I was reading this article from NASA about chemical rockets and they argue that, with a planet 50% larger than the Earth (assuming similar density, about 1.5G surface gravity), it would be impossible for chemical rockets to escape. Note that I don't care specifically about those exact values, just assume a planet slightly too big for chemical propulsion to be able to power a rocket to escape. If people were stranded on a planet like this, how would they build a means of leaving this planet.

Assume that they have no help from anyone already in orbit, so they have to get to space on their own, and a planet otherwise similar to Earth. They can know as much or as little about our spaceflight technology as is convenient for the answer.

Also, all answers must be feasible according to our current understanding of physics. Anti-gravity devices, portals, and the like are disallowed, though far-future tech can be used if/where needed.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – James
    Commented Feb 16, 2018 at 15:04
  • $\begingroup$ Important question: do the individuals need to escape, or the "species". $\endgroup$ Commented Feb 20, 2018 at 23:45

11 Answers 11


A pulsed fission engine like project Orion would have been able to move a 10 million ton ship into earth orbit. The downside is that they were achieving the propulsion with nuclear detonations. They would launch the atomic-bombs out of the back and detonate them a good distance away with a giant hemispherical "pusher-plate" which was basically a giant shock absorbing piston with a cup at the end to "catch" the energy from the blast.

enter image description here

The concept was further refined at a later date to utilize specially constructed nuclear "shaped charges" known as casaba howitzers. These nuclear devices would have been created in such a way that they focused the blast into a large tungsten slug that would vaporize into a cone, or even beam shaped blast directed at the pusher-plate. It would have made the ship even more efficient with it's thrust and obliterated/irradiated less of the surrounding countryside. enter image description here

The final on paper iteration of the plan could accelerate a space craft at 1G for 10 days. To give you an idea of how fast that is if you Accelerated at 1G for 5 days, then decelerated at 1G for 5 days you could reach Saturn in 10 days. Also it is a MASSIVE vessel. Since it's riding a series of thousands of nuclear shock waves the vessel's minimum size must be nearing the 1000 metric ton weight class just to survive launch. The vessel would have an order of magnitude larger amount of Delta-V required to achieve orbit, and as soon as it left orbit it could pretty much go anywhere in the solar system it wanted to. Shoot, if it used up all of its nuclear charges with zero regard for deceleration it could theoretically hit about 4% the speed of light.

Keep in mind the pusher plate being shown here is approx 500 meters in diameter!. Seems a bit extreme? I say irradiating an area the size of Texas is a small price to pay for progress! enter image description here

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    $\begingroup$ Added tangential bonus: Larger planet has more surface area, so you're irradiating less of the total planet for launches! $\endgroup$
    – Andon
    Commented Feb 13, 2018 at 0:28
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    $\begingroup$ @TCAT117 As much as I agree that alien life may take all sorts of forms, I think the idea of a life form that isn't bothered by close proximity to multiple nuclear detonations might be stretching it a bit, even if the smaller Orion designs were "only" using low-kiloton or sub-kiloton yields. $\endgroup$ Commented Feb 13, 2018 at 11:07
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    $\begingroup$ You say that the spacecraft could sustain 1G for 10 days (in space). If that is the maximum acceleration, it won't even be able to get far off the ground on a planet with 1.5G at the surface. Could it overcome gravity and drag and stay stable for long enough to get into orbit? $\endgroup$
    – Cyrus
    Commented Feb 13, 2018 at 21:34
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    $\begingroup$ Im saying the one nasa drew up was set at 1g of acceleration. The idea isnt that he copies NASA's design directly, simply that the engine type is excellent for large scale high output thrust. NASA was using low kiloton devices in thier math. With megaton range devices you would vastly increase thrust. Especially if built into a casaba howitzer where the configuration was less blast and more directed high energy particles. $\endgroup$
    – TCAT117
    Commented Feb 13, 2018 at 21:43
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    $\begingroup$ "I say irradiating an area the size of Texas is a small price to pay for progress!" => "I say irradiating Texas is a small price to pay for progress!" Corrected that for you, thank me later. $\endgroup$
    – plocks
    Commented Feb 14, 2018 at 8:14

With a Lofstrom launch loop.

Basically, you want to build a set of towers high enough that they can lift a train track all the way above the atmosphere. Then, in the absence of air resistance, you can accelerate your train all the way up to orbital velocity, and beyond.

Now, building a tower in high gravity may not seem like it's really any better than trying to use rockets in high gravity, and if the towers had to be supported by static forces, that would indeed be a problem. We can't build a sufficiently tall skyscraper on Earth, let alone on a heavier planet. But the towers don't have to be statically supported. They can use dynamic support. And dynamically supported structures, unlike rockets, can scale to arbitrarily large sizes, as long as you have a sufficiently powerful (and reliable!) powerplant to run them. See this video from Cody's Lab for real-world, small-scale demonstrations of the concept, one with water and one with a string.

A typical Lofstrom launch loop would work more like the string launcher than like a water-jet tower, although fountain-supported launch tracks are also potentially feasible, as long as you have enough suitable anchor locations along the track. (A loop only needs anchors that can handle compressive loads at each end, not all along the way.) Imagine a string launcher that encases the string in a stationary, frictionless tube, except the "string" is actually a telescoping steel chain, the "frictionless tube" is an active magnetic levitation track, and the "rubber wheels" are a series of linear electric motors.

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    $\begingroup$ Can you show this translates to a world with 1.5g gravity? How can dynamically supported structures scale to larger sizes than statically supported structures? Do you have any citations for any of this? $\endgroup$
    – Samuel
    Commented Feb 13, 2018 at 22:51
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    $\begingroup$ Links != citations, I saw them. Your "scientific paper" begins with a four page sci-fi story. All material strengths are limited by their chemical bonds. In your sci-fi launcher scenario the size of your vacuum tube is bounded by the vacuum sheath's ability to hold out atmospheric pressure, for example. The maximum height your structure can reach is limited by the maximum acceleration of the iron slugs (which are limited in size by your sheath), which is also a material strength issue. Sorry, all those problems don't go away because you're using magnets. $\endgroup$
    – Samuel
    Commented Feb 13, 2018 at 23:34
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    $\begingroup$ @Samuel I didn't call it a "scientific paper". It is a technical paper, and starting with four pages of motivation does not degrade the actual engineering content. Indeed, I'd say it makes it even more relevant, since this is explicitly addressing a science-fiction scenario. The hard-science tag does not require scientific papers, either. "Empirical evidence" (e.g., YouTube videos of people who have built small-scale models) and "other citations"are explicitly accepted.... $\endgroup$ Commented Feb 14, 2018 at 4:19
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    $\begingroup$ ....The height of a single stage of a space-fountain or loop-style structure is indeed limited by the acceleration that the dynamic members can support, but not the carrying capacity--you can always just add more mass to the stream, or construct multiple parallel streams. And that means you can stack stages to get more height if one stage ends up not being enough. Material limitations on the size of the vacuum sheath are not relevant, precisely because you can just build more than one in parallel if you need, and the scenario does not specify a minimum size for vehicles to be launched. $\endgroup$ Commented Feb 14, 2018 at 4:22
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    $\begingroup$ It's worth remarking that such designs become more feasible when the atmosphere is thinner – not so much in the sense of density as scale-height. And even for the same gas properties at the surface, the scale height becomes lower if gravity is stronger. For a big planet with strong gravity but an atmosphere significantly thinner than Earth's, a launch loop might be quite plausible indeed. More so when you also have less violent winds to take care of. $\endgroup$ Commented Feb 14, 2018 at 14:46

And now for a ridiculously big approach:

Build a ring around the equator. This is supported by a large number of towers. The ring is spinning well above orbital velocity (use a maglev setup, but there's a second one on top) and exerts an outward force. This is made equal to the weight of the tower beneath--thus the towers are actually hanging from the ring. (Yes, there are mountains and oceans in the way. I said this was ridiculously big--you're going to have to bore some mighty tunnels and build some pretty impressive deep sea constructs.)

Once you have this ring working do it again--this time on top of the existing one. Repeat until you're out of the atmosphere and can put your launch track on top of the whole thing. As each ring takes the load of it's layer you don't need an insanely strong tower.

I have not performed a full analysis of this but it's not needed to see that it works: Consider the extreme case with an infinite number of rings and an infinite number of towers--the materials strength requirement drops to zero. Thus it simply comes down to the required spacing.

While this is a vastly more complex engineering project than either the launch loop or space fountain approaches it doesn't have the insanely powerful turn-around magnets those approaches need. You can duplicate or triplicate all the power elements so that if there is a failure the whole thing keeps working.

As for the comment about the lack of hard science:

1) What's the force on the ring? You have an outward force between anchor points that matches the inward force exerted by the anchor points. Infinite anchor points = zero distance between them = zero force on the ring.

2) What's the force on the towers? The mass between a ring and the one below. Infinite rings = zero distance between them = zero force on the towers.

Obviously, neither can actually be infinite but they can be large enough that there's no big materials issues.

As for the Orbital Ring video in the comments:

He's talking about building it in space--something not permitted by the question. I'm talking about building up from the ground, although the basic concept is the same.

Note that his ring doesn't work--note my point #1 in response to the hard science gripe. You can't anchor that in only one point without the use of super materials.

  • $\begingroup$ Cool answer. Never thought of that before. Question: Is the tower supported by the same technology, the tower being essentially a maglev train going in the opposite direction as the track itself? $\endgroup$
    – Loduwijk
    Commented Feb 13, 2018 at 22:24
  • $\begingroup$ @Aaron No--once the ring is spinning the towers are now hanging from the ring rather than supported from the bottom. Obviously they must be engineered for both tension and compression. $\endgroup$ Commented Feb 14, 2018 at 2:18
  • $\begingroup$ Perhaps I was not clear: if the rings are rotating, then the towers cannot just sit on them normally or the towers would be traveling along with the rings. Hence my comment: if the tower is supported by maglev but going in the opposite direction from the ring's reference frame, then the tower will appear to stand still from ours. If not this, how is the tower supported by it? $\endgroup$
    – Loduwijk
    Commented Feb 14, 2018 at 13:53
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    $\begingroup$ A Single Ring would be enough: start building it on the ground ("just" bring everything to the same height around the equator) and accelerate it (in some sheet) until it "floats". If you then accelerate further and have a way to "widen" the ring (and sheet) the ring will lift up your desired height. As it lifts one can build towers (or more likely hang lifts) beneath it. $\endgroup$ Commented Feb 14, 2018 at 14:55
  • $\begingroup$ See the video Orbital Rings from Isaac Arthur for an elaborated version of this answer. $\endgroup$ Commented Feb 14, 2018 at 23:54

Aircraft launch

Use a winged vessel which uses the atmosphere as

  • a dynamic structure to carry its weight,
  • a source of oxidizer for its engines, and
  • reaction matter to provide thrust.

The aircraft's goal would be to get as high as possible, but even more importantly as fast as possible, because in air launch, speed is worth more than altitude. (If altitude were so precious, we'd launch from the Wyoming steppe, not sea level). Right now we don't have reason to throw terabuck engineering into hypersonic aircraft, but they sure would.

So this aircraft would be climbing up into the very upper limits of the atmosphere where it's thin enough to go hypersonic easily, and creating all the delta-vee it possibly can using the atmosphere as oxidizer, before detaching the first stage of rocket proper and sending it on its way.

The rocket equation would be more or less inapplicable to this mothership launcher, since its oxidizer and reaction matter is borrowed.

There are those working on this. However projects like Stratolaunch, Virgin LauncherOne, GO, Aldebaran, and MAKS are subsonic launch, IAR-111 is "mere" supersonic launch. I am proposing hypersonic launch, and the mothership doesn't need to survive separation.

  • $\begingroup$ @Aaron Good idea and done. $\endgroup$ Commented Feb 14, 2018 at 22:01
  • $\begingroup$ I was also going to propose spaceplane as a solution, however there is a problem with this. At a higher G (and assuming that air pressure at the surface is the same) the atmosphere will be thinner so you get less of an advantage with an air launch as you won't be able to fligh so high. $\endgroup$ Commented Feb 15, 2018 at 11:12
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    $\begingroup$ @JeremyFrench Having looked at air launch, height does have value, e.g. in allowing engines to be optimized for high altitude (bigger cones). But speed has a lot more value, hence my interest in hypersonic launch. So I'm not worried too much if the atmo is thin at a lower altitude. When hypersonic, the thinner the better, as long as there,s still enough to feed the engines. $\endgroup$ Commented Feb 15, 2018 at 21:40
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    $\begingroup$ And... if you were at the bottom of a difficult gravity well, powering your aircraft launch platform with something like this becomes rather more attractive than it did for us. $\endgroup$
    – J...
    Commented Feb 15, 2018 at 21:59
  • $\begingroup$ +1 Although in the quoted wiki article it is already being said that this wouldn't help too much. Even on the Superearth planes would have limitations. $\endgroup$ Commented Feb 16, 2018 at 1:21

Related to the launch loop there's the space fountain.

You build a tower to space. Of course there's nothing strong enough to build it out of so you have to take off a whole bunch of weight. You do this by building a base station that throws magnets up (in an evacuated tube) very, very fast. Each platform of the tower has generators that produce a bunch of power from the magnets flying by--in doing so energy is transferred from the magnets to the platform. That energy goes next door to the motors that are grabbing the pieces coming down (think maglev train, you can't have physical contact!) and accelerates them, likewise producing lift.

You have a very large magnet at the top that turns the pieces around and sends them back down. You have a humongous magnet at the bottom that does the same thing. So long as the paths is evacuated and everything is superconducting this costs no power once you have it set up.

Regarding the hard science gripe:


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    $\begingroup$ I wanted to come in shouting "no power once set up, but what about inefficiencies?" But I'm not sure where they would be. So you suggest the electrical conductors are perfect superconductors. The magnets are probably aimed perfectly straight and their path kept a perfect vacuum. But does this really mean no power (or at least negligible: I will grant you small amounts of input power to account for imperfections)? Even though I cannot point out the weak link, the claim in your last sentence is haunting me and intuitively feels overestimated. $\endgroup$
    – Loduwijk
    Commented Feb 13, 2018 at 22:03
  • $\begingroup$ @Aaron, no power except to compensate for inefficiencies. The magnets flying along the loop is similar to orbiting—that does not need any energy either, does it? $\endgroup$
    – Jan Hudec
    Commented Feb 13, 2018 at 22:44
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    $\begingroup$ You build an unbuildable tower by already having a tower in place? $\endgroup$
    – Samuel
    Commented Feb 13, 2018 at 22:54
  • $\begingroup$ You have to spend a huge amount of power at the bottom. At the bottom magnets start with huge upward velocity and gradually lose it to the generators and gravity at the top they are basically stopped. They then fall and pick up speed from both gravity and magnets and endup falling at huge speed. It would be energy intensive to stop them then fire them back up. Also these energy transfers on platforms would be have energy loss to resistance in the generators. $\endgroup$ Commented Feb 14, 2018 at 0:00
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    $\begingroup$ @Samuel Sort of. Build a lower version than your final one, then use what you have as a platform to extend. Then extend again... and so on. So not necessarily by already having a tower in place, but rather by building a short version of it and using it to pull itself up by its own bootstraps. Since the platforms which do the accelerating could raise or lower themselves by adding to or subtracting from the energy in the moving parts that are whizzing by, you raise it as far as seems reasonable, then use the platform to extend the vacuum tubes. $\endgroup$
    – Loduwijk
    Commented Feb 14, 2018 at 0:05

The problem posed in the article is that if planet's radius was 50% larger, current chemical rocket fuels won't allow rockets to escape earth's gravity.

Per article, this is because Rockets have a design limit on how much fuel they could carry at launch, which limits their capability to escape from a planet of certain minimum size.

But they can very well orbit.

As long as rockets are able to reach orbits with even marginal fuel remaining, we should be able to create a solution based entirely on currently available (or near-future) tech, albeit very expensive. I suppose that's not of concern for rescue of stranded people.

Consider a series of orbiting spacecrafts, that are essentially refuelling stations, lodged into orbits with some residual fuel. Installed solely to allow refuelling of final people-carrier-escape-vehicle.

A reusable rocket in people-carrier-escape-vehicle in orbit should be able to refuel from these orbiting stations sufficiently to allow escape.

  • $\begingroup$ The article said that if the planet was 50% bigger you couldn't achieve orbit with a chemical rocket. $\endgroup$
    – phil1008
    Commented Jan 9 at 11:12
  • $\begingroup$ "In the staged version of the rocket equation, the possible achievable delta-V is proportional to the number of stages when mass ratio and engine efficiency is held constant. In short, you can get any delta-V you want by adding additional stages." space.stackexchange.com/a/5383/6667 "every rocket ever used to deliver a payload into orbit has had staging of some sort" $\endgroup$
    – Mazura
    Commented Jan 9 at 12:03

One possible solution would be to turn the highest mountain into a space gun. Depending on the density of the atmosphere at that altitude, either go straight to orbit or launch a vehicle (rocket) into a low orbit and from there use propellant to get free of the gravitational well.

If we use the 9680km radius from the article and assume average density equal to earth we end up with a planet that has the following characteristics:

Earth Avg Mass: $$5.98 * 10^{24}kg / (1.33 * π * 6,378,000m^3) = 5516 kg/m^3$$ Our Planet’s Mass: $$(1.33 * π * 9,680,000m^3) * 5516 kg/m^3 = 20.9 *10 ^ {24} kg$$

Our Planet’s escape Velocity: $$( 2 * (6.67 * 10^{-11}) * (20.9 *10^{24} kg) / ( 9.680 * 10^6 ) )^{0.5} = 16.9 km/s$$

Surface gravity: $$(6.67 * 10^{-11}) * (20.9 *10^{24}) / ( 9.680 * 10^6 )^2 = 14.88 m/s^2$$

Given the increased gravity we are unlikely to see mountains as high as the ones on Earth, but let's ballpark it and assume a 5km peak maximum.

On earth we have drilled as deep as 12 km into the crust (granted with a 2 inch bit), so it is not out of the question that in the near future we will be able to dig a tunnel from the top of the mountain and continue 5km into the crust, basically creating a 10 km long barrel.

We have rockets, even tiny ones like the SS-520-5 which can achieve orbit from earth. So to prove the concept we could have the space gun place a rocket at an altitude where the escape velocity equals that of earths surface. $$ \sqrt((6.67 * 10^{-11}) * (20.9 *10^{24}) / 11.2 m/s^2) = 11,156,476m $$

Our space gun must be capable of placing the rocket at: $$11,156,476 – 9,680,000 = 1,476km $$ above the planet surface. Well that’s not going to work, since the furthest we have ever gotten with a space gun like setup is sending 180kg up 180km.

Second option would be to accelerate the rocket so it leaves the mouth of the barrel with enough velocity to make up for the increase in escape velocity. $$16.9 km/s – 11.2km/s = 5.7km/s$$ Ian McNab proposed a design for a railgun that could accelerate a 400kg projectile to 7.5km/s back in 2003.

However, I must admit the math is getting away from me when I try to build a model that accounts for atmospheric density at 5km altitude on our imaginary planet. The drag is enormous and there are huge structural challenges in accelerating an existing rocket to the tune of 165 Gees and not have it burn up once it leaves the mouth of the gun. In the end a coherent solution was beyond my ability.

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    $\begingroup$ You should be able to flesh this out. Just needs equations to show how much of your total velocity can be achieved by a realistic gun. And I'd say, for style points, you should have the space gun borred into the mountain -- preferably by the Boring Company! $\endgroup$
    – Cort Ammon
    Commented Feb 12, 2018 at 23:27
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    $\begingroup$ @kingledion after reading the tag info I must concede you are right. My answer is incomplete I’ll update based on the 9680km radius from the article. $\endgroup$ Commented Feb 12, 2018 at 23:28
  • $\begingroup$ Perhaps a good reference: en.wikipedia.org/wiki/StarTram $\endgroup$
    – NofP
    Commented Feb 12, 2018 at 23:28
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    $\begingroup$ No, a space gun will not work. All of the acceleration occurs before the projectile leaves the barrel. A rocket accelerates the whole way. A space gun would kill anyone in it. $\endgroup$ Commented Feb 13, 2018 at 0:11
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    $\begingroup$ @Pasqueflower I think that's a practicality of length problem rather than an impossibility. $\endgroup$
    – user25818
    Commented Feb 13, 2018 at 0:22

What if we use a WEAV type system of propulsion to get to low earth orbit... https://www.scientificamerican.com/article/worlds-first-flying-saucer/ This uses electrolodes and magnetic fields to create plasma that pushes the air away from the craft generating lift from any surface with few aerodynamics or moving parts involved, then you use 200 kilowatt magnetohydordynamic thrusters https://www.nasa.gov/centers/glenn/about/fs22grc.html to hit speeds close to 200,000 miles per hour with 200 (13 times that of the space shuttle) using noncondensable hydrogen plasma and electric power for fuel.

  • $\begingroup$ Cool. However, it sounds like it only works within atmospheres as it needs air to push against, so no maneuvering in space. Of course, you could get enough speed built up in-atmosphere to still greatly assist space travel. Also, I have a feeling the energy requirements will be massive if that craft lifts anything heavy. +1 $\endgroup$
    – Loduwijk
    Commented Feb 14, 2018 at 21:05

TCAT117 suggests a pulsed fission engine, but these are horribly contaminating and therefore have never been tested.

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket gives another alternative. This consists of a nuclear reactor as a source of heat, through which liquid hydrogen is heated and used as a propellant in a nozzle similar to a conventional rocket nozzle. This design was actually given some consideration and some engine tests carried out. It's much less hazarous than a pulsed fission engine, but chemical rockets are less hazardous than any of the nuclear options, so in the real world they won out.

Hydrogen is the preferred propellant as its light molecules give the highest exhaust velocity at any given temperature.

The following are highlights from the comparison in the Wikipedia article, which I have copied in here as requested:

Specific impulse 850-1000 seconds, more than double that typical for a oxygen/hydrogen powered engine. Specific impulse is the number of seconds a stage can produce a thrust equal to its initial fuel weight before fuel runs out. It is proportional to exhaust velocity. Thus the simple solid core nuclear thermal rocket is capable of double the efficiency of a chemical one.

Thrust - weight ratio achieved in apollo era (about 5:1 on a 1.5g planet.) This is much less than a chemical rocket, and means that nuclear thermal rockets are more suited to being used in upper stages where burn times are longer. The first stage (only) of a rocket needs high thrust-weight ratio as vertical takeoff means initially a lot of fuel is used fighting gravity. The sooner you can build some speed and get into a near-horizontal trajectory the better. Once this is achieved longer burn times at lower acceleration is not such a disadvantage. SNTP era (separate article) reached 30:1, a thrust-weight ratio at which engine mass ceases to be any real issue. https://en.wikipedia.org/wiki/Project_Timberwind#Space_Nuclear_Thermal_Propulsion_Program

NASA actually considered replacing the 3rd stage of Saturn V (known as Saturn IV-B) with a nuclear thermal rocket for enhanced performance.

The wikipedia article has a worked example based on the Saturn IV-B and I present a summary below. Delta V is the standard measure of efficiency of rocket in space, equal to the speed difference it is able to depart before it is depleted.

The author seems to have neglected the mass of the upper stages. If factored in, this will further favour the Nuclear Thermal Rocket on the mass/mass comparison, as the engine mass will be less significant.

Standard Saturn VI-B Hydrogen-Oxygen fueled

Fueled Mass 119800kg, dry mass 13400kg, specific impulse 475s.

Delta V (414 s × 9.81) ln(119,900/13,311), = 8900m/s

Nuclear thermal rocket, drop-in replacement matching volume/volume

Fueled Mass 38600kg, dry mass (due to increased engine mass) 17300kg, specific impulse 850s

Delta V (850×9.81) ln(38,600/17,300) = 6,700 m/s.

While the Delta V is lower, the mass of the stage is much lighter due to the hydrogen propellant being lighter than the hydrogen/oxygen bipropellant of the original stage, so the stages below will compensate.

Nuclear thermal rocket, replacement matching mass/mass

Fueled Mass 19000kg, dry mass (due to increased tankage) 38300kg, specific impulse 850s (850 s×9.81) ln(119,900/38,300), or 9,500m/s

NASA considered an even smaller stage due to constraints of the Vehicle Assembly Building : 10,429 kg empty and 53,694 kg fueled. This would improve the payload capacity of the Saturn Vf from 127,000 kg delivered to low earth orbit (LEO) to 155,000 kg.

This is a moderate improvement on chemical rockets, based on Apollo era technology and far from optimised. An example based on project Timberwind would be a much greater improvment, 1.5 to 4 times payload increase. https://en.wikipedia.org/wiki/Project_Timberwind#/media/File:SNTP_Upper_Stage_Applications.png

Note that the Space Shuttle's second stage (the main engines) fired from liftoff, though most of the initial thrust was provided by the first stage boosters. I would foresee a similar arrangement with chemical boosters around a nuclear thermal rocket core, to keep the heavy nuclear thermal rocket engine burning for the longest possible time.

An issue mentioned is that the specific impulse of nuclear thermal rockets is limited by the maximum temperature the reactor can withstand. I think a hybrid engine with a nuclear thermal core followed by oxygen injection into the hydrogen stream in an afterburner for liftoff could improve this issue to give even higher specific impulse, and would have great potential as a first stage.

  • $\begingroup$ Any idea what the numbers are here? Yes, NTRs have quite nice specific impulse when you run them in high gear, but that's due to very little mass flow. For takeoff under 1.5g gravity, it'll probably be desirable to push the thrust ratio to at least 30 N/kg. Are there any plausible designs with high specific power? $\endgroup$
    – timuzhti
    Commented Feb 15, 2018 at 9:57
  • $\begingroup$ @Alpha3031 I know little about the subject but I know that it's a more practical suggestion than others listed here. I've copied in some of the data from the Wikipedia article, which lists thrust ratios of 7:1, (approx 70N/kg). But then I've found more up to date info on project timberland with thrust ratios of 30:1, so I may tidy up my answer later. They definitely ground tested in the Apollo era. It's clear they didn't flight test in the timberland era, but not clear if they ground tested or not. Still they were proposing and designing some powerful engines. $\endgroup$ Commented Feb 17, 2018 at 4:51
  • $\begingroup$ Nice elaboration. Though, does oxygen injection increase Isp? I was under the impression that it increases thrust purely through increased mass flow. $\endgroup$
    – timuzhti
    Commented Feb 17, 2018 at 5:07
  • $\begingroup$ @Alpha3031 Oxygen injection would reduce Isp, just as running an afterburner on a fighter would reduce efficiency. But a hybrid nuclear+oxygen engine would surely give better isp than a chemical rocket, which would be the best of both worlds for liftoff. $\endgroup$ Commented Feb 17, 2018 at 5:16
  • $\begingroup$ The big question I have in my mind right now, having read about project timberland, is how do you prevent a nuclear upper stage from overheating until time to use it? Easy on the launch pad, you water cool it, but in the air or in space what do you do? For star wars applications they could be idle for months! $\endgroup$ Commented Feb 17, 2018 at 5:17

You would use a "cannon" to launch a projectile into orbit. Cannons use explosives and are not limited to the burning energy of combustible fuel.

The mathematics of orbiting a projectile based upon velocity would be the same as rockets. The only exception is the projectile accelerates under extreme forces, but the speeds would ultimately be the same.

Wiki describes the concept of a "space gun":


The challenges with launching projectiles into orbits are the forces, and materials required to keep the projectile together. A piece of technology like a satellite would be destroyed in the process, but you could package the satellite inside a hard shell. Package the satellite in a way as to have no voids, and reassible the unit into it's functional shape once in orbit.


This is a formidable problem. Let's break it down into two problems: how to get satellites into space, then how do we get people there?

For satellites, you'll want an ion propulsion system as these have a much higher specific impulse (>3000s) than chemical rockets (~450 max). The problem is that ion propulsion has low thrust and won't work in atmosphere. So, you'll need to launch the satellites into space using chemical rockets before releasing them and turning on the ion thrusters. If the gravitational pull is still low enough to enable accelerating satellites into low "earth" orbit with chemical fuels the satellites will be able to steadily escape the planet via their ion thrusters without falling back down. When the time comes to get people out you could assemble a second rocket in orbit and use that to get away.

Now, if you can't accelerate your satellites fast enough to sustain orbit you have a problem. You could install an explosive-propelled firing mechanism into the first rocket after it runs out of fuel to rapidly accelerate the satellite into a stable orbit. The advantage of this over a chemical fuel is that you can use high specific-impulse chemical compounds that would tend to explode if used in a thruster, and you would only need to accelerate the weight of the satellite rather than the rocket and fuel. It would then be physically possible to assemble a second rocket in orbit this way (using ion thrusters to adjust orbits), but putting a person into that rocket could be difficult as the acceleration from the firing mechanism would probably kill them.

Suppose we can't escape that way. Let's use our firing mechanism system to put robots in orbit instead. If you have a moon orbiting the planet you could establish a robotic base there. Alternatively, you could assemble an artificial satellite as a base. If we assume that building an artificial womb is feasible, we could install one on our base and launch a frozen embryo into orbit. It would probably take a few attempts before you can remotely raise a child to adulthood, and would be insanely expensive to sustain them, but there's no physical law preventing it.

You may now proceed to conquer the universe.

  • $\begingroup$ Hypothetically, what kind of "high specific impulse chemical compounds" are we talking about here? $\endgroup$
    – timuzhti
    Commented Feb 22, 2018 at 9:44
  • $\begingroup$ I was thinking of propellants used for artillery, which can achieve muzzle velocities of >1 km/s in atmosphere. "Specific impulse" isn't really the right word, as the launch mechanism is completely different, but there is an analogy there. $\endgroup$ Commented Feb 23, 2018 at 0:12
  • $\begingroup$ So the other 10 km/s or so of delta-v to reach low orbit will be from the standard LOX/fuel propellant? I fail to see the advantage of carrying the extra mass, TBH. $\endgroup$
    – timuzhti
    Commented Feb 23, 2018 at 0:22

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