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Summary: 1. An aerospike engine tries to overcome a problem in the traditional bell-shaped nozzles: traditional nozzle shape and size can only be optimized for

by hliyan 8y ago

Summary: 1. An aerospike engine tries to overcome a problem in the traditional bell-shaped nozzles: traditional nozzle shape and size can only be optimized for one specific pressure (e.g. either sea level or a higher altitude). At all other altitudes, it is inefficient. 2. An areospike is kind of an inverted bell -- there is a spike in the middle and the rocket exhaust flows around it. This forms a sort of virtual bell nozzle (the inner wall is the spike's surface, the outer wall is the air itself). As the atmospheric pressure varies, the virtual nozzle shape adjusts too. This makes the areospike efficient at all altitudes (but not as efficient as a bell nozzle at it's most optimal). 3. Ideal for single-stage-to-orbit 4. Work stopped (author claims) because in the space industry, tried and tested is favored. And apparently, fuel is not such a large component of the cost of a rocket launch.

wolfram74 8y ago
When sentiments like this [1] are floating around, it's an easy argument to make that companies aren't interested in fundamental research, and at most tweaking off the shelf parts. -- [1] https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2340 https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2...
- carapace 8y ago
  > Choose the subscription that is right for you Seems unrelated.
- hinkley 8y ago
  Got another article without a paywall?
  - wolfram74 8y ago
    mea culpa, I wouldn't have linked it if I'd known an hour later it would be on the wrong side of a paywall. That's a behavior I haven't encountered before.
    - leoc 8y ago
      Someone once said that the FT paywall resembled something from the Unseen University.
- valuearb 8y ago
  The fundamental research on aero-spikes has been done, and they flunk the rocket equation.
dTal 8y ago
>fuel is not such a large component of the cost of a rocket launch Surely it's a massively important cost in terms of weight? Engine efficiency isn't just about cutting costs, it's about maximising payload - and the majority of a rocket's mass is fuel. Even a 5 percent efficiency gain is a huge win for payload.
- ThenAsNow 8y ago
  > Surely it's a massively important cost in terms of weight? Engine efficiency isn't just about cutting costs, it's about maximising payload - and the majority of a rocket's mass is fuel. Even a 5 percent efficiency gain is a huge win for payload. It's not that easy an answer. This point about maximizing efficiency to maximize payload only really matters if you are trying to increase the payload delivered for an existing system of fixed size. If you have the latitude to design a new /derivative vehicle, then you have the latitude to size the tanks larger to account for increased propellant. Propellant is cheap compared to the other costs of manufacturing and operating a rocket-propelled vehicle, though as re-use becomes more and more common, the sensitivity to propellant costs might be higher. Another thing to keep in mind is that efficiency in the form of increased Isp can be traded for engine thrust-to-weight. The SpaceX Merlin engine for example, uses an "inefficient" (i.e., low Isp) gas generator kerosene cycle, but has incredible thrust-to-weight (so is very mass-efficient). Aerospike engines will tend to be heavier due to distributing the thrust chamber exhaust circumferentially as opposed to through a small central throat. In practice, with a multi-stage system it is very hard to realize a system-level performance benefit from an aerospike configuration, which is the principal reason why they have not been used. Worth noting, the original Space Shuttle Main Engine configuration Rocketdyne developed was an aerospike, which makes sense given the engine fires from ground level to orbit. NASA Marshall apparently felt this was excessively high risk, so mandated a bell nozzle. The high area-ratio SSME nozzle is operating at the hairy edge of internal separation, and visibly "twangs" as the engine starts up and the internal shock blows out of the nozzle. Very low-throttle operation at sea level involves running with the shock and separated flow inside the nozzle and results in debits against the nozzle life. Interest in the aerospike configuration waxes and wanes within the U.S. DoD & NASA, but currently the Air Force Research Lab is considering pursuing a modular aerospike engine program. The benefits are not in the realm of performance, but rather the aerospike Isp benefit is used to offset the lower thrust-to-weight. The real motivation is the desire to gang smaller thrust chambers and turbomachinery sets to provide more of a "Lego-like" low-cost way to design, test, and scale up these engines.

by hliyan 8y ago

wolfram74 8y ago

When sentiments like this [1] are floating around, it's an easy argument to make that companies aren't interested in fundamental research, and at most tweaking off the shelf parts. -- [1] https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2340 https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2...

carapace 8y ago
> Choose the subscription that is right for you Seems unrelated.
hinkley 8y ago
Got another article without a paywall?
- wolfram74 8y ago
  mea culpa, I wouldn't have linked it if I'd known an hour later it would be on the wrong side of a paywall. That's a behavior I haven't encountered before.
  - leoc 8y ago
    Someone once said that the FT paywall resembled something from the Unseen University.
valuearb 8y ago
The fundamental research on aero-spikes has been done, and they flunk the rocket equation.

dTal 8y ago

>fuel is not such a large component of the cost of a rocket launch Surely it's a massively important cost in terms of weight? Engine efficiency isn't just about cutting costs, it's about maximising payload - and the majority of a rocket's mass is fuel. Even a 5 percent efficiency gain is a huge win for payload.

ThenAsNow 8y ago
> Surely it's a massively important cost in terms of weight? Engine efficiency isn't just about cutting costs, it's about maximising payload - and the majority of a rocket's mass is fuel. Even a 5 percent efficiency gain is a huge win for payload. It's not that easy an answer. This point about maximizing efficiency to maximize payload only really matters if you are trying to increase the payload delivered for an existing system of fixed size. If you have the latitude to design a new /derivative vehicle, then you have the latitude to size the tanks larger to account for increased propellant. Propellant is cheap compared to the other costs of manufacturing and operating a rocket-propelled vehicle, though as re-use becomes more and more common, the sensitivity to propellant costs might be higher. Another thing to keep in mind is that efficiency in the form of increased Isp can be traded for engine thrust-to-weight. The SpaceX Merlin engine for example, uses an "inefficient" (i.e., low Isp) gas generator kerosene cycle, but has incredible thrust-to-weight (so is very mass-efficient). Aerospike engines will tend to be heavier due to distributing the thrust chamber exhaust circumferentially as opposed to through a small central throat. In practice, with a multi-stage system it is very hard to realize a system-level performance benefit from an aerospike configuration, which is the principal reason why they have not been used. Worth noting, the original Space Shuttle Main Engine configuration Rocketdyne developed was an aerospike, which makes sense given the engine fires from ground level to orbit. NASA Marshall apparently felt this was excessively high risk, so mandated a bell nozzle. The high area-ratio SSME nozzle is operating at the hairy edge of internal separation, and visibly "twangs" as the engine starts up and the internal shock blows out of the nozzle. Very low-throttle operation at sea level involves running with the shock and separated flow inside the nozzle and results in debits against the nozzle life. Interest in the aerospike configuration waxes and wanes within the U.S. DoD & NASA, but currently the Air Force Research Lab is considering pursuing a modular aerospike engine program. The benefits are not in the realm of performance, but rather the aerospike Isp benefit is used to offset the lower thrust-to-weight. The real motivation is the desire to gang smaller thrust chambers and turbomachinery sets to provide more of a "Lego-like" low-cost way to design, test, and scale up these engines.