Simple non destructive spaceship liftoff engine

Question

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?

Answer 1

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).

Answer 2

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.

Answer 3

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.

Answer 4

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.)

Answer 5

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.

Answer 6

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.

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

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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.

Answer 11

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.

Answer 12

Balloons.

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.

Answer 13

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.

Answer 14

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.

Answer 15

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.)

Answer 16

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.)

Answer 17

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.

https://en.wikipedia.org/wiki/Project_Pluto

Answer 18

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

Answer 19

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)

Answer 20

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.

Answer 21

Induced gamma emission

You want a long half-lived or even "non-radioactive" meta isotope which you will stimulate to release high energy gamma rays, hence lots of energy, as needed. It's a nuclear reactor you turn on and off at will. The simplest experiments have supposed that gamma rays at the same energy might do it - this hasn't seemed to hold up very well, but it implies some sort of "laser" like amplification.

Antimatter

You can't get more bang for your buck. At least, not unless everything goes perfectly right...

Copula drive

This is something more commonly termed a "space fountain", because I assume slightly different background. There are so many standard-sized packages coming down from space, full of refined resources from space industry, that your ship can simply pass around each one, using electromagnetic induction to push it down at the Earth faster, while accelerating upward itself. It is best to have some good computer piloting for this one ... and hopefully, a similar induction mechanism to catch the falling packages at the receiving dock.

Orbital tether

It's mostly a space elevator, but it doesn't stay anchored to the ground and, being shorter, has lower material requirements. You use a dinky "space tourism" rocket, latch onto the low end of the tether, and ride the whip up into low Earth orbit.

Dark Matter Hair Drive

You have developed some method to make ordinary matter interact with dark matter. Perhaps this doesn't work well enough for the ordinary low concentration of dark matter in the background, but an "astrological influence" of the stars might deliver "hairs" of dark matter, extending from the Sun or Jupiter in the direction of the Earth, which your drive might seek to align with and use. If the dark matter is made of "hot" fast moving particles, then your mysterious means of coupling with ordinary matter might allow it to be superheated and used as a near-endless source of high energy propellant, at least along the direction of the "hair".

Some of these may be more plausible than others. :)

Answer 22

Use negative matter to negate the craft's weight. Negative matter is matter with negative mass, so it is repelled by gravity. This can act like a "balloon" except that it is lighter even than a vacuum and works on airless planets. Also, the annihilation of negative matter and ordinary matter completely destroys the two particles with no by-product, unlike antimatter. (satisfies rule #1 because this is quite simple)

It negates most or all of the craft's weight (it is pulling upwards on the ship), so only a very small engine is needed, and therefore not much reaction mass. Note that the negative matter, while taking up around 50% of the ship's mass (so that it can lift the other 50% of normal matter), is not used up on each flight. It could even be able to act as defensive shields by annihilating itself with harmful weapons fire. (satisfies rule #2 because the fuel (reaction mass) is <10%; it also satisfies rule #3 because the engine is very small and cannot produce much damage)

Negative matter does not break any currently laws of physics, but it does look questionable. However, quantum vacuum fluctuations can have negative mass, but it may not be possible to harness them. (may satisfy rule #4)