Dragon LAS as BEO ascent stage

[bulkmail]

Does anybody have an estimation about the delta-v, isp, etc. parameters required so that the SuperDracos set/LAS are powerful enough for Earth soft-landing?

And a comparison with the requirements for descent/ascent stage (lifting the whole Dragon capsule - empty or full with crew or cargo) to/from Moon, NEO, Mars? (let's leave aside the issue of re-docking with the service module and/or Earth return stage).

[adrianwyard]

Good question. Although I'd add that if you could only descend to the Lunar surface before depleting fuel, that would be interesting to know as well.

There's been a lot of related discussion in the Red Dragon threads.

To start off: 8 SuperDracos should give 120K lbf of axial thrust:

http://www.spacex.com/press.php?page=20120201

Would you call a Lunar mission Blue Dragon, perhaps? (Or Cheese Dragon?)

[meekGee]

wiki's your friend:
http://en.wikipedia.org/wiki/Delta-v_budget

especially:
http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Deltavs.svg/500px-Deltavs.svg.png

[bulkmail]

Good question. Although I'd add that if you could only descend to the Lunar surface before depleting fuel, that would be interesting to know as well.

I assume that since it's (expected to be) capable enough to land on Earth, then it's capable enough to land everywhere else in the Solar System (moons and planets that aren't a gas giant) - if we disregard special environments and the delta-v required to reach there (orbit or direct descent trajectory) to be provided by service module/BEO transfer stage. Correct?

So, the question is more about ascent (whether crewed, sample return or just for re-use) - whether the engines provide enough thrust in the first place (theoretically) and whether there'll be enough propellant after the descent.

wiki's your friend:

OK, but how do these compare with the Dragon LAS capabilities when it has already utilized part of the fuel for the landing?

[Lars_J]

Good question. Although I'd add that if you could only descend to the Lunar surface before depleting fuel, that would be interesting to know as well.

I assume that since it's (expected to be) capable enough to land on Earth, then it's capable enough to land everywhere else in the Solar System (moons and planets that aren't a gas giant) - if we disregard special environments and the delta-v required to reach there (orbit or direct descent trajectory) to be provided by service module/BEO transfer stage. Correct?

Not at all. Earth's atmosphere provides the vast majority of the slowing down. Dragon's terminal velocity is apparently subsonic, and that is all that the landing thrusters need to handle.

[MP99]

Good question. Although I'd add that if you could only descend to the Lunar surface before depleting fuel, that would be interesting to know as well.

I assume that since it's (expected to be) capable enough to land on Earth, then it's capable enough to land everywhere else in the Solar System (moons and planets that aren't a gas giant)

No, because Dragon relies on friction with the atmosphere to lose the vast majority of it's orbital energy - something it can only do on Earth, Venus or Titan of your targets.

Mars gives you some benefit from it's very thin atmosphere.

Everywhere else, you have to do the whole thing via propulsion.

cheers, Martin

Edit: thick -> thin.

[charliem]

The Apollo program lunar modules had to spend half their mass in propellants to go from low lunar orbit to the surface.

Dragon, having also hypergolic engines, would need more or less the same (with cosine losses more than that).

That means that if you start with 6 mT as dry weight plus cargo, you'll need about another six of fuel.

There's not space inside a Dragon for so much propellant.

[adrianwyard]

I'm certain Martin meant to say that Mars' atmosphere was thin, not thick (it's <1% that of Earth at sea level).

As is confirmed in the wiki link, if you ignore aerobraking, an Earth LEO->surface transfer is harder than Mars (~10 km/s versus 4.1), but in reality aerobraking will do much of that delta-v for you (on the way down, at least), but the Martian atmosphere will only be ~1/100th as much help. 

I think it's fair to say that if you can land on Mars (capable of 4.1 km/s minus aerobraking), then you are likely to be able to  land on the Moon (1.6 km/s).

Supposedly Red Dragon can land 1 metric tonne on Mars, so one way to constrain the question is as follows:

A] Assuming landing on Mars is comparably hard to landing the moon (i.e. no aerobraking), and you arrive with empty tanks,

B] Is 1 tonne of propellant enough to achieve 1.6 km/s to return to LLO?

So...

A] is a very conservative assumption. Can anyone improve?
Anyone care to ball-park an answer to B] if we assume this is a stock Dragon at about 5 tonnes?

And there's always the question behind the question: why are we landing the Dragon capsule on the moon and returning it to LLO? Are you wondering if Dragon could be used to do a human return to the lunar surface? If so, there are a billion other considerations, but I suppose if we assume Red Dragon goes ahead and all the infrastructure that's needed to to pull that off, then this becomes more of a possibility.

[meekGee]

OK, but how do these compare with the Dragon LAS capabilities when it has already utilized part of the fuel for the landing?

Well, launch escape is a high-thrust, low delta-V maneuver.  (5 secs at 10g (WAG) is only 500 m/s, compared with 4100 m/s to ascend to Low Mars Orbit)

Luckily, the SD can deep throttle, so you can use the same stored impulse to loft a much lighter ascent stage.  (for the LAS burn, you can forgo the rocket equation since the delta-V is so low.  for the ascent burn, you can't)

Unluckily, the SD's are hard mounted to the Dragon, so you can't easily loft a much smaller ascent stage. The most you can do is transfer propellant to that stage, but then why not have it there to begin with?  You already know how much propellant will be used for landing.

The other part of your question could be whether SD technology can be used for a separate ascent stage.  The answer here is that many technologies can give you SSTO with 4 km/s.  Since the delta-V is low, you'll look for a light-weight engine, and worry less about ISP.  So maybe some variant of a SD will be a good choice.

[DaveH62]

Would they use an elongated second stage for braking into lunar orbit and only disconnect the Dragon just before landing to conserve fuel? One advantage of the moon vs Mars is you could (at least in theory) use the upper stage until fairly close to the surface.
As for reason, the original LEM of Apollo cost about 10bn to develop. A couple hundred million to update dragon as a landing vehicle would not be the most energy efficient process, but it would likely be the most inexpensive.

[adrianwyard]

Good point, and one which emphasizes how hard it is to do this guesswork in isolation from all the mission elements. If you were to assume that this sort of mission was enabled by the Red Dragon concept, then you already have a Falcon Heavy in the mix. That can (supposedly) throw 10 t to Mars (delta-v of ~10 km/s), so an unstretched FH upper stage will presumably have plenty of propellant to do the maneuver you suggest (LEO to LLO requiring about half the energy), even after a great deal has boiled off along the way. Of course there may be other reasons you wouldn't want to use a cryo US for this.

[Robotbeat]

Use a crasher stage. Then it closes, pretty easily. (Though now you have to get off the surface somehow...)

[docmordrid]

>.
That means that if you start with 6 mT as dry weight plus cargo, you'll need about another six of fuel.

There's not space inside a Dragon for so much propellant.

Crasher stage (tanks) + an upgraded Dragon Claw?

[bulkmail]

OK, I forgot the atmosphere drag... so, descent also becomes uncertain (e.g. adrianwyard question about "enough for just landing?" is highly relevant), so I assume there will be no fuel left for ascent (even if engines are powerful enough)... maybe if one Dragon delivers the rover/sampler/crew and another delivers fuel for the first to ascent...

Nevertheless it would be nice if somebody can make a comparison of Dragon LAS power provided vs. descent and ascent power required for Moon, NEO, Mars - together with the fuel mass required...

[edit - sorry, didn't see the multiple messages above already discussing most of that...]

[MP99]

I'm certain Martin meant to say that Mars' atmosphere was thin, not thick (it's <1% that of Earth at sea level).

Yup. 
Thanks.     <<< nearest I could get to "thanks".
Edited. 

cheers, Martin

[adrianwyard]

Another data point: the claim is that the Super Dracos on Red Dragon decelerate it from Mach 2.28 to 0, which I calculate to be about 550 m/s. (Assuming 1 mT delivered to the surface.) This matches up with meekGee's post.

If we knew the mass of the Dragon tanks when full, we could get closer to a definitive calculation, but it's looking like a descent stage is going to be needed if you wish to return to LLO. And the only reason I can think to take a full Dragon down and up again is for a minimal manned mission. Are there others?

[neilh]

If we knew the mass of the Dragon tanks when full, we could get closer to a definitive calculation, but it's looking like a descent stage is going to be needed if you wish to return to LLO. And the only reason I can think to take a full Dragon down and up again is for a minimal manned mission. Are there others?

Low-development sample return?

[docmordrid]

If we knew the mass of the Dragon tanks when full, we could get closer to a definitive calculation, but it's looking like a descent stage is going to be needed if you wish to return to LLO. And the only reason I can think to take a full Dragon down and up again is for a minimal manned mission. Are there others?

Low-development sample return?

From the recent Mars conference. Very fuzzy as its a screen cap off projected video, but you'll get the idea.

The discussion mentioned that since Dragon's pressure hull was designed for 1 atmosphere it's over-designed for Mars pressures - opening it up for fabricators to install hull mods that could hold multiple small rovers and deployment hardware. Add a sub-style missile tube up the center, ISRU etc. and poof.

[meekGee]

Neat, I had a verbal description of just that ready to go, but there it is!

And this takes us back to RoboBeat's insight on COLOX, for a no-soil ISRU mechanism.

(And of course these are non-LAS-derived designs, and thus are laughing-horse material on this thread)

[adrianwyard]

Another data point: the claim is that the Super Dracos on Red Dragon decelerate it from Mach 2.28 to 0, which I calculate to be about 550 m/s. (Assuming 1 mT delivered to the surface.) This matches up with meekGee's post.

If we knew the mass of the Dragon tanks when full, we could get closer to a definitive calculation...

For Red Dragon the claim is they'll use 1.9 t of propellant to get the 550 m/s needed for landing, so even if you use a crasher stage to get to near the Lunar surface, and all your payload is used for ascent propellant, these approx calculations suggest you don't make it back to LLO (1600 m/s) with a stock Dragon. But then I'm still not sure why you would want to do this anyway (taking its heavy heat shield, pressure vessel, thermal protection, etc. to the surface and back).