# Can cold thrusters be improved for work in space

This belter got Mr.X thinking and it is a stroke of genius this time.

What do we do with all that oxygen we get when extracting metals from their oxides? we liquify and use them as a reactive mass of course.

ISP is something like 60 seconds, so if we take something like 40% by mass to be that oxygen in our canister, then delta-v is around 300 m/s - not a lot. So we do need some massdriver on the Moon - to kickstart the can in orbit, but it is enough to make a second Hohmann maneuver to circularise the orbit.

one of the problems of low ISP is the low temperature of exhaust - so why not use sunlight, mirrors, and Al2O3 heat pipes and do heat it to some 2100K - then it possible to squeeze maybe 800m/s delta-v - not huge but definitely an improvement.

Will it work that way?

• Weight?????????? Mar 29 at 20:21
• overall mass does not matter, only the proportion of masses does matter. u may assume some in the range of 10-100-1000 tons if u need some number Mar 29 at 20:41
• I mean the extra weight of all your mirrors. and heat pipes. Mar 29 at 20:42
• @DKNguyen no extra, it in that 60% mass, made out of those metals which are extracted. maybe a slight difference may make those aluminum oxide pipes, but it not necessarily a significant percentage, +- 2 percent does not matter that much, just leave some metal bricks on the moon for local consumption. Mar 29 at 20:48

so why not use sunlight, mirrors, and Al2O3 heat pipes and do heat

You will probably be unsurprised to hear that solar thermal rockets are things that people have thought about before. Most designs generally use hydrogen as the reaction mass because of all the lovely exhaust velocity (Isp as high as 900 seconds or more in some cases!) but a high-thrust, low-Isp oxygen-fuelled rocket could be useful in some circumstances.

The basic design is potentially quite versatile. Here's a splendid illustration I found on Project Rho, who got it from Nick Stevens who seems to do a load of nice spacecraft illustration work:

Remember that rockets that use highly oxidising reaction mass won't be able to run on reducing reaction mass (eg. hydrogen) when required (because they'll corrode horribly), so you're limited in the ultimate performance of your rockets. At least you can use something like carbon monoxide to get a tiny performance boost, when it is available.

Performance will of course be disappointing... if you look at the characteristic rocket velocity equation:

$$c_* \propto \sqrt{\frac{T_t}{M_w}}$$

you'll see that because of the square root in there, O2 is only 4 times worse than H2, all else being equal, so you might get >200 seconds Isp, which isn't great given all the inconvenience of storing and handling cryogenic oxidisers.

Honestly, you'd be better off using water. Non-cryogenic, >300 seconds, readily available. Given how much water ice there's going to be out there, I feel like there's no particularly good reason for using O2 as reaction mass, but if you really felt the need I guess the option is available.

• water is in some places or not that abundant, while oxides are everywhere, so. Carbon also has other uses, oxygen however is mostly a waste in metal extraction cases. Mar 29 at 22:07
• @MolbOrg if you're mining asteroids, there will be a lot of carbonaceous chondrites about and they're surprisingly high in water. Ice is pretty much everywhere in the outer solar system. Really, the only place it is unusual is right in the inner solar system, but there's not nearly so much stuff floating around for the taking there compared to eg. the asteroid belt. And if you're close to the sun, you can use nice solar-powered electric rockets for your high Isp needs! Mar 30 at 8:45
• Belters can do whatever, "best they have is the worst there is". Mr.X related topics are mostly in the scope of bootstraping space, so early steps, creating incentives for further development, crude tech and all that, with potencial to improove. Electric engines sure they are considered option, and sure may be used extensively, but sometimes u just have to have a stronger kick for manuevers, closer to chem rockets in performance, or it takes forever or just not possible, or inconvinient. And there trash waiting to be used, so why not. Mar 30 at 9:19