What would make a good propulsion source for small engines and manouvering thrusters for spacecraft vehicles on Europa (moon of Jupiter), both for near-surface operations (vacuum) and in caves beneath the ice (low or medium pressure)?

I am only interested in spacescraft thruster propulsion systems, not wheels or tanks tracks etc. My version of europa has an extensive cave system within the ice, which will be navigated by spacecraft using thrusters. I am ignoring gas pressure and propellors, and assuming the pressure is too low to bother with aerodynamics.

Say my colonists have ample amounts of electricity from fusion power. They want to use local materials if possible. Refueling drops from other locations in the jovian system are acceptable as long as they are plausible and make economic sense...

  • $\begingroup$ SE's Q&A model is one-specific-question/one-best-answer. I see at least 3 questions, which makes the question "too broad." You can always post more questions if necessary. Please edit this post to express a single, specific question. Also, our help center states, "avoid asking subjective questions where your answer is provided along with the question." $\endgroup$ – JBH Mar 29 '19 at 16:29
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    $\begingroup$ While I agree with the above to some extent, I think the questions closely relate to each other enough that it may be considered one continuous question. Consider rewording some questions in a way to illustrate possible solutions you've thought of. I think that would clear up the question. $\endgroup$ – Cristian C. Mar 29 '19 at 16:37
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    $\begingroup$ Sometimes authors use questions rhetorically. In this case, I'm seeing one larger question with some ideas thrown out in the form of questions. $\endgroup$ – Cyn says make Monica whole Mar 29 '19 at 17:16
  • $\begingroup$ You don't need anything special, wheels or ski, powered by an electric motor will work just fine. Given the conditions on Europa, some sort of covered snowmobile would be best. $\endgroup$ – ventsyv Mar 29 '19 at 18:59
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    $\begingroup$ @ventsyv there's no snow on Europa. The surface ice is brittle and rock hard. A lunar rover-type vehicle might work if it weren't for the ultra-rough cryovolcanic landscape, but in low-gravity vacuum, good old thrusters are usually the best solution. $\endgroup$ – Adrian Hall Mar 29 '19 at 23:56

This is a big rewrite of my answer following big changes to the OP. The original stuff is still in the edit history, if you cared.

The critical question is... how light and compact are your fusion reactors? If you can fit one on your vehicle and they don't weigh too much and they make use of direct energy conversion (rather than merely being thermal power plants) then you can use something like Bussard's ARC/QED design (all regeneratively cooled quiet electric discharge). I did have an online source for the paper describing this, but the link is now dead so I can't shared it with you. It uses electron beams to heat reaction mass (which could easily be water), gets 1000 seconds $I_{sp}$ or better, could probably do SSTO on earth and so would be fine for all your needs on Europa with power to spare.

If you don't have compact portable fusion, then you have some problems.

Using the most common local resource, you have 3 basic options.

  1. Steam rocket
  2. High-test Hydrogen Peroxide monopropellant rocket
  3. Liquid hydrogen/Liquid Oxygen rocket (LH/LOX)

If you have access to carbon or hydrocarbons, you could synthesise a rocket fuel and use peroxide of liquid oxygen as the oxidiser to make a more conventional biopropellant rocket. If you had access to nitrogen, you could make hydrazine which can also be used as a monopropellant like peroxide only more efficient, toxic and hazardous to store.

(1) steam rockets have the problem that you either need to generate the steam on board your vehicle (which will require a big battery or generator) or generate it ahead of time and keep it hot. Pregenerating it won't help much for longer journeys and storing high-pressure steam requires very strong, heavy tanks, so I think pregenerated steam rockets are out.

You can generate steam on board with battery power, but you might have problems competing with peroxide rockets. The specific energy of peroxide is something like 2.7 MJ/kg, which is three times that of the best rechargeable lithium ion batteries today. Your future battery storage tech might be an order of magnitude better, eg. lithium air in which case this is achievable, but you'll need to find a good supply of lithium and I have no idea how abundant it might be on Europa or in the Jovian system.

If your powerplant and its fuel is light enough, you could generate the steam on-demand, and refuel with the aid of a pickaxe. I've not looked at how light and efficient such a power plant would need to be though, because the maths is more fiddly than I'm prepared to fight with right now. A lightweight thermal fusion plant would fit the bill, I'm sure.

(3) LH/LOX is the best for power and efficiency and so on, but storing LH is a nighmare. It is super low density (so you'll need massive fuel tanks), damages various structural metals and has such a low boiling point that even on the coldest parts of Europa you'll still need insulation and refridgeration to keep it liquid. LOX can actually freeze in the colder parts of Europa, but not as readily as peroxide so keeping it liquid isn't so hard. Lots has been written elsewhere about LH/LOX rockets, so I won't say more.

This brings us to: (2) hydrogen peroxide monopropellant rockets have high density fuel (up to 1.4 times more dense than water depending on concentration), are easy to ignite (just spray the fuel over a suitable catalyst of which there are many... silver wires are one), burn cleanly and are more or less non-toxic in the event of leakage (or at the very least, are easy to denature).

Peroxide has roughly the same melting point as water, so you'll have to work to keep it warm to use it in a rocket. With a practical $I_{sp}$ of 140 seconds it is clearly not a super efficient fuel, but you can easily make decent high-thrust rockets from it capable of flight on Earth, let alone Europa with its 1/8th gravity. Rocket belt

The RB2000 rocket belt, a modernisation of a 50s design by Bell, can actually fly an adult human around in Earth's gravity. The design should scale up OK... Bell made a 2-man stand-in vehicle design based upon their earlier work, so I'm happy that it is a plausible basis for larger, more complex things. You'll get a $\Delta_v$ of about 370m/s out of it, which isn't a whole lot, and about 4 minutes thrust time on Europa, and that's with 25% of the takeoff mass being fuel. On earth it would struggle to lift much more fuel, but there's some scope on Europa to carry larger tanks for longer trips. As you can't use lift or otherwise float on an airless world without the aid of your rockets, you'd be well advised to use a hopping strategy, with special sprung undercarriages to recover kinetic energy. Oh, and sickbags. Lots of sickbags.

Finally, non-rocket advantages of carrying around peroxide: it is like an instant life-support package. It provides heat, oxygen and water, and once you've defrosted a tiny bit (eg. with body heat) you can use the heat its decomposition generates to defrost a lot more.

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  • $\begingroup$ Thanks, this was very useful. I'll skip the bits about steam cannons since my goal is small engines and maneuvering thrusters, often in caves and canyons, but the hydrogen peroxide sounds quite plausible to me. The nitrogen pressurizer is not consumed I assume? Is the catalyst rapidly consumed, because silver will need to be imported, and may be prohibitively expensive. Any thoughts on storing H and O under high pressure, and combining them to water to power a steam rocket, or is that just too complicated? I worry about large tanks of water being required. Or just have a steam tank onboard? $\endgroup$ – Innovine Apr 1 '19 at 8:17
  • $\begingroup$ @Innovine catalysts are generally not consumed in any process. They may eventually degrade or become too contaminated to function, but this takes some time and there's plenty of scope for reprocessing them. Any pressurising gas could be used; we use nitrogen on earth because it is easily available and inert. $\endgroup$ – Starfish Prime Apr 1 '19 at 8:23
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    $\begingroup$ @Innovine storing hydrogen is hard, due to its very low density and low boiling point. If you can avoid it, you probably should. Hydrogen tanks will be much larger than water tanks, and be more awkward to make (due to hydrogen embrittlemen of some metals) and quite possibly need cryogenic cooling. A hydrogen/oxygen burning rocket is not usually referred to as a steam rocket, due to the difference between a nozzle emitting steam and one emitting multiple-thousand-degree fire. $\endgroup$ – Starfish Prime Apr 1 '19 at 8:25
  • $\begingroup$ @Innovine for an example of the hassle of hydrogen, look at all the X-prize competitors and their modern day descendants in the form of SpaceX and Virgin Galactic. If an alternative fuel to liquid hydrogen could be used, it was, because it is so hard to work with. $\endgroup$ – Starfish Prime Apr 1 '19 at 8:30
  • $\begingroup$ Hydrogen peroxide looks great so far :) $\endgroup$ – Innovine Apr 1 '19 at 9:08

Under the ice:

Under the ice on Europa, there is... more ice. The upper layer of the planet is actually a solid ice layer, similar to the upper mantle of the Earth. While solid, there is a lot of convection (on Earth this creates tectonic activity), so if you drilled a tunnel down there, it would collapse pretty quickly due to the shifting ice. Also, as you drilled down there would be lots of pressure from the gas formed by sublimating ice that you've newly exposed to the vacuum of space.

So vehicles designed to go down there would have their own drilling equipment and be able to fix themselves to the ice, maybe using giant spikes or equivalent. If and when they get down to the water level, they would probably stay as close as possible to the surface to avoid getting caught in underwater currents caused by the tides (when you orbit a gas giant, your tidal forces are very, very strong), so I doubt you'd even have a submarine-like design, just a tank that claws its way through the slushy ice-soup that saturates the top levels of the oceans.

On the surface:

Easy peasy. Europa's gravity is about 13% of Earth's, so you could propel yourself with jets of water vapor expelled from RCS thrusters:

RCS thrusters

Water vapor is easy to make when the ground is made of ice and you have nuclear power. Don't bother with hydrogen and oxygen: just superheat the water and explosively blast it out of a nozzle: the easiest way to store hydrogen and oxygen is in the form of water.

Local materials:

Well, you're good on fuel. Everything else would have to be imported or slowly fished out from the ice's mineral content through electrolysis, dew point separation, or whatever ends up being more efficient. There's a good amount of carbon down there, and plenty of hydrogen, so you could conceivably make plastics through a convoluted and power-hungry process. Some metals are present at impact sites, as well as silicon, which you could use to make ceramics. You might be better off importing though. It depends: if Europa is a fuel-mining colony, they can probably afford to filter their water for impurities as part of the refining process. If they're a research outpost, haha good luck!

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  • $\begingroup$ Remember that access to fusion power does not imply that the right sort of fusion reactor to drive a rocket is available and light enough to be used for that purpose, even on Europa. A solid core fission rocket would certainly work fine, but its fuel will be harder to come by on Europa, and it isn't an engine you can just turn on and off at will so it wouldn't really be suitable as a small engine. $\endgroup$ – Starfish Prime Apr 1 '19 at 7:00
  • $\begingroup$ The spacecraft can be battery powered. Which might have the greatest mass, a small fusion reactor, a radioisotope generator, or a bank of batteries? the idea of containing a steam boiler is not bad, but wouldnt greater amounts of gas fuel be easier to have onboard under much higher pressure? $\endgroup$ – Innovine Apr 1 '19 at 7:34
  • $\begingroup$ This answer needs to prove that storing water is more efficient than hydrogen, oxygen or both under high pressure. Or would it be best to store as H and O, and then combine to water, superheat, and eject as steam? $\endgroup$ – Innovine Apr 1 '19 at 7:38
  • $\begingroup$ @StarfishPrime Energy is generated by a fusion reactor housed in a building. The energy only has to be stored by the ship, not generated. Given that the maximum energy density of resting matter is about 10^17 joules per kg (via e=mc^2), I doubt there are any inherent issues with the energy being too heavy – the main concern is the safety of transporting it. You can superheat water with enough energy, no matter where said energy comes from. $\endgroup$ – Adrian Hall Apr 1 '19 at 15:32
  • $\begingroup$ @AdrianHall yes, it is true that you can solve the issue by handwaving an arbitrarily capable energy storage system. $\endgroup$ – Starfish Prime Apr 1 '19 at 15:36

This answers the original question as asked before edits. This answer does not work for a recently edited version of the question.

You do not need any special fuel. You said that you have plenty of electricity that you are generating, so just use electrical motors.

A good propulsion source on the surface would be wheels or legs. If the ground is solid, you use conventional means to traverse it. So, we use the same technology we have been using for thousands of years.

A good propulsion source under the surface but still in the solid volume would be a drill. This could be either a conventional mechanical drill, or, since you have ample electricity, it could drill through the ice using heat; but beware, drilling using heat will leave behind water and/or water-vapor which you may have to deal with.

A good propulsion source deeper under the surface in the volume which is believed to be liquid would require a sub.

So you see, everything will just use types of devices that we are already used to, though they may need to be adapted a bit to the specific conditions of Europa. In fact, people have been working on this for years. See the following...

Robot Submarine on Jupiter Moon Europa is 'Holy Grail' Mission for Planetary Science

An Alien-Hunting Submarine Is Being Tested in Antarctica

NASA’s New Deep-Sea Submarine Could Eventually Look for Aliens

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  • $\begingroup$ He probably things he needs a reaction engine since he is in "space" and everyone knows you use thrusters in space. $\endgroup$ – ventsyv Mar 29 '19 at 18:53
  • $\begingroup$ Europa's surface gravity is comparable to that of the Moon. Yes, you can drive on it, but the reduced traction due to lower gravity limits your speed. For longer distances, rocket-propelled hoppers are the most practical way to get around. $\endgroup$ – Mark Mar 29 '19 at 21:51
  • $\begingroup$ Yes, I need free flying craft. $\endgroup$ – Innovine Mar 30 '19 at 10:28

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