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> The aerospike engine isn't actually a new thing. Rocketdyne had been experimenting with them since the 1960s, and they're much more efficient than conventiona

> The aerospike engine isn't actually a new thing. Rocketdyne had been experimenting with them since the 1960s, and they're much more efficient than conventional rocket engines. ... kind of. Like they say, conventional bell-nozzle engines are optimized for a particular altitude, while aerospikes are not, and it's true that if you're going to use the same engine across a wide range of altitudes, aerospikes will be more efficient on average across that wide range than conventional engines will. At any particular altitude, though, a conventional engine optimized for that altitude will be more efficient than an aerospike -- they're "jack of all trades, master of none" engines. And it turns out, with contemporary multi-stage rockets, we can and do use different engine bells at different altitudes, which negates a lot of aerospikes' prospective benefits. Were single-stage-to-orbit vehicles ever to be a thing, aerospikes might have their moment, but it turns out SSTO configurations just aren't very good at lifting much mass to orbit, so that seems unlikely.

rbanffy 6y ago
SSTOs have the advantage of enormously simplified operation - no staging, no vehicle assembly building, no stage mating, just land, fix whatever is broken, refuel and launch. If the payload fraction is smaller than that of a multistage rocket, it can compensate with cheaper logistics on the ground and less time between launches.
- pfdietz 6y ago
  Making those things easier is a lot simpler than making SSTO work.
  - rbanffy 6y ago
    We thought that reusing the shuttle would be easy and that relanding boosters would be impossible and, yet, we were wrong both times. Some things are impossible until they aren't.
    - erispoe 6y ago
      No these are fundamentally different. SSTOs make things expensive because the rocket equation is unforgiving: every kilogram you haul into orbit needs fuel and propellant, which in turn makes the rocket bigger and heavier, which in turns requires more fuel and propellant, which in turns... So, you don't want to bring up anything unnecessary, like taking your first stage into orbit SSTO style. Landing boosters is an incredible engineering feat. The rocket equation is just derived from the laws of physics, it's not something you can change with great engineering.
      - evgen 6y ago
        If the rocket equation is such an unforgiving bitch then why are you only using two stages to get to orbit? By your own logic you should be using four or five or even ten stages to get to orbit because otherwise you are hauling up too much unnecessary mass on each flight. What makes two such a magic number in this case? Is it perhaps possible that there are in fact real-world tradeoffs between efficiencies in the rocket equation and engineering parameters and that sometimes advances in technique or materials shifts the optimum point?
        erispoe 6y ago
        The more stages, the less favorable your ratio fuel+propellant / dry mass. When you add stages, you add engines, tanks, systems... that you then have to haul up to the next stage, making your lower stages heavier. Rocket equation again. Sure it's possible that if you had super lightweight material for all the dry mass, 3 stages would make more sense. You're right, there is a "real-world tradeoffs between efficiencies in the rocket equation and engineering parameters". This trade-off is unfavorable to SSTOs.
        hindsightbias 6y ago
        It typically takes two to get the third stage up there, and only the third stage stays in orbit. Alternative shuttle designs, BFR and New Glenn start at two but you wouldn't want to carry around all that extra weight on a second stage unless you had a very specific mission set (launch the ISS in a couple of missions or have a stage that goes and lands on Mars). Not optimized for typical payloads.
  - vikramkr 6y ago
    Technically, making those things easier also makes SSTOs work since optimized multi stage orbits let us get to the moon where SSTOs are the default option to get off of the lunar surface. In a sense SSTOs are part of the current earth to moonbase to Mars strategy.
    - rbanffy 6y ago
      SSTO on the Moon or Mars is a lot easier.
      - sitkack 6y ago
        And make a ton of sense since there be zero to none surface support for relaunching.
        rbanffy 6y ago
        You'd probably still need to carry propellants, but yes, an Earth-capable SSTO should be able to hop around a lot on the Moon before needing to refuel. Land-anywhere and launch-anywhere are also very attractive features for places with sparse support. I wonder if SpaceX could build a demonstration landing pad, ferry some liquid methane and liquid oxygen to, say, Guam (which is kind of part of the US, making things a bit easier) and do a suborbital hop across the globe with the starship alone without the booster, refuel, and fly back to Florida.
        pfdietz 6y ago
        Or, a second stage of a two stage earth-to LEO launcher. There's no reason to make the earth-to-orbit part single stage, just because reuse in space of that upper stage would be SSTO.
      - vikramkr 6y ago
        And also already accomplished on the moon. The Apollo lunar module was SSTO during takeoff from the lunar surface.
- erispoe 6y ago
  SSTOs trade-off is very unfavorable, which is why no one is seriously considering one anymore, for Earth's gravity well. In order to avoid solving the relative simple task of efficient stage mating and assembly, an engineering challenge you have to solve once per launch system and can amortize over the whole life of the system, you end up paying the cost of hauling into orbit a huge amount of unnecessary mass, every single launch. That's a very expensive trade-off.
  - evgen 6y ago
    To take the fanboy's pride as an example, how much time is spent taking a Falcon first stage from landing to re-launch? Days? Weeks? Months? If you are expecting to use your launch infrastructure the same way we use a large commercial airliner then this isn't good enough. An SSTO could launch, orbit, return, refuel, and re-launch in the same day. For high-value cargo where the mass it not too much (c.f. humans) this might actually make sense.
    - erispoe 6y ago
      It take 2 to 3 days to reach the ISS from the ground. Getting to orbit is only 8min of that. The rest is a slow approach of the station. If you want to deliver cargo or people to a specific orbit and a specific place, say the ISS, you're not gonna be able to turnover a SSTO in a day. Even a week might not be enough for a quick return trip to and from the ISS. All this time your expensive ground to orbit capacity is idle. A first stage can launch a second stage into orbit and return to launch site in less than 15min. How fast you can turn it around for relaunch is an engineering challenge, but there's no fundamental reason you couldn't do it in a day or even in an hour.
      - mandevil 6y ago
        Note that two day approaches are not a real requirement, it's a choice that is made for operational flexibility. On Gemini 11 all the way back in 1966 NASA demonstrated docking a mere 94 minutes after launch (essentially one orbit). However, the launch window for that flight was two seconds, which is ... not ideal. By planning for a several day approach you give yourself a several hour launch window, which is much more workable. There is another reason for the leisurely approach for manned spacecraft. Studies have shown that something like 75% of spacefarers suffer from Space Acclimatization Sickness, generally lasting 1-3 days before your body gets used to zero-gee and you are fine. A leisurely approach lets an astronaut deal with that phase NOT in the shared space station that spends decades in outer space, but in the capsule which will be coming back down in a few months. SAS is a major limitation on space tourism, incidentally. You'll notice that there are very few plans between 15 minute suborbital hops and two weeks in space: that's because if even trained, physically fit astronauts take 1-3 days to adapt to space, no one is sure what a person who is not as trained will do, and if the word of mouth is "I went into space for three days and felt sick the entire time" that's not a good customer experience.
mumblemumble 6y ago
At least as a layperson, I really appreciated Everyday Astronaut's explanation for why aerospikes never caught on, and perhaps never will: https://youtu.be/D4SaofKCYwo https://youtu.be/D4SaofKCYwo It digs in much deeper than the usual offhanded, easy explanations like the, "We're stuck with bell nozzles because they were invented first," that this article offers.
- jjoonathan 6y ago
  Agreed! It's also worth highlighting the bit where he asks Elon Musk: https://youtu.be/cIQ36Kt7UVg?t=382 https://youtu.be/cIQ36Kt7UVg?t=382 I've internally asked this question so many times, like, guys, shouldn't we maybe do an aerospike? Musk focused on the high combustion efficiency of traditional combustion chambers, combined with the high efficiency of having two stages, as the reason why they won out. Jettisoning irrelevant fuel-containing mass as you go is great for efficiency and as a bonus it makes it easy to put sea-level-optimized nozzles on the bottom stage and vacuum-optimized nozzles on the top stage.
sandworm101 6y ago
The issue isn't really single stage to orbit but rather single stage to vacuum. Every first stage rocket starts at sea level but ends flight in essentially vacuum pressure (over 30km). So there might one day be a role for aerospikes on stages that don't continue all the way to orbit.
- Gibbon1 6y ago
  Every time I tried running numbers the impression I got was raw performance for the first stage isn't really that important. And the cost vs size is aprox fixedcost + log(size). You can either increase ISP or make the rocket bigger. Increasing the throw of the first stage by 10% might only cost 3% more.
  - pfdietz 6y ago
    Indeed. The first stage drops off almost immediately, so it pays little to try to optimize it. For example, it makes little sense to use LH2 in a first stage; hydrocarbons are much denser and cheaper.
nradov 6y ago
The most viable proposal for an SSTO launcher with significant payload appears to be the Reaction Engines Skylon. It doesn't use an aerospike. https://www.reactionengines.co.uk/ https://www.reactionengines.co.uk/
- pfdietz 6y ago
  I would not call it viable. The penalty imposed by using air breathing is considerable. Trade off studies of launchers with air breathing components almost invariably optimize to 100% rocket.
jsmcgd 6y ago
> SSTO configurations just aren't very good at lifting much mass to orbit, so that seems unlikely. Depends what you mean. Performance wise they are not as efficient as multi stage expendable rockets, however they offer possibly the best operational efficiency of rocket system. The venture star has a launch capability of 20 tons to LEO. Larger lifters like Boeing's proposed 'Big Onion' had a lift capability of 227 tons to LEO. Despite its large size, its fully reusable nature and simple design would have meant its operation would have been considerably cheaper than anything that has flown to date. I suggest that it would be prudent to test at least one SSTO before the industry writes off the technology. Especially if we had one already 90% complete!
- nradov 6y ago
  The VentureStar has a launch capability of 0 tons to LEO because the fuel tank was too heavy.
- valuearb 6y ago
  No need. Anything a SSTO can do, staging can do better. Staging will always produce higher payloads to orbit.
jandrese 6y ago
Ultimately SSTOs just don't work well on Earth given our currently available propellants. If we lived on Mars the equation would be different, but Earth's gravity well is just too deep for the engines we can make today. It's not just about finding a new more energy dense but lighter weight propellant either, you need previously nonexistent alloys that can survive the intense heat, pressure, abrasive, and corrosive effects of a running rocket chamber. One of the big challenges with aerospike engines is keeping them from melting. It's a difficult challenge for bell shaped rocket engines too, but aerospikes are more heavily impacted.

rbanffy 6y ago

SSTOs have the advantage of enormously simplified operation - no staging, no vehicle assembly building, no stage mating, just land, fix whatever is broken, refuel and launch. If the payload fraction is smaller than that of a multistage rocket, it can compensate with cheaper logistics on the ground and less time between launches.

pfdietz 6y ago
Making those things easier is a lot simpler than making SSTO work.
- rbanffy 6y ago
  We thought that reusing the shuttle would be easy and that relanding boosters would be impossible and, yet, we were wrong both times. Some things are impossible until they aren't.
  - erispoe 6y ago
    No these are fundamentally different. SSTOs make things expensive because the rocket equation is unforgiving: every kilogram you haul into orbit needs fuel and propellant, which in turn makes the rocket bigger and heavier, which in turns requires more fuel and propellant, which in turns... So, you don't want to bring up anything unnecessary, like taking your first stage into orbit SSTO style. Landing boosters is an incredible engineering feat. The rocket equation is just derived from the laws of physics, it's not something you can change with great engineering.
    - evgen 6y ago
      If the rocket equation is such an unforgiving bitch then why are you only using two stages to get to orbit? By your own logic you should be using four or five or even ten stages to get to orbit because otherwise you are hauling up too much unnecessary mass on each flight. What makes two such a magic number in this case? Is it perhaps possible that there are in fact real-world tradeoffs between efficiencies in the rocket equation and engineering parameters and that sometimes advances in technique or materials shifts the optimum point?
      - erispoe 6y ago
        The more stages, the less favorable your ratio fuel+propellant / dry mass. When you add stages, you add engines, tanks, systems... that you then have to haul up to the next stage, making your lower stages heavier. Rocket equation again. Sure it's possible that if you had super lightweight material for all the dry mass, 3 stages would make more sense. You're right, there is a "real-world tradeoffs between efficiencies in the rocket equation and engineering parameters". This trade-off is unfavorable to SSTOs.
      - hindsightbias 6y ago
        It typically takes two to get the third stage up there, and only the third stage stays in orbit. Alternative shuttle designs, BFR and New Glenn start at two but you wouldn't want to carry around all that extra weight on a second stage unless you had a very specific mission set (launch the ISS in a couple of missions or have a stage that goes and lands on Mars). Not optimized for typical payloads.
- vikramkr 6y ago
  Technically, making those things easier also makes SSTOs work since optimized multi stage orbits let us get to the moon where SSTOs are the default option to get off of the lunar surface. In a sense SSTOs are part of the current earth to moonbase to Mars strategy.
  - rbanffy 6y ago
    SSTO on the Moon or Mars is a lot easier.
    - sitkack 6y ago
      And make a ton of sense since there be zero to none surface support for relaunching.
      - rbanffy 6y ago
        You'd probably still need to carry propellants, but yes, an Earth-capable SSTO should be able to hop around a lot on the Moon before needing to refuel. Land-anywhere and launch-anywhere are also very attractive features for places with sparse support. I wonder if SpaceX could build a demonstration landing pad, ferry some liquid methane and liquid oxygen to, say, Guam (which is kind of part of the US, making things a bit easier) and do a suborbital hop across the globe with the starship alone without the booster, refuel, and fly back to Florida.
        pfdietz 6y ago
        Or, a second stage of a two stage earth-to LEO launcher. There's no reason to make the earth-to-orbit part single stage, just because reuse in space of that upper stage would be SSTO.
    - vikramkr 6y ago
      And also already accomplished on the moon. The Apollo lunar module was SSTO during takeoff from the lunar surface.
erispoe 6y ago
SSTOs trade-off is very unfavorable, which is why no one is seriously considering one anymore, for Earth's gravity well. In order to avoid solving the relative simple task of efficient stage mating and assembly, an engineering challenge you have to solve once per launch system and can amortize over the whole life of the system, you end up paying the cost of hauling into orbit a huge amount of unnecessary mass, every single launch. That's a very expensive trade-off.
- evgen 6y ago
  To take the fanboy's pride as an example, how much time is spent taking a Falcon first stage from landing to re-launch? Days? Weeks? Months? If you are expecting to use your launch infrastructure the same way we use a large commercial airliner then this isn't good enough. An SSTO could launch, orbit, return, refuel, and re-launch in the same day. For high-value cargo where the mass it not too much (c.f. humans) this might actually make sense.
  - erispoe 6y ago
    It take 2 to 3 days to reach the ISS from the ground. Getting to orbit is only 8min of that. The rest is a slow approach of the station. If you want to deliver cargo or people to a specific orbit and a specific place, say the ISS, you're not gonna be able to turnover a SSTO in a day. Even a week might not be enough for a quick return trip to and from the ISS. All this time your expensive ground to orbit capacity is idle. A first stage can launch a second stage into orbit and return to launch site in less than 15min. How fast you can turn it around for relaunch is an engineering challenge, but there's no fundamental reason you couldn't do it in a day or even in an hour.
    - mandevil 6y ago
      Note that two day approaches are not a real requirement, it's a choice that is made for operational flexibility. On Gemini 11 all the way back in 1966 NASA demonstrated docking a mere 94 minutes after launch (essentially one orbit). However, the launch window for that flight was two seconds, which is ... not ideal. By planning for a several day approach you give yourself a several hour launch window, which is much more workable. There is another reason for the leisurely approach for manned spacecraft. Studies have shown that something like 75% of spacefarers suffer from Space Acclimatization Sickness, generally lasting 1-3 days before your body gets used to zero-gee and you are fine. A leisurely approach lets an astronaut deal with that phase NOT in the shared space station that spends decades in outer space, but in the capsule which will be coming back down in a few months. SAS is a major limitation on space tourism, incidentally. You'll notice that there are very few plans between 15 minute suborbital hops and two weeks in space: that's because if even trained, physically fit astronauts take 1-3 days to adapt to space, no one is sure what a person who is not as trained will do, and if the word of mouth is "I went into space for three days and felt sick the entire time" that's not a good customer experience.

mumblemumble 6y ago

At least as a layperson, I really appreciated Everyday Astronaut's explanation for why aerospikes never caught on, and perhaps never will: https://youtu.be/D4SaofKCYwo https://youtu.be/D4SaofKCYwo It digs in much deeper than the usual offhanded, easy explanations like the, "We're stuck with bell nozzles because they were invented first," that this article offers.

jjoonathan 6y ago
Agreed! It's also worth highlighting the bit where he asks Elon Musk: https://youtu.be/cIQ36Kt7UVg?t=382 https://youtu.be/cIQ36Kt7UVg?t=382 I've internally asked this question so many times, like, guys, shouldn't we maybe do an aerospike? Musk focused on the high combustion efficiency of traditional combustion chambers, combined with the high efficiency of having two stages, as the reason why they won out. Jettisoning irrelevant fuel-containing mass as you go is great for efficiency and as a bonus it makes it easy to put sea-level-optimized nozzles on the bottom stage and vacuum-optimized nozzles on the top stage.

sandworm101 6y ago

The issue isn't really single stage to orbit but rather single stage to vacuum. Every first stage rocket starts at sea level but ends flight in essentially vacuum pressure (over 30km). So there might one day be a role for aerospikes on stages that don't continue all the way to orbit.

Gibbon1 6y ago
Every time I tried running numbers the impression I got was raw performance for the first stage isn't really that important. And the cost vs size is aprox fixedcost + log(size). You can either increase ISP or make the rocket bigger. Increasing the throw of the first stage by 10% might only cost 3% more.
- pfdietz 6y ago
  Indeed. The first stage drops off almost immediately, so it pays little to try to optimize it. For example, it makes little sense to use LH2 in a first stage; hydrocarbons are much denser and cheaper.

nradov 6y ago

The most viable proposal for an SSTO launcher with significant payload appears to be the Reaction Engines Skylon. It doesn't use an aerospike. https://www.reactionengines.co.uk/ https://www.reactionengines.co.uk/

pfdietz 6y ago
I would not call it viable. The penalty imposed by using air breathing is considerable. Trade off studies of launchers with air breathing components almost invariably optimize to 100% rocket.

jsmcgd 6y ago

> SSTO configurations just aren't very good at lifting much mass to orbit, so that seems unlikely. Depends what you mean. Performance wise they are not as efficient as multi stage expendable rockets, however they offer possibly the best operational efficiency of rocket system. The venture star has a launch capability of 20 tons to LEO. Larger lifters like Boeing's proposed 'Big Onion' had a lift capability of 227 tons to LEO. Despite its large size, its fully reusable nature and simple design would have meant its operation would have been considerably cheaper than anything that has flown to date. I suggest that it would be prudent to test at least one SSTO before the industry writes off the technology. Especially if we had one already 90% complete!

nradov 6y ago
The VentureStar has a launch capability of 0 tons to LEO because the fuel tank was too heavy.
valuearb 6y ago
No need. Anything a SSTO can do, staging can do better. Staging will always produce higher payloads to orbit.

jandrese 6y ago

Ultimately SSTOs just don't work well on Earth given our currently available propellants. If we lived on Mars the equation would be different, but Earth's gravity well is just too deep for the engines we can make today. It's not just about finding a new more energy dense but lighter weight propellant either, you need previously nonexistent alloys that can survive the intense heat, pressure, abrasive, and corrosive effects of a running rocket chamber. One of the big challenges with aerospike engines is keeping them from melting. It's a difficult challenge for bell shaped rocket engines too, but aerospikes are more heavily impacted.