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Old October 3 2012, 08:14 PM   #18
gturner
Admiral
 
Location: Kentucky
Re: SpaceX's Grasshopper

newtype_alpha wrote: View Post
YellowSubmarine wrote: View Post
I'm not sure if the SuperDraco engines provide enough thrust for landing on the Moon
They don't. Depending on the fuel loads, their delta-vee is around 400m/s: just enough to escape from an exploding rocket, or enough to slow to a stop from a terminal velocity of around 250m/s and then hover for a second or two just before touchdown. That, combined with Dragon's RCS thrusters, would give you about 700m/s, about a third of what you'd need for lunar deorbit and controlled landing (The Apollo LEM needed just under 2100m/s on its way down and the ascent stage needed 1500 on its way up).
I think we're saying "thrust" where we mean either impulse or delta-V. Since the SuperDracos are abort engines, they have enough thrust to accelerate the Dragon capsule away from an accelerating Falcon. The DragonRider has 8 SuperDracos, each with about 15,000 lbs thrust, but they are mounted at an angle so cosine losses drop the total to about 100,000 lbs, which on a 20,000 lb Dragon in launch configuration should give you about 5 G's acceleration. But there's not much fuel on board, so the mass ratio is very poor, as is the total delta-V.

The engines would probably have an ISP of around 300 (I assume they're tuned for a a sea-level expansion ratio instead of vacuum), but the cosine losses would again reduce this to about 260 or so. The full-thottle mass flow rate would be about 380 pounds of fuel per second.

Since the fuel-fraction is so low, I'll skip the rocket equation and just note that you'd get 10 seconds of burn time on 3800 pounds of onboard fuel, and 10 seconds at 5 G's gives you a delta-V of only 490 m/sec. You could of course double the amount of onboard fuel but it's eating into the payload, so the Dragon would definitely need a seperate descent stage (even if it was just an external tank), and possibly an ascent stage external tank.

The Apollo Descent Engine only had a tenth as much thrust as the Dragon abort engines, so the engines are already serious overkill. They need one SuperDraco (already 50% more thrust than the Apollo DPS) and they've got eight. The change they sould make is eliminating the cosine losses (which directly reduce the ISP) by realigning the engines or using a seperate vertical SuperDraco for ascent and descent.
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