6 ms·

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 havi

by spiek 13y ago

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.

Game_Ender 13y ago
It's elegant because you "just" have to strap landing gear and some parachutes to your existing rockets and you can reuse them. You lose some payload capacity, but you get to use your rocket again (which is the expensive part). We have tried engineering more complex re-usable lifting body type designs, only the shuttle ended up flying, and it wasn't cost effective.
- adrianpike 13y ago
  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.
  - jccooper 13y ago
    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. (It could fall slower, maybe even 50 m/s, on its side or with parachutes, but parachutes are heavy and complicated, and falling sideways may be difficult to control, but seems quite possible.) Say an F9 "1.1" first stage weighs 28,000 kg. The landing components mostly are already there: the engines are already gimballed and the additional software weighs nothing. Unless you're so awesome you can land in a cradle--which is not impossible--you need some landing gear, so let's add 3000kg, for a dry weight of 31,000 kg. Amongst all its engines, our F9 has some 300,000kg of thrust. Just one engine is around 33,000kg thrust, so we'll probably only use one or two. Two engines will get us easily about 10m/s slower per second, so we need to retro-fire for minimum 10 seconds. You probably want some margin over that; let's reserve 20 seconds fuel. F9 1.1 stage 1 will have a burn time of 185s; our retro burn is only at about 25% capacity, so 20s of two engines is worth, oh, 5s of normal burn time. Say about 3% of the fuel. Of course, it's not that simple, because (1) we've made the stage heavier with the landing gear, which requires more fuel and (2) to avoid losing performance we'll want to add enough fuel to add our extra burn time, which also requires more fuel, and (3) we need more fuel to lift all that fuel and mass we added. The rocket equation strikes again! (Of course, it may not be that bad, because usually first stages have plenty of margin, and most payloads are not max. Maybe if you encounter a really heavy payload you just agree to throw away the rocket that time; use an old one or something.) Most of my assumptions are pessimistic (I think) but in the real world they may turn out optimistic. I would guess you're looking at 3-5% of the fuel load for a recoverable VTOL stage. A lot depends on details, as with rockets weight scales very badly against performance.
    - jccooper 13y ago
      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.
cryptoz 13y ago
It's extremely difficult to land large payloads on Mars unless you have a controlled rocket descent like SpaceX is building with Grasshopper. Check out the way NASA lands small rovers on Mars and imagine trying to build a colony of 80,000 people like that: not gonna happen.
- spiek 13y ago
  Interesting. It does seem far less crazy than the '7 minutes of terror' from the recent mars rover. Thank you for the explanation.
- rbanffy 13y ago
  If you build your supply landers (and your supplies, of course) capable of withstanding high g's you can probably crash-land large things on airbags. NASA hasn't done it because the rovers are somewhat delicate, but if you are shipping frozen-solid food containers, I believe it would be fine.
mikeash 13y ago
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 reused, and the Shuttle boosters required enough refurbishment afterwards as to make it not really worth recovering them. You can land on land, but then you need some seriously hefty landing gear, or retro-rockets. The Soviets use retro-rockets fired at the last moment to recover their capsules on land. A rocket has problems that a capsule does not, though, namely that you need to keep it from tipping over once it lands! You can make a capsule fat, but a rocket really needs to be tall and thin, and any sort of horizontal velocity will have it falling on its side. You also have very little control over where the rocket comes to a rest. Again, tolerable for a capsule, but dropping your rocket into a forest is not going to be conducive to reusability. You can add wings, like the Shuttle did, and land horizontally. Trouble with that is that wings are extremely heavy. Each pound of wing in the Shuttle was a pound of payload it couldn't carry. If you try to fit every stage of a rocket with wings so it can come back and land, you'll end up not having much excess capacity left over to actually carry a payload. Wings are great for aircraft, when you can use them to efficiently fight gravity for the whole flight. But they suck for spacecraft, where they don't help at all on the way up, and are just dead weight for most of the flight. So landing a rocket vertically on its engines isn't really all that bad in the end. Inelegant, perhaps, but it has a lot of advantages. You get absolute control over where the rocket lands. You can land it as softly as you like. You can ensure that it touches down with no horizontal velocity so as not to tip over. You use the same engines that you used for the launch, so there's no extra weight there. The only real downsides are the need for some sort of landing gear, and the need to carry a bit of extra fuel. Neither one is all that bad. When considering the stability, don't make the common mistake of comparing it to balancing a pencil. When the rocket turns, the motors turn as well, so it is not inherently unstable the way you might think. Rather, the stability is neutral: if you perturb the rocket, it will just stay in its new position, not returning to how it was before, but neither does it run farther away. A rocket that's slightly tilted will stay slightly tilted (gradually accelerating sideways, of course), it won't tilt over further the way a pencil does. Note that Robert Goddard made this same error. He built his first rocket with the engines at the top, believing this would help with stability. So if you were thinking that way, don't feel bad: the father of modern rocketry did the same thing!
InclinedPlane 13y ago
Almost all of the cost of orbital launch is tied up in manufacturing cost and operational complexity. Fuel cost is about $200k for each Falcon 9. The most important factor in reusability is making sure that the vehicle gets back to someplace convenient without any significant damage. If you crash-land using parachutes somewhere in the ocean or in a desert, that's not going to happen. So you need a controlled landing at a prepared site. The overhead necessary to return to the original landing site is generally more than made up by the fact that you get the rocket back exactly where you need it within a matter of hours and without any damage other than the normal wear and tear caused by every flight.

by spiek 13y ago

Game_Ender 13y ago

It's elegant because you "just" have to strap landing gear and some parachutes to your existing rockets and you can reuse them. You lose some payload capacity, but you get to use your rocket again (which is the expensive part). We have tried engineering more complex re-usable lifting body type designs, only the shuttle ended up flying, and it wasn't cost effective.

adrianpike 13y ago
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.
- jccooper 13y ago
  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. (It could fall slower, maybe even 50 m/s, on its side or with parachutes, but parachutes are heavy and complicated, and falling sideways may be difficult to control, but seems quite possible.) Say an F9 "1.1" first stage weighs 28,000 kg. The landing components mostly are already there: the engines are already gimballed and the additional software weighs nothing. Unless you're so awesome you can land in a cradle--which is not impossible--you need some landing gear, so let's add 3000kg, for a dry weight of 31,000 kg. Amongst all its engines, our F9 has some 300,000kg of thrust. Just one engine is around 33,000kg thrust, so we'll probably only use one or two. Two engines will get us easily about 10m/s slower per second, so we need to retro-fire for minimum 10 seconds. You probably want some margin over that; let's reserve 20 seconds fuel. F9 1.1 stage 1 will have a burn time of 185s; our retro burn is only at about 25% capacity, so 20s of two engines is worth, oh, 5s of normal burn time. Say about 3% of the fuel. Of course, it's not that simple, because (1) we've made the stage heavier with the landing gear, which requires more fuel and (2) to avoid losing performance we'll want to add enough fuel to add our extra burn time, which also requires more fuel, and (3) we need more fuel to lift all that fuel and mass we added. The rocket equation strikes again! (Of course, it may not be that bad, because usually first stages have plenty of margin, and most payloads are not max. Maybe if you encounter a really heavy payload you just agree to throw away the rocket that time; use an old one or something.) Most of my assumptions are pessimistic (I think) but in the real world they may turn out optimistic. I would guess you're looking at 3-5% of the fuel load for a recoverable VTOL stage. A lot depends on details, as with rockets weight scales very badly against performance.
  - jccooper 13y ago
    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.

cryptoz 13y ago

It's extremely difficult to land large payloads on Mars unless you have a controlled rocket descent like SpaceX is building with Grasshopper. Check out the way NASA lands small rovers on Mars and imagine trying to build a colony of 80,000 people like that: not gonna happen.

spiek 13y ago
Interesting. It does seem far less crazy than the '7 minutes of terror' from the recent mars rover. Thank you for the explanation.
rbanffy 13y ago
If you build your supply landers (and your supplies, of course) capable of withstanding high g's you can probably crash-land large things on airbags. NASA hasn't done it because the rovers are somewhat delicate, but if you are shipping frozen-solid food containers, I believe it would be fine.

mikeash 13y ago

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 reused, and the Shuttle boosters required enough refurbishment afterwards as to make it not really worth recovering them. You can land on land, but then you need some seriously hefty landing gear, or retro-rockets. The Soviets use retro-rockets fired at the last moment to recover their capsules on land. A rocket has problems that a capsule does not, though, namely that you need to keep it from tipping over once it lands! You can make a capsule fat, but a rocket really needs to be tall and thin, and any sort of horizontal velocity will have it falling on its side. You also have very little control over where the rocket comes to a rest. Again, tolerable for a capsule, but dropping your rocket into a forest is not going to be conducive to reusability. You can add wings, like the Shuttle did, and land horizontally. Trouble with that is that wings are extremely heavy. Each pound of wing in the Shuttle was a pound of payload it couldn't carry. If you try to fit every stage of a rocket with wings so it can come back and land, you'll end up not having much excess capacity left over to actually carry a payload. Wings are great for aircraft, when you can use them to efficiently fight gravity for the whole flight. But they suck for spacecraft, where they don't help at all on the way up, and are just dead weight for most of the flight. So landing a rocket vertically on its engines isn't really all that bad in the end. Inelegant, perhaps, but it has a lot of advantages. You get absolute control over where the rocket lands. You can land it as softly as you like. You can ensure that it touches down with no horizontal velocity so as not to tip over. You use the same engines that you used for the launch, so there's no extra weight there. The only real downsides are the need for some sort of landing gear, and the need to carry a bit of extra fuel. Neither one is all that bad. When considering the stability, don't make the common mistake of comparing it to balancing a pencil. When the rocket turns, the motors turn as well, so it is not inherently unstable the way you might think. Rather, the stability is neutral: if you perturb the rocket, it will just stay in its new position, not returning to how it was before, but neither does it run farther away. A rocket that's slightly tilted will stay slightly tilted (gradually accelerating sideways, of course), it won't tilt over further the way a pencil does. Note that Robert Goddard made this same error. He built his first rocket with the engines at the top, believing this would help with stability. So if you were thinking that way, don't feel bad: the father of modern rocketry did the same thing!

InclinedPlane 13y ago

Almost all of the cost of orbital launch is tied up in manufacturing cost and operational complexity. Fuel cost is about $200k for each Falcon 9. The most important factor in reusability is making sure that the vehicle gets back to someplace convenient without any significant damage. If you crash-land using parachutes somewhere in the ocean or in a desert, that's not going to happen. So you need a controlled landing at a prepared site. The overhead necessary to return to the original landing site is generally more than made up by the fact that you get the rocket back exactly where you need it within a matter of hours and without any damage other than the normal wear and tear caused by every flight.