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The abstract brings up SSTOs, but has there been anything in recent invention that will make them anything other than the white whale people have been chasing s

The abstract brings up SSTOs, but has there been anything in recent invention that will make them anything other than the white whale people have been chasing since forever?

PaulHoule 1y ago
The 1990s were a lost decade for reusable space flight because instead of chasing incremental improvements to the Space Shuttle (an orbiter with reusable tiles that could be turned around in days, not months) or something like the Falcoln 9 or the fly-back version of Saturn V that O'Neill's students drew in 1979, it was all about SSTO. SSTO is just marginally possible, if it is possible you need exotic materials and engines and you're never going to get a good payload fraction and adding wings, horizontal takeoff, horizontal landing and such just makes it worse. The one good thing about it is that you get closer to "aircraft-like operations" because in principle you can inspect it, refill it, and relaunch it -- whereas something like the STS or Falcoln 9 or Starship will require stacking up multiple parts for each launch. My guess is aerospikes are making a comeback though because of interest in hypersonic weapons system. I could also see them being useful for the second stage of something like Starship which mostly operates at high altitudes but has to land at low altitudes. There are a lot of other technical problems, like the thermal management system, which really have to be solved before worrying about that optimization.
- cubefox 1y ago
  Currently the Starship upper stage simply has two different sets of bell nozzles: Three engines with nozzles for atmospheric pressure, and three for vacuum. I wonder how inefficient this really is compared to having just aerospike nozzles.
  - psunavy03 1y ago
    That's the same as the genesis of the question I asked above. SSTOs are a concept, but given their complete lack of market share, I assume as a non-aerospace engineer that there are valid reasons smart people have not been able to design a competitive one yet. Similarly, I assume there are valid reasons SpaceX has chosen not to use aerospike Raptors, especially given their well-earned reputation for innovating things everyone else swore couldn't be done. If even they haven't been able to make it work, that's a strong data point as to the state of the art.
    - PaulHoule 1y ago
      I'd argue that the brilliance of SpaceX is the opposite. They stick to technology and markets that are proven and use technically conservative approaches. Falcon 9 is about relentless improvement in small ways, not bold new ideas -- unless you count not getting caught up in the politics and psychology of bold new ideas as a bold new idea. Sure, they talk about Mars, and in-space refueling seems radical, but they've yet to succeed at doing anything radical... yet. Rumor has it they were struggling with the payload fraction w/ the first generation of Starship and they switched to a second generation that struggles with blowing up. A big advantage of the two-stage architecture is that you can develop the two stages independently. Presumably they will eventually get Starship to orbit and bring it home, they will have plenty of time to improve it get the payload fraction up just as they did with F9.
      - dcminter 1y ago
        Landing and re-using their Falcon first stages was pretty radical though.
        wongarsu 1y ago
        As is landing rockets on the launch tower (or as SpaceX would say: catching them). And I might be wrong, but I believe they are the first to use a crane on the launch tower to stack the rocket. Usually you do that before you roll it out to the pad. They were also the first to fly a full-flow staged-combustion engine. Maybe that one was less radical because prototypes have been around for 60 years, but SpaceX were still the first to actually fly one
        WJW 1y ago
        I don't think that's true, at least it wasn't conceptually radical. People have noticed the cost of "throwing away" the lower stages for ages, and many approaches have been thought of how not to do that. Take the (partly) renewable SSRBs of the space shuttle program for example, which came down by parachute. Landing a rocket on its tail is also quite an old idea. NASA had several demonstrators demonstrating the concept in flight. SpaceX took a lot of ideas which had been individually proven before, and then put in the work to perfect them and integrate them in a production ready spacecraft. That is important work and good engineering, but not radical. An aerospike had literally never been flown to orbit at that time (I think still not), so it would have been a way worse fit for the SpaceX method of developing the Falcon 9.
- margalabargala 1y ago
  > SSTO is just marginally possible, if it is possible Looking at the specs it would appear the first stage of a Falcon 9 plus a nosecone could get itself to orbit with no cargo. Barely.
d_silin 1y ago
There has been some progress on scramjet propulsion.
- bryanlarsen 1y ago
  This. In my very uninformed opinion the only way we'll get useful SSTO is if we can get a meaningful amount of oxygen from the atmosphere rather than carrying it up in heavy tanks. The failure of Reaction Engines with their SABRE engine is disappointing on this front.
  - PaulHoule 1y ago
    Aren't rockets more powerful (as in energy/time) than rocket engines in that they are getting compressed/liquified oxygen out of a tank as opposed to taking the comparably tiny amount that passes into the intake of an engine?
    - goku12 1y ago
      There are two performance parameters for a rocket/jet engine. The first is thrust and the second is specific impulse. You are thinking about thrust. The others in the tread are talking about specific impulse. Thrust is important for some stages, especially the early stage booster engines (as opposed to later stage sustainer engines). As a simple example, any space rocket will need a first stage with an enormous thrust so that it can lift itself, the subsequent stages and the payload off the launch pad. Additionally, the rocket has to finish its initial vertical climb as fast as possible. Otherwise the propellants will be wasted in just lifting off (this is called gravity loss). That will also require a high initial thrust. However, the requirement of the high thrust disappears once you finish the vertical climb. There's no danger of falling back to ground once you reach orbit. What you need at this stage instead, is to add velocity (deltav) to the craft to change its orbit/trajectory. This can be done even at very low thrust, because you have all the time you need. The limiting factor now is that you have only a finite amount of propellant onboard. You want to add as much deltav as possible before it runs out. A high thrust doesn't help because the engine will simply consume the propellant faster and exhaust it before you get the required deltav. This is where specific impulse comes into play. The maximum deltav you can get is proportional to the specific impulse of the engine (see rocket equation for details). As you can imagine, high specific impulse is critical for space missions requiring high deltav, like the New Horizons spacecraft that imaged Pluto or the Parker solar probe (interestingly, getting to the sun is harder than escaping the solar system). Rockets/jets with low thrust and high specific impulse are called sustainers. The general trend seen is that specific impulse drops off as thrust increases. For example, the space shuttle solid booster has Tmax = 15 MN, Isp = 268s, and space shuttle orbiter cryogenic engine RS25 has Tmax = 2.28 MN, Isp = 452s. Meanwhile, the NEXT xenon ion thruster used in the DART mission has Tmax = 236 mN and Isp = 4200s. Note that the thrust has changed from Mega newtons to milli newtons. You would hardly recognize it if the ion engine thrusted against your body.
ordinaryradical 1y ago
Source: worked at a startup that took over the patents for the X-33 next gen shuttle and VentureStar SSTO (aerospike design!) The Columbia disaster really set back SSTO appetite. Probably the whole reason we got the patents, truly. SSTOs are, like everything else going to orbit, delimited by weight. If you are going to make the fuel tanks internal to the vehicle and not something that falls off and sheds their weight mid-flight, you have to get vehicle weight to the absolute minimum. Losing weight has second order effects because it means you now have to carry less fuel so you now have a smaller fuel tank which means the tank weighs less which means you get to carry less fuel… etc. The key, IMO, is material science advancements, specifically around plastics and composites. Very efficient engine design is matters too, but if you can just bring less mass up with you you can start to approach an achievable fuel weight. It’s a hard job, you need plastics that can handle orbital temperature cycling (+300 to -300 F every 30 mins), atomic oxygen (nasty corrosion), UV with no atmospheric protection, FST for crew exposure… Exotic metal alloys can get you around some of these problems, but they can be difficult and expensive to work with. Same issue with high-performance polymers. No free lunches here. With 3D printing of metals and high-performance composites, you can probably remove additional weight so there’s some light in that tunnel. But all in all it’s very hard to get out of the gravity well with your fuel in tow and survive the extremes of space. My belief is the first vehicle to pull it off will look like a Swiss cheese of voids and lattices from printing / honeycombs and be made almost entirely out of plastic and carbon fiber.
- deleted 1y ago
  [deleted]
pfdietz 1y ago
Why make an SSTO when you can make a TSTO? First stage recovery is a solved problem and will always greatly relax the engineering problems over making a SSTO.
- avmich 1y ago
  Because of course SSTO has benefits over TSTO, simplicity of operation being one example.
  - pfdietz 1y ago
    The argument for SSTOs was that staging was too scary. But experience since then shows this argument was bogus. Staging can be made highly reliable. Even a small amount of delta V provided by a first stage makes the job of the "almost SSTO" second stage much easier. And a low delta V first stage can be rugged, with high high safety factors, and is easy to recover at the launch site. Put another way: if you have an SSTO, its payload increases dramatically if you stack it on a very low performance recoverable first stage. I don't see any way SSTOs are going to be preferable to TSTOs, especially if the SSTO has to use hydrogen to get off the ground.
    - avmich 1y ago
      Everybody may have their own criteria of preferences, so the same solution with fail for one and succeed for another.
      - pfdietz 1y ago
        Sure, you can cook up whatever BS criteria you want, but when it hits the market SSTO doesn't make sense.
        avmich 1y ago
        You're so sure you proved yourself right, huh?

The abstract brings up SSTOs, but has there been anything in recent invention that will make them anything other than the white whale people have been chasing since forever?

PaulHoule 1y ago

The 1990s were a lost decade for reusable space flight because instead of chasing incremental improvements to the Space Shuttle (an orbiter with reusable tiles that could be turned around in days, not months) or something like the Falcoln 9 or the fly-back version of Saturn V that O'Neill's students drew in 1979, it was all about SSTO. SSTO is just marginally possible, if it is possible you need exotic materials and engines and you're never going to get a good payload fraction and adding wings, horizontal takeoff, horizontal landing and such just makes it worse. The one good thing about it is that you get closer to "aircraft-like operations" because in principle you can inspect it, refill it, and relaunch it -- whereas something like the STS or Falcoln 9 or Starship will require stacking up multiple parts for each launch. My guess is aerospikes are making a comeback though because of interest in hypersonic weapons system. I could also see them being useful for the second stage of something like Starship which mostly operates at high altitudes but has to land at low altitudes. There are a lot of other technical problems, like the thermal management system, which really have to be solved before worrying about that optimization.

cubefox 1y ago
Currently the Starship upper stage simply has two different sets of bell nozzles: Three engines with nozzles for atmospheric pressure, and three for vacuum. I wonder how inefficient this really is compared to having just aerospike nozzles.
- psunavy03 1y ago
  That's the same as the genesis of the question I asked above. SSTOs are a concept, but given their complete lack of market share, I assume as a non-aerospace engineer that there are valid reasons smart people have not been able to design a competitive one yet. Similarly, I assume there are valid reasons SpaceX has chosen not to use aerospike Raptors, especially given their well-earned reputation for innovating things everyone else swore couldn't be done. If even they haven't been able to make it work, that's a strong data point as to the state of the art.
  - PaulHoule 1y ago
    I'd argue that the brilliance of SpaceX is the opposite. They stick to technology and markets that are proven and use technically conservative approaches. Falcon 9 is about relentless improvement in small ways, not bold new ideas -- unless you count not getting caught up in the politics and psychology of bold new ideas as a bold new idea. Sure, they talk about Mars, and in-space refueling seems radical, but they've yet to succeed at doing anything radical... yet. Rumor has it they were struggling with the payload fraction w/ the first generation of Starship and they switched to a second generation that struggles with blowing up. A big advantage of the two-stage architecture is that you can develop the two stages independently. Presumably they will eventually get Starship to orbit and bring it home, they will have plenty of time to improve it get the payload fraction up just as they did with F9.
    - dcminter 1y ago
      Landing and re-using their Falcon first stages was pretty radical though.
      - wongarsu 1y ago
        As is landing rockets on the launch tower (or as SpaceX would say: catching them). And I might be wrong, but I believe they are the first to use a crane on the launch tower to stack the rocket. Usually you do that before you roll it out to the pad. They were also the first to fly a full-flow staged-combustion engine. Maybe that one was less radical because prototypes have been around for 60 years, but SpaceX were still the first to actually fly one
      - WJW 1y ago
        I don't think that's true, at least it wasn't conceptually radical. People have noticed the cost of "throwing away" the lower stages for ages, and many approaches have been thought of how not to do that. Take the (partly) renewable SSRBs of the space shuttle program for example, which came down by parachute. Landing a rocket on its tail is also quite an old idea. NASA had several demonstrators demonstrating the concept in flight. SpaceX took a lot of ideas which had been individually proven before, and then put in the work to perfect them and integrate them in a production ready spacecraft. That is important work and good engineering, but not radical. An aerospike had literally never been flown to orbit at that time (I think still not), so it would have been a way worse fit for the SpaceX method of developing the Falcon 9.
margalabargala 1y ago
> SSTO is just marginally possible, if it is possible Looking at the specs it would appear the first stage of a Falcon 9 plus a nosecone could get itself to orbit with no cargo. Barely.

d_silin 1y ago

There has been some progress on scramjet propulsion.

bryanlarsen 1y ago
This. In my very uninformed opinion the only way we'll get useful SSTO is if we can get a meaningful amount of oxygen from the atmosphere rather than carrying it up in heavy tanks. The failure of Reaction Engines with their SABRE engine is disappointing on this front.
- PaulHoule 1y ago
  Aren't rockets more powerful (as in energy/time) than rocket engines in that they are getting compressed/liquified oxygen out of a tank as opposed to taking the comparably tiny amount that passes into the intake of an engine?
  - goku12 1y ago
    There are two performance parameters for a rocket/jet engine. The first is thrust and the second is specific impulse. You are thinking about thrust. The others in the tread are talking about specific impulse. Thrust is important for some stages, especially the early stage booster engines (as opposed to later stage sustainer engines). As a simple example, any space rocket will need a first stage with an enormous thrust so that it can lift itself, the subsequent stages and the payload off the launch pad. Additionally, the rocket has to finish its initial vertical climb as fast as possible. Otherwise the propellants will be wasted in just lifting off (this is called gravity loss). That will also require a high initial thrust. However, the requirement of the high thrust disappears once you finish the vertical climb. There's no danger of falling back to ground once you reach orbit. What you need at this stage instead, is to add velocity (deltav) to the craft to change its orbit/trajectory. This can be done even at very low thrust, because you have all the time you need. The limiting factor now is that you have only a finite amount of propellant onboard. You want to add as much deltav as possible before it runs out. A high thrust doesn't help because the engine will simply consume the propellant faster and exhaust it before you get the required deltav. This is where specific impulse comes into play. The maximum deltav you can get is proportional to the specific impulse of the engine (see rocket equation for details). As you can imagine, high specific impulse is critical for space missions requiring high deltav, like the New Horizons spacecraft that imaged Pluto or the Parker solar probe (interestingly, getting to the sun is harder than escaping the solar system). Rockets/jets with low thrust and high specific impulse are called sustainers. The general trend seen is that specific impulse drops off as thrust increases. For example, the space shuttle solid booster has Tmax = 15 MN, Isp = 268s, and space shuttle orbiter cryogenic engine RS25 has Tmax = 2.28 MN, Isp = 452s. Meanwhile, the NEXT xenon ion thruster used in the DART mission has Tmax = 236 mN and Isp = 4200s. Note that the thrust has changed from Mega newtons to milli newtons. You would hardly recognize it if the ion engine thrusted against your body.

ordinaryradical 1y ago

Source: worked at a startup that took over the patents for the X-33 next gen shuttle and VentureStar SSTO (aerospike design!) The Columbia disaster really set back SSTO appetite. Probably the whole reason we got the patents, truly. SSTOs are, like everything else going to orbit, delimited by weight. If you are going to make the fuel tanks internal to the vehicle and not something that falls off and sheds their weight mid-flight, you have to get vehicle weight to the absolute minimum. Losing weight has second order effects because it means you now have to carry less fuel so you now have a smaller fuel tank which means the tank weighs less which means you get to carry less fuel… etc. The key, IMO, is material science advancements, specifically around plastics and composites. Very efficient engine design is matters too, but if you can just bring less mass up with you you can start to approach an achievable fuel weight. It’s a hard job, you need plastics that can handle orbital temperature cycling (+300 to -300 F every 30 mins), atomic oxygen (nasty corrosion), UV with no atmospheric protection, FST for crew exposure… Exotic metal alloys can get you around some of these problems, but they can be difficult and expensive to work with. Same issue with high-performance polymers. No free lunches here. With 3D printing of metals and high-performance composites, you can probably remove additional weight so there’s some light in that tunnel. But all in all it’s very hard to get out of the gravity well with your fuel in tow and survive the extremes of space. My belief is the first vehicle to pull it off will look like a Swiss cheese of voids and lattices from printing / honeycombs and be made almost entirely out of plastic and carbon fiber.

deleted 1y ago
[deleted]

pfdietz 1y ago

Why make an SSTO when you can make a TSTO? First stage recovery is a solved problem and will always greatly relax the engineering problems over making a SSTO.

avmich 1y ago
Because of course SSTO has benefits over TSTO, simplicity of operation being one example.
- pfdietz 1y ago
  The argument for SSTOs was that staging was too scary. But experience since then shows this argument was bogus. Staging can be made highly reliable. Even a small amount of delta V provided by a first stage makes the job of the "almost SSTO" second stage much easier. And a low delta V first stage can be rugged, with high high safety factors, and is easy to recover at the launch site. Put another way: if you have an SSTO, its payload increases dramatically if you stack it on a very low performance recoverable first stage. I don't see any way SSTOs are going to be preferable to TSTOs, especially if the SSTO has to use hydrogen to get off the ground.
  - avmich 1y ago
    Everybody may have their own criteria of preferences, so the same solution with fail for one and succeed for another.
    - pfdietz 1y ago
      Sure, you can cook up whatever BS criteria you want, but when it hits the market SSTO doesn't make sense.
      - avmich 1y ago
        You're so sure you proved yourself right, huh?