For spaceships working on or near Ceres, what would be the best option for locally produced fuels?

Let's assume that there's plenty of water ice available, and other chemicals as long as its plausibly realistic. There is lots of electricity due to fusion reactors on the surface. Spaceships use batteries, fuel cells, and present-day tech [more or less]. Is there one particular fuel which would be dominant due to it being cheap, effective, easy to handle, and easy to produce?

The goal is for short trips, for example, a surface to orbit, then plane change to another orbit, then back to the surface again with some delta-v to spare, for a craft with a mass of around 3000kg. Would this fuel source be used for both an RCS and main engine, or just one of them?

  • $\begingroup$ It seems that much of this comes down to what is more difficult or dangerous to store, high test h2o2 or cryogenic H2... any thoughts on that? $\endgroup$ – Innovine Aug 17 at 16:38

With lots of water (ice) on hand, and cheap electricity, steam is a good short-trip thruster choice. The performance is completely inadequate for interplanetary distances, but if you're only going a few hundred kilometers (say, Ceres surface to orbit and back, hence anywhere on Ceres once) a pressurize water tank with bladder to control ullage, and a high-rate heater driven from batteries should do the job.

If you need a bit more (say, a 180 degree plane change, which takes another 2x orbital velocity), steam might not do the job -- but hydrogen peroxide will. Same elements, you just have to do a little work on the water. Peroxide was used in the Rocket Pack that James Bond used in one of his movies -- 1G+ thrust for (in those days) 20+ seconds, that adds up to 200+ m/s for a tank a human could carry on his back. Build a ship with 3:1 or 4:1 mass ratio, and you could easily lift to orbit, reverse orbit, and still land with some reserve.

  • $\begingroup$ At what point does it become less attractive? I would like to do something like a surface to orbit, a big plane change, and a return to the surface, without refueling. Maybe have a nice bit of delta v in reserve. For a 3000kg craft. Still attractive with steam or time to look for something better? I'ln update the question with this data $\endgroup$ – Innovine Aug 15 at 16:34
  • $\begingroup$ You could/should look this figure up, but as I recall, Ceres has an orbital velocity of only a few hundred m/s (3% surface gravity, near enough, and escape velocity of 514 m/s, so orbital speed under 400 m/s -- Mad Mike Hughes could almost get into orbit with his steam rocket). With a little care in design, this is well within the capability of a steam thruster. $\endgroup$ – Zeiss Ikon Aug 15 at 16:59
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    $\begingroup$ Steam alone is going to have low Isp. You want to go supercritical steam to keep efficiency up. $\endgroup$ – stix Aug 15 at 17:06
  • $\begingroup$ H2o2 has been on my mind so far. Why would hydrogen-oxygen not be used? $\endgroup$ – Innovine Aug 15 at 17:07
  • $\begingroup$ A comparison of supercritical steam, and h2o2 would be very welcome. Can h2o2 be stored in a frozen state? $\endgroup$ – Innovine Aug 15 at 17:08

The answer I gave to a previous question of yours remains relevant (too lazy; didn't follow link: peroxide is great, use peroxide) but as you've already read all of that ages ago, have an alternative.

Peroxide is of course a little unstable, and in its highly concentrated form is somewhat hazardous. It'd be great to just have an inert propellant, ideally one that didn't have to be stored under any pressure, either. You've already stated that you have ample amounts of electricity available from surface fusion plants, so I suggest using beamed power in one of three forms, in increasing levels of sophistication and potential hazard.

  1. Remotely powered electrothermal.

    Mount your spacecraft with rectennae and beam microwaves at them. Mount big phased-array microwave emitters on your fusion plants, allowing you to do beam-forming and power multiple small ships or fewer larger ships at once. Use whatever engine seems best... resistojets or arcjets using water as reaction mass are the first ones that spring to mind, but I'm sure there are many others. You probably don't need super-high efficiency, high $I_{sp}$ drives here, but you might be able to plasma drives too.

  2. Laser thermal

    Mount big lasers on your fusion plants. Mount mirrors on your ships. Point relatively diffuse lasers at your ships, they use the mirrors to focus them down through suitably heat resistant windows (eg. quartz) to turn water into steam and generate thrust. Not massive amounts of thrust mind you, but it should be enough.

  3. Laser ablative

    Make a suitably-shaped mass of your solid propellant... water ice would certainly work, but other materials (possibly up to and including plain old rock and regolith) should work fine. Use big, serious, high-power, short-pulse laser cannon mounted on your fusion plants to selectively vapourise the reaction mass, generating plasma and a decent amount of thrust with a pretty reasonable $I_{sp}$ to boot. This is a great way to to surface-to-orbit with even quite big payloads, and by way of a bonus you can use your lasers to remove all kinds of debris from your orbit, including the meaty kind you disapprove of.

In all cases, your spacecraft can be extremely simple, with just some attitude control and communication systems. Traffic control can be substantially handed over to external authorities... scope for malicious manoevers is limited if ground control can simply turn off your main engine (which can be quite bad if your orbit intersects something solid...). No need for excessively engineered high-pressure fuel tanks or dangerous chemicals. Not even any need for any on-board power source more fancy than a battery. Quite useful, on the whole.

Other important things to consider: with an excess of cheap electricity on the surface, electromagnetic launch catapults can be used to boost spacecraft into a variety of sub-orbital or escape trajectories as required, which can substantially reduce $\Delta_v$ requirements and hence mass of propellant required. You'll still need some to inject yourself into a closed orbit, of course, but even that can be done using externally-powered systems such as a magbeam. The magbeam installations can help you change orbit to do useful rendevous, and break orbit to land back on ceres or enter interplanetary space. It may even be practical to run magbeams on the surface of ceres itself to brake incoming craft, though there are perhaps safety issues there...

The most efficient solution, perhaps, is a space elevator. You could build one on Ceres out of kevlar using current-day engineering techniques, for example. I don't doubt that a scifi spacefaring humanity could make something pretty substantial here, and multiple elevators to boot, so simply loading your little 3 tonne ships into a cable climber for ascent and descent could be entirely practical. You could then use pretty much any rocket to get around, probably even including compressed gas.

  • $\begingroup$ +1 for alterative methods of generating steam. I'd give a second +1, if I could, for elevators -- didn't even occur to me. $\endgroup$ – Zeiss Ikon Aug 16 at 11:17

The current Cube satellites use water as an impulse reaction jet - FEMTA thrusters by name - the technology is a micro-electromechanical system, or a MEMS, which are tiny machines that contain components measured on the scale of microns, or millionths of a meter. The thruster demonstrated a thrust-to-power ratio of 230 micronewtons per watt for impulses lasting 80 seconds.

Basically they rely on the surface tension of water and very small tubule apertures to prevent the water from jetting unless driven by the tiny internal heaters - from an excellent article from Purdue University [link below]:

The new system, called a Film-Evaporation MEMS Tunable Array, or FEMTA thruster, uses capillaries small enough to harness the microscopic properties of water. Because the capillaries are only about 10 micrometers in diameter, the surface tension of the fluid keeps it from flowing out, even in the vacuum of space. Activating small heaters located near the ends of the capillaries creates water vapor and provides thrust. In this way, the capillaries become valves that can be turned on and off by activating the heaters. The technology is similar to an inkjet printer, which uses heaters to push out droplets of ink.

So one can imagine a huge array of FEMTA thrusters (say 1500 of them running in concert) providing sufficient thrust for orbital maneuvers without trouble, and having the advantage of the source water not boiling off even through tank insulated walls as would happen with ammonia if in space for years.

There's also a lot of ongoing discussion in the space development community about PV-based electrolysis and local water from moons or asteroids as a ready source of in-situ hydrogen and oxygen for hydrox thrusters - the amazing advantage being that gathering water and then cracking it in situ removes that vehicle from the tyrannical constraints of the rocketry lemma - the ever-increasing fuel requirements mandated by needing to carry all reactant through all phases of a journey.

So short answer: Water.

Purdue FEMTA Thruster Article

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    $\begingroup$ So, ink-jet propulsion, without the dye? Brilliant! $\endgroup$ – Zeiss Ikon Aug 16 at 11:18

I would use the standard H2/O2 mix you can get from elecrolysing (molten) surface ice; this mix is multipurpose.

You can expell the H2 from supercooled tanks trough RCS thrusters, you can use standard fuel cells with H2/O2 to provide electricity, you can burn the H2 under the oxigen for main thrust, and you can breathe the O2. Bonus effect: the water produced from the fuel cell can be drunk, and used to make dried foods edible again.

  • $\begingroup$ There seems to be some concern regarding storage of h2? $\endgroup$ – Innovine Aug 16 at 14:42
  • $\begingroup$ @Innovine given that H2/O2 is a rocket fuel we use right now (that's what the space shuttle main engines used), I think the storage problems can be described as solved. In fact, on Ceres, where exterior temperatures are already far lower than they are on Earth, keeping the liquid h2 below its boiling point will be even easier. $\endgroup$ – Gryphon - Reinstate Monica Aug 16 at 16:00

If you have some spare CO2, you can make very nice fuel pair (with impulse up to 300s): peroxide and methane.

  • $\begingroup$ Peroxide monopropellant gets you 140s, which is enough in Ceres' shallow gravity well. There's no shortage of carbonate minerals on the surface though, so bulk hydrocarbon synthesis should be straightfoward. $\endgroup$ – Starfish Prime Aug 16 at 10:46

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