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SpaceX's Grasshopper Successfully Flew 325 Metres

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Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#41

Post by mikeash »

InclinedPlane wrote: post #33
grinich wrote: post #17

Earlier quoted context omitted.

ICBMs already do this, and we've been precisely deorbiting satellites for decades. Landing safely is the really tricky part. During the Corona spy satellite days, they used to catch the parachuting pods with a plane+hook. http://upload.wikimedia.org/wikipedia/commons/7/76/Keyhole_c...

ICBM warheads have about a 90% chance of landing within a circle with a radius of about half a kilometer, at least according to declassified info, the actual accuracy is likely higher. Even so, that hardly compares to a precision landing on a small landing pad.

I think your info on ICBM accuracy may be a bit out of date. Some searching around shows modern American missiles with a CEP of 100 meters or better. I'm not quite sure how to translate a CEP (50% chance of landing in the circle) to a 90% figure, but I wouldn't think it would be a factor of 5.

(I specified American because the Soviets and Russians have historically lagged behind here, and it looks like that's still the case. I don't know what the story is for other ICBM-equipped powers. The Soviets compensated for their lesser accuracy by fielding larger bombs.)

Also keep in mind that the major factor limiting ICBM accuracy these days is the need to be completely self contained, since they're intended to operate in the middle of World War III, and no external navigation sources can be counted on. Slap a JDAM guidance system on the warhead and your ICBM can deliver it to within whatever square meter you're most interested in.

ICBMs armed with conventional bombs have been considered, since accuracy can be enough for that to work now. As far as I know, the only reasons these haven't been deployed are cost (using up an entire ICBM to deliver a chemical bomb seems wasteful) and, probably more importantly, the fact that there's no way to tell whether a given ICBM is carrying a nuclear warhead or not, thus the strong possibility that using such a weapon could trigger a nuclear war by accident. See:

http://en.wikipedia.org/wiki/Prompt_Global_Strike

Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#42

Post by mikeash »

jlgreco wrote: post #40
mikeash wrote: post #28

Earlier quoted context omitted.

Well, how else do you recover a rocket intact? Parachutes have lots of trouble. The rate of descent they achieve is still fairly fast, so you need something to brake the descent at the end. This is why various American capsules splashed into the ocean, as well as the Shuttle boosters, because the water acts as a cushion. However, dunking your delicate rocket in salt water is a bad idea. The American capsules weren't…

More on this, if anybody else is interested: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

Neat, I didn't realize it had a name, let alone a Wikipedia entry. Thanks!

Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#43

Post by InclinedPlane »

mikeash wrote: post #41
InclinedPlane wrote: post #33

Earlier quoted context omitted.

ICBM warheads have about a 90% chance of landing within a circle with a radius of about half a kilometer, at least according to declassified info, the actual accuracy is likely higher. Even so, that hardly compares to a precision landing on a small landing pad.

I think your info on ICBM accuracy may be a bit out of date. Some searching around shows modern American missiles with a CEP of 100 meters or better. I'm not quite sure how to translate a CEP (50% chance of landing in the circle) to a 90% figure, but I wouldn't think it would be a factor of 5. (I specified American because the Soviets and Russians have historically lagged behind here, and it looks like that's still t…

The Minuteman III (the backbone of the US ICBM fleet) has a listed CEP of 200 m. The Trident D5 has a CEP of as low as 90 m, but it's an SLBM not strictly an ICBM, and it varies depending on range.

The CEP figure represents the radius of a circle where there is a 50% chance of the bomb landing within that circle. The CEP figure is basically 2/3 of a standard deviation.

Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#44

Post by mikeash »

InclinedPlane wrote: post #43
mikeash wrote: post #41

Earlier quoted context omitted.

I think your info on ICBM accuracy may be a bit out of date. Some searching around shows modern American missiles with a CEP of 100 meters or better. I'm not quite sure how to translate a CEP (50% chance of landing in the circle) to a 90% figure, but I wouldn't think it would be a factor of 5. (I specified American because the Soviets and Russians have historically lagged behind here, and it looks like that's still t…

The Minuteman III (the backbone of the US ICBM fleet) has a listed CEP of 200 m. The Trident D5 has a CEP of as low as 90 m, but it's an SLBM not strictly an ICBM, and it varies depending on range. The CEP figure represents the radius of a circle where there is a 50% chance of the bomb landing within that circle. The CEP figure is basically 2/3 of a standard deviation.

I suppose I should have said "some". Anyway, when looking for the state of the art, we should look at the best value available, rather than the average or typical. The Peacekeeper missile lists a CEP of 120m, although it's now retired. As for Trident, with a 11,000km range, I think we can put it in the same category when looking at reentry accuracy.

In any case, accuracy is gated by navigational accuracy, not so much guidance, as long as your initial aim isn't too bad. ICBMs have a 100m CEP because they're using inertial navigation all the way. Switch to GPS and you'll have no trouble doing better. In the context of SpaceX, getting the rocket to within a few hundred meters of the landing pad is perfectly fine initially, because it can then maneuver to the pad more precisely.

Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#45

Post by sehugg »

karpathy wrote: post #36
mikeash wrote: post #28

Earlier quoted context omitted.

Well, how else do you recover a rocket intact? Parachutes have lots of trouble. The rate of descent they achieve is still fairly fast, so you need something to brake the descent at the end. This is why various American capsules splashed into the ocean, as well as the Shuttle boosters, because the water acts as a cushion. However, dunking your delicate rocket in salt water is a bad idea. The American capsules weren't…

This is great write up, thanks for the explanation. I've studied physics in undergrad but I still made the mistake you describe in your last paragraph. The Grasshopper intuitively feels dynamically like an inverted pendulum but even harder because all you have are rockets at the bottom and you can adjust their rotation (I assume?). On the other hand, there is no fixed pivot point around which you get torque, is this…

Here's a good article on Lunar Module stability, which is a similar problem with the exception that the Grasshopper has a much longer longitudinal axis, and relies solely on gimbaling (thrust vectoring) with no separate attitude control thrusters.

http://www.clavius.org/techlmstab.html

Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#46

Post by jccooper »

jccooper wrote: post #37
adrianpike wrote: post #27

Earlier quoted context omitted.

How much fuel does it take to control a descent from the altitude they separate the stage? I mean, obviously it's worth it, but I'm curious just how much of a game changer this is.

No one quite knows, as no such system exists yet, though I imagine SpaceX has some internal numbers. You need enough to slow your mostly-empty (and thus fairly light) stage from its terminal velocity to 0, and however many seconds margin you need to be comfortable. Let's do some very rough numbers to get a ballpark... Say terminal velocity is 100 m/s, which is fairly fast, but a rocket is a skinny streamlined thing.…

Oh, and I should also point out that Armadillo Aerospace originally considered rocket-only recovery. I don't know if they posted any numbers on the necessary fuel reserves; I did look briefly but didn't see anything, and don't remember any specific numbers. May or may not be any published.

They have lately been working on parachute-only recovery, apparently having decided that the mass/complexity penalty of parachutes was better than that of retro-fire fuel. They've lost several vehicles already to minor parachute problems. Parachutes aren't easy.

SpaceX, working at a different scale (though not a different flight regime, at least for the first stage) still thinks retro-rocket landing to be the answer. I imagine an Armadillo-size rocket is less likely to have its various parts crushed at a parachute-speed descent than an F9 stage. Armadillo does occasionally get a STIG back in one piece, and parachute-into-water F9 stages are known to be completely trashed. Dry weight would seem to make a difference here.

Re: SpaceX's Grasshopper Successfully Flew 325 Metres

#47

Post by spiek »

mikeash wrote: post #28
spiek wrote: post #22

Could anyone explain why this is a good way to fly a spacecraft/what they are aiming for with this? It seems somewhat inelegant, considering the challenges having to do with stabilizing a vertical rocket.

Well, how else do you recover a rocket intact? Parachutes have lots of trouble. The rate of descent they achieve is still fairly fast, so you need something to brake the descent at the end. This is why various American capsules splashed into the ocean, as well as the Shuttle boosters, because the water acts as a cushion. However, dunking your delicate rocket in salt water is a bad idea. The American capsules weren't…

This is such a great and thorough explanation. Thank you!
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