So, several times over the years I’ve thought me and Elon Musk should sit down and have a discussion – which would probably, I readily admit, turn into a argument. But we have a lot of things in common, interest wise.
I am often struck by how Elon and Tesla both have the same sense of showmanship, and the same tendency to make utterly batshit claims occasionally.
Elon’s got some great ideas (reusable spacecraft, electric cars, possibly even underground transit) and some reeeeeally bad ones (hyperloop, semiballiastic orbital travel for point to point on earth). But I want today to address a particularly nutty idea he’s come up with – that we’d be able to do a manned Mars mission by 2026. (Claim made here).
This is absolutely true – if we don’t mind that it’s a one way trip, and that we’ll be sending people to die on Mars in a few weeks or months.
However, the tradition in American spaceflight is we don’t consider the crew expendable. Which means a more reasonable timeline is 2046 if we use conventional fuels, or 2036 if we use a NERVA.
Now, I know in general the world is against NERVAs, and I don’t deny that they’re a bit risky – if we did use a NERVA we’d probably have to send up the fuel rods encapsulated in the best tech we could put them in, and assemble them in space. I do feel like various forces have overstated the risk of using nuclear fueled spacecraft, and that’s another whole topic. But, I want to make the case for using a NERVA here.
If we used a NERVA, we would not need combustible fuel, just ‘reaction mass’. THis means any liquid or gas that can be liquified would do! This makes it easy to guarantee a return flight, assuming the spacecraft arrives in one piece on mars, because you can
A: Use the same reactor fuel to run a reactor that compresses Mars’s atmosphere into the reaction mass tanks
B: Use the same reactor fuel to power heat & light onboard the spacecraft and a connected ‘Hab’ a la the Martian
C: It’s very likely a NERVA would not suffer from many of the ‘cold soak’ issues that a conventional rocket engine does, because NERVA designs often involve very few moving parts and by definition the reactor itself is going to get the whole system plenty hot by the time the jet fires
However, given that we’ve shelved NERVA technology, it would still be many years, even if we used a NERVA, before we could talk about going to mars and returning. I don’t know if Musk is outrageously optimistic, or if he’s doing the ‘just one more hill and we’ll be at the top of the mountain’ technique of pushing humanity along.
Anyway, without a NERVA, let’s talk about the challenges, so we understand why 2026 is ludicrous
#1: With a combustible fuel rocket, carrying enough fuel for a two way journey is a non-starter. It makes the fuel load *the square* of the amount you need for a one way journey, because you must carry the fuel to move the fuel.
#2: With a combustible fuel rocket, generally after a burn adequate to reach escape velocity on earth, some maintenance must be done before another burn can occur. Very likely this would have to be done in orbit above mars. Fixing things in space is *tricky* and takes months of simulations and practice to pull off.
#3: With a combustible fuel rocket, a very real concern is ‘cold soak’.
a) Most spacecraft to date have avoided the challenge of trying to ignite something that has been sitting at -200 degrees by using hypergolic fuels – that is to say, fuels that when two dissimilar parts are mixed together, ignition happens automatically. However, hypergolic fuels are much less efficient than oxidized fuels AND they’re incredibly toxic. They are not suitable for a escape velocity rocket anywhere much bigger than the moon.
b) If not using hypergolic fuels, you actually have to figure out starting up the fuel and oxidizer pumps, then igniting the mix coming out of them. Bunch of moving parts to get all working together.
#4: We’ve often spoken of using a combustible fuel rocket and “making the fuel on mars”. If we can find water, we can electrolyze it and make hydrogen and oxygen, then liquify them and make fuel. OK, I agree, all that is true, but there’s a lot less sunlight falling on Mars than on Earth. Doing that with a solar array is going to take a loooong time (and at the same time we’ll also have to make enough power for heat & light for the crew). So we’re probably going to have to send a nuclear reactor to Mars anyway, just to make the fuel, unless we want to try to keep men alive on Mars for a year on our first foray – something that is also likely to end in a dead crew.
