SpaceX's Grasshopper

Discussion in 'Science and Technology' started by Wanderlust, Sep 22, 2012.

  1. Wanderlust

    Wanderlust Captain Captain

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  2. gturner

    gturner Admiral

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    Ooooo..... :)
     
  3. gturner

    gturner Admiral

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    It needs a memorable quote.

    "That's one small hop for a rocket, one giant leap for ... um, cheaper rocketry."
     
  4. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

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    Ironically, between the Falcon 9 and the Dragon capsule, this means SpaceX is closer to a manned moon landing than NASA is now.
     
  5. MANT!

    MANT! Rear Admiral Rear Admiral

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  6. YellowSubmarine

    YellowSubmarine Rear Admiral Rear Admiral

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    I think closer is an understatement, SpaceX would soon have hardware that's almost capable of a Moon landing. And if the Grasshopper achieves its stated goals dropping the price of launches low enough, they could do a Moon landing with their own money. Regularly.

    I'm not sure if the SuperDraco engines provide enough thrust for landing on the Moon, but that aside, they can do an unmanned in 2013-2014 if they have $200-300M to waste. Or even less if they utilise the Falcon Heavy demo flight due to a lack of customers.

    By the way, I'm not sure if gturner's post is intended to be serious or humorous, but it is spot on. The Grasshopper and SuperDraco will be SpaceX's first* technical firsts. Being the first private company in orbit is something, true, but so far they have been doing what has been already done by governments. On the other hand, nobody has ever landed a rocket, or even a capsule propulsively. Which is the reason I'm excited for SpaceX much more than I would be for a random company that would provide launch capability and a capsule as requested.

    Even that feeble hop in the video in the OP is an example of an imaginative technical first, because I'm pretty sure that nobody has done that either.

    * Well, there were probably some small ones in the design of the rockets and capsule, including the launch abort, but nothing substantial.
     
  7. Scroogourner

    Scroogourner Admiral Admiral

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    ^You're totally forgetting DC-X and about 3 other companies that have been building propulsive landing rockets for about the last 10 years. Now, when and if they get it into service, THAT will be a first, if one of the other companies don't beat them to it.

    As for the Superdraco having enough thrust to land on the moon, you do remember it's designed to land the dragon capsule on earth right? Earth having higher gravity?
     
  8. YellowSubmarine

    YellowSubmarine Rear Admiral Rear Admiral

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    Oh, I didn't know about DC-X, I actually like how it looks. But as far as I can tell all the tests have been limited in duration, and it neither has actually gone to orbit, nor it has had a big enough version capable of doing so?

    As for the SuperDraco and the Moon –
    I don't think that gravity is as important as air drag here. For landing on Earth, the delta-v required from the engines is far less, because the atmosphere does most of the job. For the Moon, you have less gravitational acceleration to deal with, but your initial speed will be a problem anyway. So unless you pick some crazy trajectory, I have some doubts that the Earth SuperDraco engines will do it, but I'm sure they can be upgraded for the Moon.

    Unless I'm missing something?

    Edit: Hm, I guess someone who has worked more with celestial mechanics could clarify that, but now that I think about it, when Dragon is inserted into lunar orbit, it has to deal mostly with cutting horizontal speed, at least initally, and if I understand correctly you would have more time to do it, so thrust might be not as important as fuel? I'm still not sure that it would be enough though.

    Edit 2: Right, I think I figured that out, the lack of air would make almost no difference, except for the fuel expenses. Gosh, I'm stupid. You only need thrust force to cancel out the force of gravity. If it can do it on Earth, the Moon would be a piece of cake. At the speed you'd be touching down, air resistance doesn't play a role at all. Feel free to laugh at me.

    Now, having learnt this, I'm betting on a private moon landing in 2013. :lol:
     
    Last edited: Sep 26, 2012
  9. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

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    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).

    Actually, though, I was thinking that the grasshopper's landing system wouldn't be hard to modify into a lunar landing vehicle. The grasshopper stage is the same diameter as Dragon's trunk, after all, so mounting the landing system to a Dragon capsule along with propellant tanks and life support equipment is a natural evolution path.

    True that. If SpaceX continues to meet its technical goals the way it has in the past, they WILL supplant NASA as America's front line space program. It's fair to say that NASA's big dreams and long-range ambitions are slightly more impressive, but they're not doing the grunt work it takes to REACH those goals in a way that makes sense. It's like a kid who never does his homework and never shows up to class and studies really really hard for the final exam because he wants to get into Harvard.
     
  10. YellowSubmarine

    YellowSubmarine Rear Admiral Rear Admiral

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    Given how SpaceX designs things, I'm half sure that's already an intended part of the design. :p
     
  11. Scroogourner

    Scroogourner Admiral Admiral

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    all they would really have to do is cut a short version of the tankage. A bigger problem with grasshopper might be how deeply the Merlin engine can throttle for a moon landing.
     
  12. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

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    AFAIK, the Merlin isn't throttlable, but I could be wrong about that.

    More importantly, the Merlin runs on RP-1 and Liquid oxygen, a precarious mix for a spacecraft trying to land on the moon. That, plus the lighter gravity of the moon means you'd be better off using the landing platform with a cluster of five or six superdracos or a modified Kestrel engine fueled by hydrazine.

    If SpaceX was smart, though, they would probably take a page out of the old Soviet playbook and have the landing platform and its propellant tanks as a separate unit from the actual engine. This way, the ship blasts off from the landing platform using the ascent/descent engine and leaves the spent propellant tanks and the landing legs behind. Saves weight on the descent that way.
     
  13. Scroogourner

    Scroogourner Admiral Admiral

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    SpaceX has a version of Merlin-1D that is throttle-able. Otherwise, no grasshopper. This variant will also be capable of at least 2 restarts.
     
  14. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

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    But again, they're going to need an engine that can run on hydrazine, while the Merlin is still burning RP-1/LOX, propellants which are NOT ideal for a lunar landing vehicle despite the higher ISP.
     
  15. Scroogourner

    Scroogourner Admiral Admiral

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    Oh, most likely they wouldn't use Merlin for a lunar lander. I was just correcting the throttle remark.
     
  16. Alpha_Geek

    Alpha_Geek Commodore Commodore

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    "I would like to die on Mars; just not on impact."- Elon Musk
     
  17. publiusr

    publiusr Rear Admiral Rear Admiral

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    Well, that's the internet generation for you, no long form reading. ;)

    How about this:

    "X-number of years ago, Neil Armstrong made a giant leap for all mankind. In his memory...we have made another."

    That's how its done.
     
  18. gturner

    gturner Admiral

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    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.
     
  19. gturner

    gturner Admiral

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    Ooo... Much better. And the Grasshopper is an awfully lot like the lunar lander trainer (the flying bedpost) that Neil flew.

    When they actually return to the moon, though, it would be hard to top the Onion's version of Neil's statement from their book "Our Dumb Century."

    Or something like that.
     
  20. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

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    First of all, I knew exactly what you were saying. I am saying that the Delta-v for the Superdracos in an abort configuration is a little over 400m/s. It could be considerably less for a controlled landing because terminal velocity near sea level is going to be quite a bit less than that, but for a range of plausible ISPs plus how much fuel they've got on board (not to mention what an abort thruster would have to be able to produce in order to be useful in that role) it's no less than 400 and no more than 600.

    Second of all, when you start running the rocket equation to figure these things out, the only thing that really matters is the amount of propellant on board the craft. Even an increase or decrease in ISP doesn't make a huge difference unless you double or triple it. The cosine losses don't count for much; the only way to get a lot of movement for the SuperDracos in terms of performance is to change the fuel they're using and increase the combustion chamber pressure, and once you've done that, you've basically developed a whole new engine.

    They wish. 270 is probably pushing it, 210 is more realistic considering the superdracos won't have comparatively large nozzles.

    I didn't skip the rocket equation, I spent a couple of days last year going through these calculations.

    But the descent MODULE had ten times the fuel capacity and a slightly higher ISP, thus a larger overall delta-v. Significantly: even if you used the DragonRider with half of its superdracos firing and throttled way down to about 1.6m/s^2 (enough to hover over the surface of the moon) they would only be able produce that low acceleration for a finite period of time (250 seconds, with a D-V of 400m/s).

    The thing is, the Dragon is coming to a landing on the lunar surface from orbital velocity of about 1600m/s. That means that even if all eight of those superdracos fire at maximum thrust and burn until they're bone dry, Dragon will still be moving way too fast to land safely. Even more importantly, if you somehow added enough fuel to cancel its orbital velocity entirely, you still need another minute or so of hover time to give your pilots time to zero in on a landing site, and another 1000 or more in reserve if you have to abort your landing and jump back into orbit.

    That would change nothing, since even a 70% increase in ISP -- way more than you'd get from the cosine losses -- would still leave the Dragon about 1000m/s short of a safe landing.

    What they actually need is about 7 times more propellant than the Dragon can carry internally; the lunar descent stage carried 8 tons of hypergolic propellants to the Dragon's 1200km and the Dragonrider's (probable) 2000kg. So Dragon either needs to add some gigantic propellant tanks under its heat shield (like the Soviet lunar landers; their ascent/descent engine was on the capsule and the landing stage was just fuel and legs) or it needs to get about 90% lighter.