leftaroundabout Dec 13 '16 at 18:58
  • $\begingroup$ Actually, we do know both. Heavy because they are cold dark matter and dynamics of forming the large scale structure we see. Particles because of quantization: particles pop out of the math for any possible interaction, which ultimatly takes place as an all-or-nothing lump. $\endgroup$ – JDługosz Dec 14 '16 at 6:40
  • $\begingroup$ Particles pop out of the math assuming some kind of QFT holds. (Which is certainly a reasonable theoretical assumption, but still – anything you build with dark matter is necessarily more speculative than sticking to nowadays measurable particles.) $\endgroup$ – leftaroundabout Dec 14 '16 at 10:06
  • $\begingroup$ It applies to larger families than just QFT. Given the uncertainty principle, the idea necessarily infects everything that touches what already has it. So even if DM were amorphus or otherwise played by different physics, it would appear as particles when interacting with us even if only through gravity. Overthrowing physics as we know it while still keeping valid the rules we do know is way beyond the simple hack needed by the premise. $\endgroup$ – JDługosz Dec 14 '16 at 10:25
  • $\begingroup$ I’ve added clarification and links to indicate the specific models I’m referring to in the DM section. Even if it turns out to be wrong, it’s smart science fiction consistent with what we know. $\endgroup$ – JDługosz Dec 14 '16 at 10:39

If you didn't have to worry about acceleration, why not just have your rocket push itself off the planet--like with a giant spring or very strong stick.

The problem is if you want non-computer intelligences to survive there has to be some kind of protection against the acceleration that would be required--if you think about it you'd have to complete your acceleration within the length of your "Spring" or "Stick" is that you are using to push yourself off the planet. The longer the stick, the less g's you need to deal with.

So part 1 of the solution is a very long stick--What about a liquid metal like in Terminator 2 that can form itself into a pseudopod to push you--this could go quite far if you assume your metal to be very strong with very little mass (Probably using some kind of rigid-but-mostly-hollow self-created internal structure like "aerogel" (fascinating material). The nice thing about a self-shaping metal like this it is retracted into the ship it would have endless additional uses--reconfiguring internal rooms, extra insulation, radiation shielding, etc. It would even make a great heat-shield which you would need during take-off.

Part 2 would be some system of "hardening" the squishy bits. Freezing everyone into solid blocks of ice might do it, or you could do what they did to Hans Solo. Pierce Anthony had a really interesting (if far-fetched) solution in Macroscope where he melted the people down into a protoplasmic goo that as a liquid could resist any amount of pressure, then after the trip they were re-constituted to their original selves.

The more likely (although not very fun from a story-writing POV) solution to part 2 is to just make all your spaceships unmanned drones that are hardened to a very high acceleration.

Part 2 would also allow much faster conventional space travel--unless you have some magic like a "Warp drive" this is a serious issue.

Note that the pseudopod solution would also allow you to land with minimal energy wasted--in fact it might be able to recover some of the energy it absorbs as it decelerates the ship, and imagine it reaching out and grabbing an asteroid to make a slingshot maneuver with a much lighter object than gravity would allow.

Man I'm really starting to like this idea.

  • $\begingroup$ It sounds goofy at first but I actually really like this idea. If you had a small "ship" with an ultralight, durable shaft for it to slide through attached to it, it could accelerate up the shaft over a great distance, then take the shaft with it when it leaves. Then it could reorient itself and decelerate for landing via the reverse procedure. $\endgroup$ – Devsman Dec 13 '16 at 20:38
  • $\begingroup$ I originally pictured it as 3 shafts like landing gear, but hadn't thought of them being a LOT longer than the ship, so I thought of the nanotech metal instead. It could really also be a spring (or 3, or...?) with a mechanism that locks when fully compressed so that taking off is just a matter of unlocking the spring and holding on to your britches.. I'm not sure but the energy stored in the spring might avoid a lot of the heat problems of dealing with energy released as you decelerate. Also--kicking off asteroids! $\endgroup$ – Bill K Dec 13 '16 at 21:07
  • $\begingroup$ That is good. The magic metal u talking about are carbon nanotubes. But the rest is kinda resembles what I meant in the comment to the question. Play with that idea, it really opens interesting perspectives and changes the way we imagine space ships and how they act in the space. Second useful moment may be active supporting structures, and you may try to watch on youtube mega-structures play list from Isaac Arthur $\endgroup$ – MolbOrg Dec 13 '16 at 22:36

Use an RF resonant cavity thruster.

This is an experimental technology that has been gaining attention. Some physicists believe it is impossible because it appears to violate Newton's laws of motion, and yet some tests have resulted in thrust being produced.

I've read a few papers explaining how different physicists think it might work, and it's difficult for me to follow, but "you put microwaves in it and it generates thrust" is about the level of detail an average audience looks for in a scientific explanation.

As with all experiments, these positive results could be obtained in error, but also as with all scientific models, Newton's could be incomplete or misunderstood.

What anyone believes is up to them, but at the very least, this is is going to be as close as you're going to get to fulfilling the requirements set forth by the question.

  1. It should be simple to "understand" for the audience - You put microwaves in and it generates thrust.
  2. It should not consume more than 10% of the ships mass in fuel - No fuel at all.
  3. It should not destroy the place of liftoff or landing - Should be good.
  4. It should not violate fundamental laws of physiscs conspicuous even to a 12 year old with some interest in physics. - The thruster itself does this, so as worded this is a violation and yet the device is still a topic of interest and continued research.
  • $\begingroup$ Probably the best option for OP, from all answers presented so far. $\endgroup$ – MolbOrg Dec 13 '16 at 22:32
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    $\begingroup$ It does appear to generate thrust, but not much of it, and it takes a lot of power to do so. To take off you need a high energy nuclear reactor and output enough waste heat to cook your surroundings. $\endgroup$ – Donald Hobson Dec 13 '16 at 22:35


This gets around #1, #3 and mostly #4, but does not address the issues other answers have provided with #2. Assuming you handwave the technology to produce high-velocity exhaust with sufficient thrust, the balloons will get around the problematic issue of destroying whatever the thrust is pointed at.

The engineering requirements on balloons sufficient to lift a spaceship is another problem, but nothing that most 12-year-olds would wonder about.

  • $\begingroup$ You mean you lift off with a balloon to get far enough away before firing the rocket? $\endgroup$ – JDługosz Dec 14 '16 at 6:46
  • $\begingroup$ Okay, and to lift off you fill your balloon with...? $\endgroup$ – Steve Taylor Dec 14 '16 at 13:04
  • $\begingroup$ @JDługosz Yes, lift off far enough that your rocket doesn't destroy the launch site. $\endgroup$ – Logan Pickup Dec 14 '16 at 17:46
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    $\begingroup$ @SteveTaylor Anything less dense than the atmosphere. Clearly it will not work on a planet without atmosphere, but for most atmospheres hydrogen or helium will do. The same gas can be re-used as well; re-compressed and stored in the ship when not needed. I do not know the math to work out how likely that is. $\endgroup$ – Logan Pickup Dec 14 '16 at 17:55
  • $\begingroup$ There's a reason we moved away from hydrogen balloons... also, as you get higher you'd get a severe risk of the balloon popping, without much atmosphere to press in on it from the outside! $\endgroup$ – Steve Taylor Dec 14 '16 at 19:32

Laser propulsion has another cool advantage, you don't even need ablator if you're in a planet with atmosphere. You can actually use a design that uses air as its propellant for what is effectively the first stage. Such a proposed design is called a lightcraft.

This has a few major downsides. The first problem is that we have no idea how to build a laser of the scale required. A reasonably near term proposal uses a cubesat payload with a payload of 40kg and still finds that it would take a 350 MW laser. This would require 7200 commercial 20kw lasers in an array. I'd hate to see the monster capable of launching something serious.

It also isn't as efficient as you want. It still needs more than 10 percent of its mass as fuel. The wet mass is still eight times that of the payload. While it doesn't need fuel, it needs to carry propellant to get the rest of the way into orbit. Once the craft hits 35 km, it no longer gets enough airflow to serve as a useful propellant. At this point it switches to water.

The next big problem is that of power generation. That tiny cubesat version would still take most of the electrical output an entire aircraft carrier(because lasers are not very efficient, which is also why space based solar basically won't work). Something big enough for a serious payload would require a few dozen nuclear reactors.

Another side effect is that it would violate the treaty against laser blinding weapons, as the reflection would be bright enough to blind onlookers. More seriously it would also be a political problem in the same sense as nuclear weapons given that level of power output could be militarized for other things.

Also, I would not suggest the RF thruster unless you want to look foolish in the future. It is almost certainly impossible.

  • $\begingroup$ Welcome to WorldBuilding.SE! There's nothing wrong with answering old questions as long as your your answer is good, and yours looks good to me! $\endgroup$ – F1Krazy Nov 27 '19 at 9:38

I know this isn't feasible given our current knowledge, but just like we can use a lens to focus light waves, radio waves, sound waves, etc., I seem to recall reading a story (perhaps one of Asimov's short stories) where one used a gravity lens for propulsion. For example, to lift off from Earth, point the lens at the sun and the focal point is the back of space vehicle. So the vehicle, from bottom to top, is a focus mass, living quarters, and the lens. Once in space you can point the lens towards another celestial body to take you in that direction or offset your current azimuth and steer towards your other destination after building up speed.

To land you could change aperture as to how much gravity you let into the lens. Let the planet's gravity be just slightly higher than the amount of gravity you 'gather' from the star or other body.

  • $\begingroup$ This violates #4, and I'm not sure it meets #1. $\endgroup$ – a CVn Dec 12 '16 at 20:15
  • $\begingroup$ @MichaelKjörling Do you think focusing gravitons like we do photons is literally impossible? If so, then yes, I agree, it would violate #4. $\endgroup$ – Tracy Cramer Dec 12 '16 at 20:17
  • $\begingroup$ See here and here. $\endgroup$ – JDługosz Dec 12 '16 at 21:35
  • $\begingroup$ @JDługosz, thanks for the links but I'm not sure they qualify as "conspicuous even to a 12 year old". I'm not 12, have an interest in physics, a few college courses under my belt, my own reading, and much of those answers go over my head. That's not to say there aren't some precocious and/or brilliant 12 year old's out there, but if you're writing a pre-teen/teen book (my assumption based on this comment) that's probably not your intended audience. Nevertheless, it would appear that this is, based on our current knowledge base, impossible. $\endgroup$ – Tracy Cramer Dec 12 '16 at 23:12
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Tracy Cramer Dec 13 '16 at 2:23

The <10% fuel requirement means we need a high exhaust velocity. Far beyond what we can do with chemical rockets.

The non-destructive nature is far more of a problem. You are liberating vast energies when you push a rocket into the sky, that energy is going to bounce off the ground and be quite destructive.

What can we eject from our rocket that goes at very high velocity and doesn't tear up the ground? Neutrinos!

Now, I have no idea how we are supposed to generate this flood of neutrinos or direct them in the right direction but you didn't ask for something known to work, but just something that doesn't obviously not work. Assuming you had an appropriate neutrino generator you would have a rocket that could take off from your lawn without killing the grass (except where it was squashed by the landing legs.)

  • $\begingroup$ Did you read the other answers (e.g. mine) before posting? Where I even explain how neutrino beams are generated today, rather than asking another question? $\endgroup$ – JDługosz Dec 14 '16 at 6:42
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    $\begingroup$ @JDługosz Somehow I missed your addressing of neutrinos. $\endgroup$ – Loren Pechtel Dec 14 '16 at 17:48

For rule #3, we need to not destroy the place of liftoff. Let's say that exhaust velocities of 3 km/s are OK, but going higher would destroy the launch site. (The Falcon 9's effective exhaust velocity is 2.77 km/s, and the actual velocity is higher, so this seems reasonable.)

Escape velocity is 11.2 km/s, so the rocket equation says the required mass ratio is $\exp(11.2/3) \approx 42,$ but we want a mass ratio at most $10/9 \approx 1.11.$ I propose a solution: the spaceship refills reaction mass locally. "All" they need to do is compress and store matter from the planet weighing 50 times the mass of their ship, then accelerate it to the required speed. That way they don't consume matter from the ship's mass, as required.

What's left? First, they need a power source which can produce the required energy -- 63 billion joules per tonne of the ship's mass -- without consuming more than 10% of the ship's (usual) mass. DT fusion releases 340 billion joules of energy per gram, and so a hypothetical technology able to extract most of this (which is hard, the bulk is in fast neutrons which pass through most things) could do this easily.

Second, they need to have a way to use this energy to accelerate the bulk matter to very high speeds. Presumably there's a filtration process by which they pick up only the most suitable of the available matter. Compared to fusion this should be easy.

Third, they need a way to compress the matter ~100 times denser than the density of their ship (at which point it would require half the ship's volume to store). This might be difficult for us, but again this should be very easy for a civilization having mastered fusion.

I think this meets the requirements of the question:

  1. I don't think the audience would understand or expect to understand fusion itself; I think that they would generally be content to accept a fusion reactor as a primitive. More sophisticated audience members might appreciate that you've stayed within the hard constraints of nature, even if you've pushed closer to them than we have any hope of achieving in the near future.
  2. None of the ship's mass is expelled as reaction mass, and in principle only a very small percentage is used up, less than a tenth of a percent of the ship's mass for a fusion reactor capturing 0.1% of the total energy. (Getting to 0.1% efficiency, as I understand, would be very difficult.)
  3. It does not destroy the liftoff area any more than current rockets... admittedly, a somewhat generous interpretation.
  4. As far as I know it does not violate any laws of physics, just pushes them to their extreme. (Whether this is possible or not is more a question of engineering than physics, in other words.)

A nuclear ramjet could conceivably fulfil all of criteria except for the need to work without an atmosphere. Whilst current technology would be damaging (with the radiation) a theoretical fusion reactor-based design could avoid much of that.



So, I'm going to be slightly silly here because I do not have a physics background and wouldn't even know where to begin, but you could get a little steampunky with this: Balloons!

Hear me out. There are Balloons that can get all the way up to the very edge of space. They rely on Buoyancy, so they do not need reaction mass until you get almost all the way up anyway. They will be already aloft when you fire up the engines, so the landing pad will take no damage at all. On the way up, you could have some sort of intake that will be gathering atmospheric gases to use as reaction mass as you get higher, meaning you save on reaction mass.

Now all you need is to be able to accelerate the particles at near relativistic speeds as you reel in the balloons and re-compress the helium. There is nothing left but to have fun in space.

Yes, this relies on a currently impossible energy source, but once we get to taming fusion....Hey, doesn't fusion create Helium?!?!?! Balloons


Use an impulser engine.

An impulser pushes off space-time, generating a ripple that carries equal and opposite momentum, therefore it doesn't violate Newton's laws of motion. (rule #4)

Impulsers only require an electric power input (no propellant) - no mass is ejected from the ship. (rule #2)

The amount of momentum imparted by the ripple is equivalent to the momentum imparted to the ship, therefore it shouldn't cause any more kinetic damage to a launchpad than a regular engine. Even better, it doesn't have all the flames and noise associated with a traditional rocket launch and therefore the damage is much lower. (rule #3)

The concept is simple: An impulser turns space-time into a medium of exchange for momentum, eliminating the need for propellant as an intermediate. (rule #1)


Since you aren't requiring that your ship obey the prime directive, have its robots construct a mag-lev launch platform with a massive power source at the landing site. Mining and refining raw materials from the planet, the ship could create everything needed to throw itself back up to an altitude where its thrusters could be engaged safely. After the ship has left, sacrificial robots could dismantle the construct and bury the parts.

If suitably advanced technology is available, substitute the word "nanite" for "robot" in the paragraph above.


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