An article at Technology Review about Elon’s plans for next year.
13 thoughts on “Reusable Rockets”
I read that Elon has given up on 2nd stage reusability. I wonder if it’s because of price or because of total payload to LEO. Because even a 2mT RLV would be extremely useful for launching propellant, which is what SpaceX is going to have to do anyway if they want to go to Mars.
I think he’s given up on it for Falcon 9. Not sure about upcoming vehicles (e.g., heavy) with more performance to spare.
I think he said he’s given up on resuability for FH too, and only the MCT will be fully reusable, eventually. He also mentioned it’s very hard to get something back from GTO. I’m not sure that’s true, but LEO seems far more important for commercial development of space and easier too.
Musk says he’s not planning on full reuse until after the Falcons:
The next generation vehicles after the Falcon architecture will be designed for full reusability. I don’t expect the Falcon 9 to have a reusable upper stage, just because the – with a kerosene-based system, the specific impulse isn’t really high enough to do that, and a lot of the missions we do for commercial satellite deployment are geostationary missions. So, we’re really going very far out. These are high delta-velocity missions, so to try to get something back from that is really difficult. But, with the next generation of vehicles, which is going to be a sub-cooled methane/oxygen system where the propellants are cooled close to their freezing temperature to increase the density, we could definitely do full reusability – and that system is intended to be a fully reusable Mars transportation system. So, not merely to low Earth orbit but all the way to Mars and back, with full reusability. [Within 3 years?] Ha. I am an optimistic person, but – I think we could expect to see some test flights in the five or six year time frame. But, we’re talking about a much bigger vehicle, and we’re also going to be upgrading to a new generation – a harder engine cycle, which is a full-flow staged combustion.
He’s exaggerating the problem a bit. There are plenty of two stage (Falcon 9 is one) and two stage and a half (Soyuz 2 is one) vehicles using LOX/Kerosene and I doubt going LOX/Methane will make the number of stages go down to less than two. The main issue is it is much harder to design a LOX/Kerosene staged combustion engine. Most of the cases where the Russians use 3 stages for their LOX/Kerosene rockets is because the launch base is too far from the equator to begin with.
I have thought for a long time the solution has to pass by techniques like staged-combustion and some sort of altitude compensating nozzle be it TAN or whatever. Then there is the problem of making a light, cheap, and reusable heatshield for the second stage. If that is still not enough to get performance you have to consider putting some of the energy outside the aircraft with power beaming e.g. ALP or whatever.
The conversation you are talking about seems rather weird because AFAIK the temperature interval where methane and oxygen are in liquid form isn’t that large to begin with and I have always heard about storing them at the same temperature in order to have a common bulkhead to save on launch mass. Quite benign in fact. Unlike LOX/Slush H2 and things like that which are great in theory but in practice are good at clogging up pipes.
They might be planning to use a mixture of methane and other hydrocarbons. A eutectic mixture of methane and ethane, for example, remains liquid at 77K.
When did he say he gave up on these things?
I think at MIT a few days ago.
“Musk, in Friday’s interview, indicated that SpaceX wasn’t pursuing a reusable second sage for the Falcon 9. “I don’t expect the Falcon 9 to have a reusable upper stage,” he said. “With a kerosene-based system, the specific impulse isn’t really high enough to do that.” He added that it would be difficult to recover the stage on missions to deploy satellites in geostationary orbit.
However, he indicated a future generation of launch vehicles will be designed to reuse all its stages. “The next generation vehicles after the Falcon architecture will be designed for full reusability,” he said. Those vehicles will use “densified methalox” propulsion, liquid methane and oxygen cooled to near their freezing points, which will provide additional performance.
Those vehicles, though, are still well into the future. “I think we could start to see some test flights in the five- to six-year timeframe,” he said. ”
* How long does it take to move a 300’x170′ floating platform from Louisiana to a position off Cape Canaveral?
* Does the Falcon 9 first stage return to a predetermined GPS coordinate, or does it use some sort of terminal guidance (e.g. radar, computer vision, manual)?
* Has SpaceX figured out how to keep the first stage camera from icing over?
Here’s hoping for calm seas on CRS-5 launch day.
Recovering an upper stage is difficult but some cases are worse than others. Recovering from LEO is challenging but I think possible. Recovery from GTO is an interesting thought experiment. I think the best way would be to do a retroburn at apogee to lower the perigee into the atmosphere. That shouldn’t take more than a hundred meters per second or so of delta-v. The problem is surviving a very high velocity reentry. Without running any numbers, I’d guess the stage would be going well over 20,000 MPH at atmospheric entry. Perhaps they could bleed off the speed with multiple passes by keeping the perigee high enough to keep from immediately burning in but low enough to shed some serious velocity with each pass. To do that, the upper stage attitude control system, TT&C system, etc. would need to function for days or even weeks. Maybe some disposable solar cells on the rocket body could do the job.
Reading between the lines I think that Elon is planning for an SSTO.
Evidence:
* working on a higher ISP engine
* building really big (SSTO works better at scale)
* abandoning second stage reusability work, while saying full reusability is the next thing
It is Also possible that a Falcon Heavy type configuration ( 1.5 stages ) is the next thing.
The one items that is yet to be demonstrated would be can a full sized first stage be designed to survive reentry heating from orbital speeds.
I don’t believe that a reusable SSTO is possible with chemical propulsion, from the Earth. SSTO is, but with very, very small payload/GLOW ratio. On the other hand, a 1.5 stage rocket where the engines are jettisoned (the Atlas was a prime example) could be made fully reusable without too much difficulty. It takes designing it in from the beginning, however.
I read that Elon has given up on 2nd stage reusability. I wonder if it’s because of price or because of total payload to LEO. Because even a 2mT RLV would be extremely useful for launching propellant, which is what SpaceX is going to have to do anyway if they want to go to Mars.
I think he’s given up on it for Falcon 9. Not sure about upcoming vehicles (e.g., heavy) with more performance to spare.
I think he said he’s given up on resuability for FH too, and only the MCT will be fully reusable, eventually. He also mentioned it’s very hard to get something back from GTO. I’m not sure that’s true, but LEO seems far more important for commercial development of space and easier too.
Musk says he’s not planning on full reuse until after the Falcons:
He’s exaggerating the problem a bit. There are plenty of two stage (Falcon 9 is one) and two stage and a half (Soyuz 2 is one) vehicles using LOX/Kerosene and I doubt going LOX/Methane will make the number of stages go down to less than two. The main issue is it is much harder to design a LOX/Kerosene staged combustion engine. Most of the cases where the Russians use 3 stages for their LOX/Kerosene rockets is because the launch base is too far from the equator to begin with.
I have thought for a long time the solution has to pass by techniques like staged-combustion and some sort of altitude compensating nozzle be it TAN or whatever. Then there is the problem of making a light, cheap, and reusable heatshield for the second stage. If that is still not enough to get performance you have to consider putting some of the energy outside the aircraft with power beaming e.g. ALP or whatever.
The conversation you are talking about seems rather weird because AFAIK the temperature interval where methane and oxygen are in liquid form isn’t that large to begin with and I have always heard about storing them at the same temperature in order to have a common bulkhead to save on launch mass. Quite benign in fact. Unlike LOX/Slush H2 and things like that which are great in theory but in practice are good at clogging up pipes.
They might be planning to use a mixture of methane and other hydrocarbons. A eutectic mixture of methane and ethane, for example, remains liquid at 77K.
When did he say he gave up on these things?
I think at MIT a few days ago.
“Musk, in Friday’s interview, indicated that SpaceX wasn’t pursuing a reusable second sage for the Falcon 9. “I don’t expect the Falcon 9 to have a reusable upper stage,” he said. “With a kerosene-based system, the specific impulse isn’t really high enough to do that.” He added that it would be difficult to recover the stage on missions to deploy satellites in geostationary orbit.
However, he indicated a future generation of launch vehicles will be designed to reuse all its stages. “The next generation vehicles after the Falcon architecture will be designed for full reusability,” he said. Those vehicles will use “densified methalox” propulsion, liquid methane and oxygen cooled to near their freezing points, which will provide additional performance.
Those vehicles, though, are still well into the future. “I think we could start to see some test flights in the five- to six-year timeframe,” he said. ”
http://www.thespacereview.com/article/2628/1
Random things I’m curious about:
* How long does it take to move a 300’x170′ floating platform from Louisiana to a position off Cape Canaveral?
* Does the Falcon 9 first stage return to a predetermined GPS coordinate, or does it use some sort of terminal guidance (e.g. radar, computer vision, manual)?
* Has SpaceX figured out how to keep the first stage camera from icing over?
Here’s hoping for calm seas on CRS-5 launch day.
Recovering an upper stage is difficult but some cases are worse than others. Recovering from LEO is challenging but I think possible. Recovery from GTO is an interesting thought experiment. I think the best way would be to do a retroburn at apogee to lower the perigee into the atmosphere. That shouldn’t take more than a hundred meters per second or so of delta-v. The problem is surviving a very high velocity reentry. Without running any numbers, I’d guess the stage would be going well over 20,000 MPH at atmospheric entry. Perhaps they could bleed off the speed with multiple passes by keeping the perigee high enough to keep from immediately burning in but low enough to shed some serious velocity with each pass. To do that, the upper stage attitude control system, TT&C system, etc. would need to function for days or even weeks. Maybe some disposable solar cells on the rocket body could do the job.
Reading between the lines I think that Elon is planning for an SSTO.
Evidence:
* working on a higher ISP engine
* building really big (SSTO works better at scale)
* abandoning second stage reusability work, while saying full reusability is the next thing
It is Also possible that a Falcon Heavy type configuration ( 1.5 stages ) is the next thing.
The one items that is yet to be demonstrated would be can a full sized first stage be designed to survive reentry heating from orbital speeds.
I don’t believe that a reusable SSTO is possible with chemical propulsion, from the Earth. SSTO is, but with very, very small payload/GLOW ratio. On the other hand, a 1.5 stage rocket where the engines are jettisoned (the Atlas was a prime example) could be made fully reusable without too much difficulty. It takes designing it in from the beginning, however.