Why is it such a big deal for SpaceX to land its used booster rocket on a floating platform rather than just having the booster parachute down into the ocean and then be retrieved?
Salt water is VERY UNKIND to precision metal machinery like rocket engines. Also the tank has such thin walls that chaotic wave action will destroy it.
Hm, this isn’t intuitive to me. How could a rocket that was designed to withstand the pressures and conditions of space not be able to take some salt water? And what about just adding a layer of coating that would protect it?
When it’s closed and pressurized you have a very hard time crushing it. The internal pressure is converted to a force of tension that resists deformation. Once it’s been opened, you can crush it with one hand from the side. But it’s much stronger along the axis of the cylinder, since the force is directed through all the material rather than deforming it inwards.
A rocket if scaled down to the size of a coke can has walls much thinner than a coke can, and is much longer relative to its width. You can create great torques by hitting the sides to bend it, or crush it inwards. Imagine the force of tens of tons of water suddenly slapping onto the side of this tank as waves lap around, unevenly across multiple parts of the tank.
Consider a rocket.
It must, with the least possible amount of mass, generate a high acceleration along its direction of motion while subtending a very small surface area in that direction of motion. This dictates that it is long and thin, and able to withstand high forces along that long axis. But every kilogram you add to its mass is one kilogram you can’t get to orbit, or a couple more kilograms of fuel. You make it withstand the forces of its operational environment well. Other forces not so much. As far as I know most rockets get a good deal of their shear-strength perpendicular to the long axis from pressure within their tanks—some cannot even be held sideways without being pressurized. I’ve seen figures to the effect that more than half of the structural strength of a falcon 9 tank comes from it being pressurized by inert gases during operation.
Optimizing for its operational environment plus mass puts limits on how much you can optimize for other things. There is no one abstract ‘durability’ factor.
Salt water does not play nice with metals, especially fine tubes and components that will be channeling ridiculous energies and god knows how many RPM and very fine precision. It corrodes, it dries up and gums joints, etc. Shuttle SRBs were a different story—they were big, dumb, thick, wide metal tubes filled with fuel that burned like a sparkler, not fine actuators of active control. If someone could find a simple coating that could protect complex powerful machines from the damage of salt water without tradeoffs, I’m sure the navy would LOVE to know about it.
So I (now) understand that rockets are designed to be thin and light to reduce drag and gravitational forces. As far as the cost-benefit of adding a protective layer, the cost is the added drag/gravitational forces, but the benefit seems to be huge (able to land it in water). Is it really that hard to generate the necessary propulsion forces?
Obviously the answer is “yes”, but that goes against what makes intuitive sense to me. Can you explain?
I think part of the problem is a fundamental misunderstanding of what parachuting into the ocean does to a rocket motor. The motors are the expensive part of the first stage; I don’t know exact numbers, but they are the complicated, intricate, extremely-high-precision parts that must be exactly right or everything goes boom. The tank, by comparison, is an aluminum can.
The last landing attempt failed because a rocket motor’s throttle valve had a bit more static friction than it should, and stuck open a moment too long. SpaceX’s third launch attempt—the last failed launch they’ve had, many years ago with the Falcon 1 - was because the motor didn’t shut off instantly before stage separation, like it should have. As far as I know, people still don’t know why Orbital ATK (FKA Orbital Science)’s last launch attempt failed, except that it was obviously an engine failure. We talk about rocket science, but honestly the theoretical aspects of rocketry aren’t that complicated. Rocket engineering, though, that’s a bloody nightmare. You get everything as close to perfect as you can, and sometimes it still fails catastrophically and blows away more value than most of the people reading this thread will earn in their lifetimes, leaving virtually nothing to tell the tale of what happened.
What does all that have to do with parachute recovery of booster stages? Well, once you’ve dunked those motors in saltwater, they’re a write-off. They can’t be trusted to ever again operate perfectly without fairly literally rebuilding them, which defeats most of the purpose of recovering the booster.
There’s nothing you could coat a rocket motor with that would both survive that motor operating and make it economical to re-use the motor after plunging into the ocean. The closest thing I can think of would be some kind of protective bubble that expands to protect the motors from the ocean once their job is done. It would need to be watertight, impact-resistant (the rocket still hits the water pretty hard, even with parachutes), able to deploy around the motors reliably, avoid causing a bending moment that collapses the tank (which has minimal pressure, because its fuel is depleted and any excess pressurizing agent you carry is wasted weight to orbit), and able to operate after being exposed to the environment in close proximity to a medium-lift rocket’s primary launch motors. Maybe it’s possible, but I can’t think of how to do reliably enough to be worth the added cost on launch.
Each additional kilogram of the rocket is probably extremely expensive. I don’t how much weight would that extra protective layer add, but I can imagine it could more than double the weight of the rocket, and the weight of the fuel it needs, etc.
Look at the Tsiolkovsky rocket equation—a rocket’s delta-v (velocity change potential) is proportional to the log of its mass ratio (its mass with fuel divided by its mass without fuel). For modern rockets, that means about twenty kilos of fuel for every kilo of anything else (the rocket included). You really don’t want to add structural mass if there’s any way to avoid it.
Just guessing: In space the pressure comes from inside, in water from outside; it is easier to protect in one direction than in both. In space the pressure difference is at most one atmosphere, in water it depends on how deep you submerge. The salt water also attacks the metal chemically.
It’s a step toward landing it back at the launch site for rapid reuse.
The project’s long-term objectives include returning a launch vehicle first stage to the launch site in minutes and to return a second stage to the launch pad following orbital realignment with the launch site and atmospheric reentry in up to 24 hours. Both stages will be designed to allow reuse a few hours after return.
1) Why is it important that the rocket that just landed take off again so soon? I’ve always had the impression that space missions aren’t too frequent.
2) Does transporting the rocket from the ocean back to the launch site cost a lot of money? Is avoiding this a big benefit of the reusable launch system?
Why is it important that the rocket that just landed take off again so soon?
While that’s worded in customer benefit, I think the actual reason is supply-side: hovering is costly, and so landing the stages as cheaply as possible implies doing it quickly.
I’ve always had the impression that space missions aren’t too frequent.
This may be because they are so expensive; if reusable rockets decrease the launch costs significantly enough, there may be many more launches.
The Space Shuttle did something like this, the rocket boosters were landed in the ocean with parachutes and reused. I found a PDF from NASA which describes the procedure. They disassembled the entire thing into parts, inspected each part for damage, and then restored and reused the parts as appropriate. By contrast, I think what SpaceX is aiming for is more like an airplane, you just fill the tank with new fuel and launch it again.
(The PDF claims that the refurbishment program is cost effective, but word of mouth has it that if you factor in the cost of retrieving the boosters, the whole thing cost more than just manufacturing new ones from scratch. See also this thread in the KSP forum.)
You’re also talking about fundamentally different kinds of rocket boosters. The Space Shuttle used solid fuel boosters, which are basically nothing except a tube packed full of energetically burning material, an igniter to light said material, and a nozzle for the generated gases to come out. They couldn’t throttle, couldn’t gimbal, couldn’t shut off or restart, didn’t use cryogenic fuel so didn’t need insulation, didn’t rely on pressurized fuel so they didn’t need turbopumps… In fact, as far as I know they basically didn’t have any moving parts at all!
You ever flown a model rocket, like an Estes? That little tube of solid grey gritty stuff that you use to launch the rocket is basically a miniature version of the solid fuel boosters on the Space Shuttle. The shuttle boosters were obviously bigger, and were a lot tougher (which made them unacceptably heavy for something like the Falcon 9′s first stage) so they could survive the water landing, but fundamentally they were basically just cylindrical metal tubes with a nozzle at the bottom.
Despite that, reconditioning them for re-use was still so expensive that it’s unclear if the cost was worth it. Now, of course, they cost a lot less to build than a Falcon 9 first stage, but every one of the Falcon 9 first stage’s nine Merlin 1D engines is many times as complicated as the entire solid booster used on the Space Shuttle. Even the first stage tank is much more complicated, since it needs to take cryogenic fuels and massive internal pressurization.
This isn’t all that relevant, but the Shuttle SRBs were gimbaled (Wikipedia, NASA 1, NASA 2).
(I was thinking that there is probably at least a mechanical component to arming the ignition and/or range safety systems, but research turned up this big obvious part.)
Whoops, you’re right. I thought the gimbaling was just on the SSMEs (attached to the orbiter) but in retrospect it’s obvious that the SRBs had to have some control of their flight path. I’m now actually rather curious about the range safety stuff for the SRBs—one of the dangers of an SRB is that there’s basically no way to shut it down, and indeed they kept going for some time after Challenger blew up—but the gimbaling is indeed an obvious sign that I should have checked my memory/assumptions. Thanks.
I’m now actually rather curious about the range safety stuff for the SRBs—one of the dangers of an SRB is that there’s basically no way to shut it down, and indeed they kept going for some time after Challenger blew up
What I’ve heard (no research) is that thrust termination for a solid rocket works by charges opening the top end, so that the exhaust exits from both ends and the thrust mostly cancels itself out, or perhaps by splitting along the length of the side (destroying all integrity). In any case, the fuel still burns, but you can stop it from accelerating further.
Hm. A solid rocket burns from one end, opening up the nose will do nothing to the thrust. Splitting a side, I would guess, will lead to uncontrolled acceleration with chaotic flight path, but not zero acceleration.
Apparently that’s true of some model rocket motors, but the SRBs have a hollow through the entire length of the propellant, so that it burns from the center out to the casing along the entire length at the same time.
That exposes the maximum surface area for combustion, I guess (the surface area actually increases as the propellant is burned, interestingly) so blowing the top would work, yeah.
Why is it such a big deal for SpaceX to land its used booster rocket on a floating platform rather than just having the booster parachute down into the ocean and then be retrieved?
Salt water is VERY UNKIND to precision metal machinery like rocket engines. Also the tank has such thin walls that chaotic wave action will destroy it.
Hm, this isn’t intuitive to me. How could a rocket that was designed to withstand the pressures and conditions of space not be able to take some salt water? And what about just adding a layer of coating that would protect it?
Consider a Coke can.
When it’s closed and pressurized you have a very hard time crushing it. The internal pressure is converted to a force of tension that resists deformation. Once it’s been opened, you can crush it with one hand from the side. But it’s much stronger along the axis of the cylinder, since the force is directed through all the material rather than deforming it inwards.
A rocket if scaled down to the size of a coke can has walls much thinner than a coke can, and is much longer relative to its width. You can create great torques by hitting the sides to bend it, or crush it inwards. Imagine the force of tens of tons of water suddenly slapping onto the side of this tank as waves lap around, unevenly across multiple parts of the tank.
Consider a rocket.
It must, with the least possible amount of mass, generate a high acceleration along its direction of motion while subtending a very small surface area in that direction of motion. This dictates that it is long and thin, and able to withstand high forces along that long axis. But every kilogram you add to its mass is one kilogram you can’t get to orbit, or a couple more kilograms of fuel. You make it withstand the forces of its operational environment well. Other forces not so much. As far as I know most rockets get a good deal of their shear-strength perpendicular to the long axis from pressure within their tanks—some cannot even be held sideways without being pressurized. I’ve seen figures to the effect that more than half of the structural strength of a falcon 9 tank comes from it being pressurized by inert gases during operation.
Optimizing for its operational environment plus mass puts limits on how much you can optimize for other things. There is no one abstract ‘durability’ factor.
Salt water does not play nice with metals, especially fine tubes and components that will be channeling ridiculous energies and god knows how many RPM and very fine precision. It corrodes, it dries up and gums joints, etc. Shuttle SRBs were a different story—they were big, dumb, thick, wide metal tubes filled with fuel that burned like a sparkler, not fine actuators of active control. If someone could find a simple coating that could protect complex powerful machines from the damage of salt water without tradeoffs, I’m sure the navy would LOVE to know about it.
Great answer!
I see, that makes much more sense now. Thank you!
So I (now) understand that rockets are designed to be thin and light to reduce drag and gravitational forces. As far as the cost-benefit of adding a protective layer, the cost is the added drag/gravitational forces, but the benefit seems to be huge (able to land it in water). Is it really that hard to generate the necessary propulsion forces?
Obviously the answer is “yes”, but that goes against what makes intuitive sense to me. Can you explain?
I think part of the problem is a fundamental misunderstanding of what parachuting into the ocean does to a rocket motor. The motors are the expensive part of the first stage; I don’t know exact numbers, but they are the complicated, intricate, extremely-high-precision parts that must be exactly right or everything goes boom. The tank, by comparison, is an aluminum can.
The last landing attempt failed because a rocket motor’s throttle valve had a bit more static friction than it should, and stuck open a moment too long. SpaceX’s third launch attempt—the last failed launch they’ve had, many years ago with the Falcon 1 - was because the motor didn’t shut off instantly before stage separation, like it should have. As far as I know, people still don’t know why Orbital ATK (FKA Orbital Science)’s last launch attempt failed, except that it was obviously an engine failure. We talk about rocket science, but honestly the theoretical aspects of rocketry aren’t that complicated. Rocket engineering, though, that’s a bloody nightmare. You get everything as close to perfect as you can, and sometimes it still fails catastrophically and blows away more value than most of the people reading this thread will earn in their lifetimes, leaving virtually nothing to tell the tale of what happened.
What does all that have to do with parachute recovery of booster stages? Well, once you’ve dunked those motors in saltwater, they’re a write-off. They can’t be trusted to ever again operate perfectly without fairly literally rebuilding them, which defeats most of the purpose of recovering the booster.
There’s nothing you could coat a rocket motor with that would both survive that motor operating and make it economical to re-use the motor after plunging into the ocean. The closest thing I can think of would be some kind of protective bubble that expands to protect the motors from the ocean once their job is done. It would need to be watertight, impact-resistant (the rocket still hits the water pretty hard, even with parachutes), able to deploy around the motors reliably, avoid causing a bending moment that collapses the tank (which has minimal pressure, because its fuel is depleted and any excess pressurizing agent you carry is wasted weight to orbit), and able to operate after being exposed to the environment in close proximity to a medium-lift rocket’s primary launch motors. Maybe it’s possible, but I can’t think of how to do reliably enough to be worth the added cost on launch.
Each additional kilogram of the rocket is probably extremely expensive. I don’t how much weight would that extra protective layer add, but I can imagine it could more than double the weight of the rocket, and the weight of the fuel it needs, etc.
Look at the Tsiolkovsky rocket equation—a rocket’s delta-v (velocity change potential) is proportional to the log of its mass ratio (its mass with fuel divided by its mass without fuel). For modern rockets, that means about twenty kilos of fuel for every kilo of anything else (the rocket included). You really don’t want to add structural mass if there’s any way to avoid it.
Just guessing: In space the pressure comes from inside, in water from outside; it is easier to protect in one direction than in both. In space the pressure difference is at most one atmosphere, in water it depends on how deep you submerge. The salt water also attacks the metal chemically.
It’s a step toward landing it back at the launch site for rapid reuse.
— https://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_development_program
1) Why is it important that the rocket that just landed take off again so soon? I’ve always had the impression that space missions aren’t too frequent.
2) Does transporting the rocket from the ocean back to the launch site cost a lot of money? Is avoiding this a big benefit of the reusable launch system?
While that’s worded in customer benefit, I think the actual reason is supply-side: hovering is costly, and so landing the stages as cheaply as possible implies doing it quickly.
This may be because they are so expensive; if reusable rockets decrease the launch costs significantly enough, there may be many more launches.
I don’t think there will be, because sattelites themselves usually are much more expensive than their launch.
The Space Shuttle did something like this, the rocket boosters were landed in the ocean with parachutes and reused. I found a PDF from NASA which describes the procedure. They disassembled the entire thing into parts, inspected each part for damage, and then restored and reused the parts as appropriate. By contrast, I think what SpaceX is aiming for is more like an airplane, you just fill the tank with new fuel and launch it again.
(The PDF claims that the refurbishment program is cost effective, but word of mouth has it that if you factor in the cost of retrieving the boosters, the whole thing cost more than just manufacturing new ones from scratch. See also this thread in the KSP forum.)
You’re also talking about fundamentally different kinds of rocket boosters. The Space Shuttle used solid fuel boosters, which are basically nothing except a tube packed full of energetically burning material, an igniter to light said material, and a nozzle for the generated gases to come out. They couldn’t throttle, couldn’t gimbal, couldn’t shut off or restart, didn’t use cryogenic fuel so didn’t need insulation, didn’t rely on pressurized fuel so they didn’t need turbopumps… In fact, as far as I know they basically didn’t have any moving parts at all!
You ever flown a model rocket, like an Estes? That little tube of solid grey gritty stuff that you use to launch the rocket is basically a miniature version of the solid fuel boosters on the Space Shuttle. The shuttle boosters were obviously bigger, and were a lot tougher (which made them unacceptably heavy for something like the Falcon 9′s first stage) so they could survive the water landing, but fundamentally they were basically just cylindrical metal tubes with a nozzle at the bottom.
Despite that, reconditioning them for re-use was still so expensive that it’s unclear if the cost was worth it. Now, of course, they cost a lot less to build than a Falcon 9 first stage, but every one of the Falcon 9 first stage’s nine Merlin 1D engines is many times as complicated as the entire solid booster used on the Space Shuttle. Even the first stage tank is much more complicated, since it needs to take cryogenic fuels and massive internal pressurization.
This isn’t all that relevant, but the Shuttle SRBs were gimbaled (Wikipedia, NASA 1, NASA 2).
(I was thinking that there is probably at least a mechanical component to arming the ignition and/or range safety systems, but research turned up this big obvious part.)
Whoops, you’re right. I thought the gimbaling was just on the SSMEs (attached to the orbiter) but in retrospect it’s obvious that the SRBs had to have some control of their flight path. I’m now actually rather curious about the range safety stuff for the SRBs—one of the dangers of an SRB is that there’s basically no way to shut it down, and indeed they kept going for some time after Challenger blew up—but the gimbaling is indeed an obvious sign that I should have checked my memory/assumptions. Thanks.
What I’ve heard (no research) is that thrust termination for a solid rocket works by charges opening the top end, so that the exhaust exits from both ends and the thrust mostly cancels itself out, or perhaps by splitting along the length of the side (destroying all integrity). In any case, the fuel still burns, but you can stop it from accelerating further.
Hm. A solid rocket burns from one end, opening up the nose will do nothing to the thrust. Splitting a side, I would guess, will lead to uncontrolled acceleration with chaotic flight path, but not zero acceleration.
Apparently that’s true of some model rocket motors, but the SRBs have a hollow through the entire length of the propellant, so that it burns from the center out to the casing along the entire length at the same time.
That exposes the maximum surface area for combustion, I guess (the surface area actually increases as the propellant is burned, interestingly) so blowing the top would work, yeah.
What fubarobfusco said, and also because the ocean deals a huge amount of damage to complicated machinery, which is very expensive and slow to fix.