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The problem with single-stage-to-orbit (SSTO) is mainly weight. I think using composite tanks is enough to make a SSTO rocket. X-33 was supposed to use a compos

by nawitus 13y ago

The problem with single-stage-to-orbit (SSTO) is mainly weight. I think using composite tanks is enough to make a SSTO rocket. X-33 was supposed to use a composite tank, but the fuel tank failed during testing in 1999. Recently, NASA has succesfully tested a composite tank[1]. 1. http://nasawatch.com/archives/2013/07/composite-cryog.html http://nasawatch.com/archives/2013/07/composite-cryog.html

lobster_johnson 13y ago
I was confused by this video [1], which looks like it shows three stages. All three stages are able to land on their own. Can you explain? [1] http://www.youtube.com/watch?v=abLC1l3loFA http://www.youtube.com/watch?v=abLC1l3loFA
- jlgreco 13y ago
  The Dragon capsule is more "payload" than "stage". The Falcon 9 will have two stages (hopefully both eventually landing themselves). The Dragon (not always used. Sometimes they may just stick a satellite or something on top) will also be able to land itself. Sort of like how the Saturn V is considered a 3-stage rocket, even though during the later Apollo missions it had on top a service module, a command module, and a (two stage!) lunar module.
  - lobster_johnson 13y ago
    So what modules does the video show? Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all? Why not just make the rocket capable of flying to the ISS, let the whole thing dock to the ISS, exchange astronauts, then detach and land back on Earth? (Of course, if there are tons of missions happening around the same time, all requiring a first stage rocket, and each mission has a period that doesn't require any rocket, then I see the point of letting the rocket go back and be used for other things.)
    - nawitus 13y ago
      >Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all? Because that requires a single-stage-to-orbit vehicle, which has never been done before. Yes, in the future that might be possible.
    - jlgreco 13y ago
      The video shows the lower stage and upper stage of a Falcon 9 taking off, then landing. These two stages are shown launching a Dragon spacecraft. The Dragon spacecraft is shown docking with the ISS. The pressurized half of the Dragon spacecraft is then shown landing (the unpressurized trunk section, with the solar panels attached, burns up in the atmosphere (though this is not shown in the video)). Staging is done because not doing it ("SSTO" or "single stage to orbit") is extremely hard. In fact, it has never been done before from earth. When that second stage burns, it is only accelerating the mass of itself and the Dragon spacecraft on top. Having it accelerate the mass of the lower stage as well would be massively inefficient, requiring much more fuel to be used. So much more fuel that there most likely would no be any mass left over for the Dragon spacecraft. Without staging we wouldn't be able to put things in orbit at all (today and in the past anyway), let alone reuse rockets.
      - lobster_johnson 13y ago
        Thanks. I assumed the section with the solar panels somehow retracted the panels before re-entering the atmosphere, but they skip that part, and I now see that the part re-entering is actually a bit smaller.
        jlgreco 13y ago
        Yup, that part burns up. It's not a complete waste though. I believe at the ISS they usually pack those sorts of parts (parts that burn up that is. Other examples being the Russian "Progress" spacecraft or the ESA's ATV) full of garbage before sending them off. Intact reentry volume/mass is valuable (currently only the Soyuz and the Dragon can do that) so they just let all of their trash burn up. Here's a neat picture of the first ATV, carrying a bunch of ISS trash, burning up: http://en.wikipedia.org/wiki/File:Jules_Verne_Automated_Transfer_Vehicle_re-enters_Earth%27s_atmosphere.jpg http://en.wikipedia.org/wiki/File:Jules_Verne_Automated_Tran...
pixie_ 13y ago
I feel like a lot of good technology was lost with the cancellation of the X-33/VentureStar. They were very close, and a few test failures caused the entire program to be shut down. It's really a shame.
- nkoren 13y ago
  Well... even more bad technology was lost with the cancellation of the X-33. It was a horrifically over-complicated design which had everything including the kitchen sink thrown into it, strictly to maximise its potential for congressional pork. I'm serious. I was actually at a Lockheed-Martin briefing prior to the X-33 competition, wherein they presented a slide which looked like this: TECHNICAL FEATURES * Linear aerospike engine [first ever tried] * Conformal composite LH2 tank [first ever tried] * Integrated advanced metallic thermal protection [first ever tried] * Subcontractors in 38 states and 122 congressional districts The level of complexity in the Venturestar design meant that it could never have achieved the lightweight mass fractions needed for an SSTO. If there is ever to be an SSTO, it will be simple and elegant. But "simple" and "infinitely divisible into a large number of congressional districts" are mutually incompatible, which is why the US government chose the Lockheed Martin design over the vastly more realistic Macdonald Douglas DC-Y: http://www.astronautix.com/lvs/dcy.htm http://www.astronautix.com/lvs/dcy.htm
  - thom 13y ago
    It's a shame that they decided to run back this same approach for the F-35.
  - Gravityloss 13y ago
    A leap too far. You could test and mature those technologies further independently (in the lab, at test stands, flying tests with existing vehicles) at much lower cost before trying to integrate them into a design that costs billions.
Gravityloss 13y ago
You don't need composite tanks for SSTO. There have been normal rocket stages that have had high enough mass ratio and specific impulse big enough for SSTO, like in Titan, but they weren't used as SSTO. One of the secrets is dense propellants which increases mass ratio considerably. It also makes higher thrust to weight ratio engines easier to implement (because pumps primarily pump volume rather than mass), and with identical initial acceleration, the dense propellant higher mass ratio lower impulse vehicle gets lighter faster and thus has higher acceleration towards the end of the flight, meaning less gravity losses. (Also air drag is less with a smaller vehicle, but it's not a big effect, though it is proportionally bigger for smaller vehicles.) Hydrogen has the best specific impulse but it is non-dense. It is also hard cryogenic, meaning potential complexity in the vehicle, ground infrastructure and operations. Also, big vehicles might be more expensive, even if their contents (the propellants) don't weigh much.

by nawitus 13y ago

lobster_johnson 13y ago

I was confused by this video [1], which looks like it shows three stages. All three stages are able to land on their own. Can you explain? [1] http://www.youtube.com/watch?v=abLC1l3loFA http://www.youtube.com/watch?v=abLC1l3loFA

jlgreco 13y ago
The Dragon capsule is more "payload" than "stage". The Falcon 9 will have two stages (hopefully both eventually landing themselves). The Dragon (not always used. Sometimes they may just stick a satellite or something on top) will also be able to land itself. Sort of like how the Saturn V is considered a 3-stage rocket, even though during the later Apollo missions it had on top a service module, a command module, and a (two stage!) lunar module.
- lobster_johnson 13y ago
  So what modules does the video show? Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all? Why not just make the rocket capable of flying to the ISS, let the whole thing dock to the ISS, exchange astronauts, then detach and land back on Earth? (Of course, if there are tons of missions happening around the same time, all requiring a first stage rocket, and each mission has a period that doesn't require any rocket, then I see the point of letting the rocket go back and be used for other things.)
  - nawitus 13y ago
    >Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all? Because that requires a single-stage-to-orbit vehicle, which has never been done before. Yes, in the future that might be possible.
  - jlgreco 13y ago
    The video shows the lower stage and upper stage of a Falcon 9 taking off, then landing. These two stages are shown launching a Dragon spacecraft. The Dragon spacecraft is shown docking with the ISS. The pressurized half of the Dragon spacecraft is then shown landing (the unpressurized trunk section, with the solar panels attached, burns up in the atmosphere (though this is not shown in the video)). Staging is done because not doing it ("SSTO" or "single stage to orbit") is extremely hard. In fact, it has never been done before from earth. When that second stage burns, it is only accelerating the mass of itself and the Dragon spacecraft on top. Having it accelerate the mass of the lower stage as well would be massively inefficient, requiring much more fuel to be used. So much more fuel that there most likely would no be any mass left over for the Dragon spacecraft. Without staging we wouldn't be able to put things in orbit at all (today and in the past anyway), let alone reuse rockets.
    - lobster_johnson 13y ago
      Thanks. I assumed the section with the solar panels somehow retracted the panels before re-entering the atmosphere, but they skip that part, and I now see that the part re-entering is actually a bit smaller.
      - jlgreco 13y ago
        Yup, that part burns up. It's not a complete waste though. I believe at the ISS they usually pack those sorts of parts (parts that burn up that is. Other examples being the Russian "Progress" spacecraft or the ESA's ATV) full of garbage before sending them off. Intact reentry volume/mass is valuable (currently only the Soyuz and the Dragon can do that) so they just let all of their trash burn up. Here's a neat picture of the first ATV, carrying a bunch of ISS trash, burning up: http://en.wikipedia.org/wiki/File:Jules_Verne_Automated_Transfer_Vehicle_re-enters_Earth%27s_atmosphere.jpg http://en.wikipedia.org/wiki/File:Jules_Verne_Automated_Tran...

pixie_ 13y ago

I feel like a lot of good technology was lost with the cancellation of the X-33/VentureStar. They were very close, and a few test failures caused the entire program to be shut down. It's really a shame.

nkoren 13y ago
Well... even more bad technology was lost with the cancellation of the X-33. It was a horrifically over-complicated design which had everything including the kitchen sink thrown into it, strictly to maximise its potential for congressional pork. I'm serious. I was actually at a Lockheed-Martin briefing prior to the X-33 competition, wherein they presented a slide which looked like this: TECHNICAL FEATURES * Linear aerospike engine [first ever tried] * Conformal composite LH2 tank [first ever tried] * Integrated advanced metallic thermal protection [first ever tried] * Subcontractors in 38 states and 122 congressional districts The level of complexity in the Venturestar design meant that it could never have achieved the lightweight mass fractions needed for an SSTO. If there is ever to be an SSTO, it will be simple and elegant. But "simple" and "infinitely divisible into a large number of congressional districts" are mutually incompatible, which is why the US government chose the Lockheed Martin design over the vastly more realistic Macdonald Douglas DC-Y: http://www.astronautix.com/lvs/dcy.htm http://www.astronautix.com/lvs/dcy.htm
- thom 13y ago
  It's a shame that they decided to run back this same approach for the F-35.
- Gravityloss 13y ago
  A leap too far. You could test and mature those technologies further independently (in the lab, at test stands, flying tests with existing vehicles) at much lower cost before trying to integrate them into a design that costs billions.

Gravityloss 13y ago

You don't need composite tanks for SSTO. There have been normal rocket stages that have had high enough mass ratio and specific impulse big enough for SSTO, like in Titan, but they weren't used as SSTO. One of the secrets is dense propellants which increases mass ratio considerably. It also makes higher thrust to weight ratio engines easier to implement (because pumps primarily pump volume rather than mass), and with identical initial acceleration, the dense propellant higher mass ratio lower impulse vehicle gets lighter faster and thus has higher acceleration towards the end of the flight, meaning less gravity losses. (Also air drag is less with a smaller vehicle, but it's not a big effect, though it is proportionally bigger for smaller vehicles.) Hydrogen has the best specific impulse but it is non-dense. It is also hard cryogenic, meaning potential complexity in the vehicle, ground infrastructure and operations. Also, big vehicles might be more expensive, even if their contents (the propellants) don't weigh much.