The VASMIR/"The Martian" tactic to getting to Duna or other planets

[Laie]

the intent is to prepare enough LH2 for the desired burn duration, including reactor spin-down.

That's a lot of LH2. No matter how I think of it, you can either make it as fast as it's consumed, or you need to deal with storage.

[Norcalplanner]

If you have lots of time to plan ahead, try the so-called Mangalyaan Maneuver. It's one of the best ways to ensure you can do an ejection burn with even the weakest of propulsion systems, it requires almost no math to go through, and you get away without having to estimate or measure your ejection angle. It works by repeatedly periapsis-kicking your orbit at a specific date ahead of the transfer window where your periapsis is aligned with the sun, so you cannot possibly miss the right angle. Then you just wait until the actual transfer window arrives to burn the final few dozen m/s towards your destination. Details in the linked thread.

(Just keep your growing orbital period in mind while raising your orbit.)

But Streetwind, getting back to the Hermes, wouldn't that be next to impossible with an acceleration of only 2 mm per second? By my math, that works out to an acceleration of 0.0002 g. Assuming a 90 minute orbital period for LEO, and assuming that you're willing to accept the steering losses associated with burning 45 degrees in front of and behind the desired periapsis for a burn time of 22.5 minutes per orbit, that means that you'll add only 2.7 m/s per orbit. As the periapsis speed goes ever higher, the period of time per orbit available for periapsis kicks will get even smaller. If the orbital period stayed the same (which it won't) then the Hermes would take 794 periapsis kicks to get up to 4.3 km/s for the mars transfer, and the increasing orbital period would mean that it would take many months to actually reach escape velocity.

Maybe I'm just dense, but I'm having a heck of a time figuring out how a vehicle with thrust that low can break out of the earth's gravity well in anything approaching a reasonable timeframe and without things getting weird due to the Moon's gravity well.

Edited September 20, 2015 by Norcalplanner
typos

[Rune]

There's a video where the author shows the exact path the Hermes takes, although I agree that the whole "breaking earth orbit" part is a bit hand-wavy.

Here it is. The interesting part starts at 12 minutes in.

That's weird. The part when he mentions the implausibility of the dust storm scene, and then he mentions he had other ideas for it? Well, pretty much word for word, that is what I offered in I-don't-remember-what-thread here! Complete with the fuel leak on the MAV forcing the immediate abort! Ok, I said it was a helium leak, but still...

Rune. I was going to say "great minds think alike", but instead I'm going with "then why did you pick the stupid version!"

- - - Updated - - -

You'd be surprised... after an issue that cropped up recently with Near Future, I did some math, and it turns out nuclear engines are far less exciting than I thought they would be. Zero-boil-off cryogenic tanks kill them to the point where hypergolics have better total dV unless you really improve the specific impulse of NTRs significantly. =/

Ok, that's interesting. As in, I don't often hear it mentioned, the supposed 'Achilles heel' of nuclear propulsion. I say supposed, because both NERVA and the russian RD-0410 were tested with a variety of storable propellants largely without specifically re-engineering them to handle the different propellants. It's also the reason the Discovery (the real one, from the book, not the bastardized child of Kubrick) used ammonia for fuel, too. Isp ranges from 450-500s (water propellant, marginally better than H2/LOX propulsion but you can't beat it in propellant storability), to a very decent 650-700s for methane, a soft cryogen. Ammonia is kind of a middle ground, and CO2, CO and N2 give worse specific impulse than most chemical rockets. Both H2O and CH4 propellant would allow for single-stagers to Mars and back, although the one using water propellant would probably have to employ aerobraking (if it can't remass along the way).

Rune. The really funny part is when people assume H2/LOX propulsion after discarding NTR as "too technically challenging". Ha! H2 storage seems easy only because we haven't seriously tried it.

[Streetwind]

But Streetwind, getting back to the Hermes, wouldn't that be next to impossible with an acceleration of only 2 mm per second? By my math, that works out to an acceleration of 0.0002 g. Assuming a 90 minute orbital period for LEO, and assuming that you're willing to accept the steering losses associated with burning 45 degrees in front of and behind the desired periapsis for a burn time of 22.5 minutes per orbit, that means that you'll add only 2.7 m/s per orbit. As the periapsis speed goes ever higher, the period of time per orbit available for periapsis kicks will get even smaller. If the orbital period stayed the same (which it won't) then the Hermes would take 794 periapsis kicks to get up to 4.3 km/s for the mars transfer, and the increasing orbital period would mean that it would take many months to actually reach escape velocity.

Maybe I'm just dense, but I'm having a heck of a time figuring out how a vehicle with thrust that low can break out of the earth's gravity well in anything approaching a reasonable timeframe and without things getting weird due to the Moon's gravity well.

I was talking about an actual KSP application here. I've not read "The Martian" yet and don't know about the Hermes. If we're talking RL electric propulsion with significant payload, though, then you can pretty much forget periapsis kicking; you'll need a spiral trajectory because you'll need to be burning for multiple months just to scrounge together the ejection dV.

The Mangalyaan probe, for which the maneuver is named, had a 0.44 kN liquid fueled engine and weighed 1350 kg, for an initial TWR of 0.033. It performed six periapsis kicks over the course of 16 days, and could have been even faster but there was an issue during one burn that required one long extra loop and one extra burn, taking several days.

Ok, that's interesting. As in, I don't often hear it mentioned, the supposed 'Achilles heel' of nuclear propulsion. I say supposed, because both NERVA and the russian RD-0410 were tested with a variety of storable propellants largely without specifically re-engineering them to handle the different propellants. It's also the reason the Discovery (the real one, from the book, not the bastardized child of Kubrick) used ammonia for fuel, too. Isp ranges from 450-500s (water propellant, marginally better than H2/LOX propulsion but you can't beat it in propellant storability), to a very decent 650-700s for methane, a soft cryogen. Ammonia is kind of a middle ground, and CO2, CO and N2 give worse specific impulse than most chemical rockets. Both H2O and CH4 propellant would allow for single-stagers to Mars and back, although the one using water propellant would probably have to employ aerobraking (if it can't remass along the way).

Yeah, that's why I said at the end that I'd love to see the numbers with ammonia, but I have no data on the required tankage.

[Captain H@dock]

Maybe I'm just dense, but I'm having a heck of a time figuring out how a vehicle with thrust that low can break out of the earth's gravity well in anything approaching a reasonable timeframe and without things getting weird due to the Moon's gravity well.

I think you have to stop thinking in term of punctual burns. Such engines force you to use spiral orbits. So I'm assuming you'll continuously burn prograde which will make your circular orbit grow bigger and bigger (yet still circular) until you're out of the earth gravity well.

Granted, I don't know how that would allow you to use the earth orbital velocity (sound like you'd be wasting it since you can't eject...)...

Rune. I was going to say "great minds think alike", but instead I'm going with "then why did you pick the stupid version!"

He says in that (another?) interview that he really wanted this to be a fight against nature.

[Frozen_Heart]

There has been quite a bit of progress in storing hydrogen in ammonia. Doesn't drift through the tank walls and require cryogenic temperatures.

[Archgeek]

There has been quite a bit of progress in storing hydrogen in ammonia. Doesn't drift through the tank walls and require cryogenic temperatures.

Neat, that would allow for a lot more hydrogen per unit mass. Though it may make tanks and processing equipment smell like cat pee.

[Laie]

There has been quite a bit of progress in storing hydrogen in ammonia. Doesn't drift through the tank walls and require cryogenic temperatures.

Unless you can get at least a 9:1 mass ratio, that's not worthwhile. That is, every single ammonia molecule would need to cling to 9 hydrogen molecules. Me no believe.

In principle, hydrogen reliquefaction should be a thing. In practice, I can't even start to imagine how to go about it. And that doesn't even take into account hydrogen's tendency to seep through fittings, welds and even sheet metal. Keeping (most) hydrogen for a week or three weeks is doable. A lunar shuttle should work well enough. But returning from mars with hydrogen you brought along, no way.

As Rune said above, methane gives you about 2/3rds of hydrogen's ISP and is much easier to keep.