28 ms·

Aero spikes only make sense for single stage to orbit, and because of the rocket equation, SSTO doesn’t make much sense itself. First, Aerospikes only have bet

Aero spikes only make sense for single stage to orbit, and because of the rocket equation, SSTO doesn’t make much sense itself. First, Aerospikes only have better efficiency over staged rockets over part of the launch to orbit. At sea level it has vurtually no advantage, it’s not until Tye rocket ge s to thin air that the Aerospike efficiency is higher. But guess what happens shortly thereafter? The first stage is dropped and a vacuum optimized second stage takes over, again reducing the aerospike efficiency advantage to near zero. And that slightly better efficiency over part of the flight isn’t free, it costs extra weight over bell shaped nozzles, So the big win is for Aerospikes is SSTO, that’s where it’s far more efficient than fixed nozzles. But because of the rocket equation, SSTO doesn’t make sense. Dropping off a big chunk of mass half way up (the first stage) provides substantial efficiency gains. Musk has shown that the key to making space flight affordable isn’t breakthroughs in engine performance, and its not reusability, at least not yet. Before SpaceX ever landed a booster they were 30x cheaper than the Shuttle, and 20x cheaper than the SLS. How did they do it when the Falcon 9 engine has very average performance, and when they didn’t reuse any of them? The reason is same as Henry Fords, assembly line manufacturing. They mastered making Merlins cheaply by building them in high volumes and constantly improving at it. Nine engines per rocket made that possible, just as it’s a key to their making reuse work.

ThenAsNow 8y ago
> Musk has shown that the key to making space flight affordable isn’t breakthroughs in engine performance, and its not reusability, at least not yet. Before SpaceX ever landed a booster they were 30x cheaper than the Shuttle, and 20x cheaper than the SLS. How did they do it when the Falcon 9 engine has very average performance, and when they didn’t reuse any of them? > The reason is same as Henry Fords, assembly line manufacturing. They mastered making Merlins cheaply by building them in high volumes and constantly improving at it. Nine engines per rocket made that possible, just as it’s a key to their making reuse work. This is one ingredient, and may be the largest, but there are several others as well. To start with, engine R&D costs were significantly reduced because Elon was able to build atop a well-established foundation by Tom Mueller whom he hired from TRW. That's not a dig - it has proven to be a very smart move. Vertical integration, and not having to pay Aerojet Rocketdyne their margin on each engine also helps. Another is that a lot of his initial R&D was done via the Falcon 1, sponsored in significant part by DARPA. Yet another is that the legacy rocket companies do business with the traditional "Mission Assurance" mindset that accompanies national security space missions as well as high-value commercial missions (e.g., GEO sats). Starting in the late 2000s free of that legacy enabled a completely different culture that was far less plodding, risk-averse, and legacy-driven. Additional mission assurance was then driven in after failures (like the early Falcon 1 flights) and more when SpaceX put themselves through the USAF launch certification process. Don't underestimate the impact of the legacy mission assurance mindset. The US has a major Federally-Funded Research and Development Center, The Aerospace Corporation where likely the largest component of its income is being involved in DoD mission assurance.
ThenAsNow 8y ago
Not that I'm expecting any replies given the thread's age, but for future reference of any interested party... > At sea level it has vurtually no advantage, it’s not until Tye rocket ge s to thin air that the Aerospike efficiency is higher. There's more to it than this. Almost all practical rocket engines will have nozzle area ratios sized for altitude rather than sea level. That is to say, actual design nozzle area ratios will be significantly higher than the optimal ratio for ground level. This implies overexpansion losses at low altitude. The higher the area ratio, the worse this effect is. Rockets are designed this way because when you consider the mission-average Isp, it's better to reduce the underexpansion losses at altitude through higher area ratio and eat the overexpansion losses at low altitude than to optimize for liftoff conditions. An ideal aerospike lets you run a very high geometric area ratio with reduced overexpansion penalties. So it's conditional as to whether the aerospike "has an advantage" at sea level. If you are running a large enough area ratio, then compared to an equivalent area ratio bell, the aerospike will minimize the overexpansion losses at lower altitude, and additionally it will have reduced unsteady pressure loads due to avoidance of internal shock-induced separation, the way you would with a bell. The limiting area ratio for a bell nozzle is usually this internal shock separation criterion, whereas that constraint is removed for the aerospike. So at high enough area ratio, the aerospike could indeed have a low altitude benefit over an equivalent area ratio bell. As you point out, in practice, this effect doesn't tend to buy you a large net difference in mission-averaged Isp, particularly with staged rockets. Also as you correctly indicate, the largest mission-average Isp benefit for an aerospike is when the engine fires over a large enough range of altitude (e.g., SSTO) and when you can maximize the geometric area ratio. Such a case would also include the Shuttle, where an aerospike could have made a meaningful mission-average Isp difference, though for the same cycle parameters as the RS-25/SSME it likely would have increased engine weight. Also note that assuming you truncate the spike (as all practical aerospike engines would be designed), a high area ratio aerospike configuration would be shorter than the equivalent area ratio bell.
- valuearb 8y ago
  Good stuff, thanks for the insight.

ThenAsNow 8y ago

> Musk has shown that the key to making space flight affordable isn’t breakthroughs in engine performance, and its not reusability, at least not yet. Before SpaceX ever landed a booster they were 30x cheaper than the Shuttle, and 20x cheaper than the SLS. How did they do it when the Falcon 9 engine has very average performance, and when they didn’t reuse any of them? > The reason is same as Henry Fords, assembly line manufacturing. They mastered making Merlins cheaply by building them in high volumes and constantly improving at it. Nine engines per rocket made that possible, just as it’s a key to their making reuse work. This is one ingredient, and may be the largest, but there are several others as well. To start with, engine R&D costs were significantly reduced because Elon was able to build atop a well-established foundation by Tom Mueller whom he hired from TRW. That's not a dig - it has proven to be a very smart move. Vertical integration, and not having to pay Aerojet Rocketdyne their margin on each engine also helps. Another is that a lot of his initial R&D was done via the Falcon 1, sponsored in significant part by DARPA. Yet another is that the legacy rocket companies do business with the traditional "Mission Assurance" mindset that accompanies national security space missions as well as high-value commercial missions (e.g., GEO sats). Starting in the late 2000s free of that legacy enabled a completely different culture that was far less plodding, risk-averse, and legacy-driven. Additional mission assurance was then driven in after failures (like the early Falcon 1 flights) and more when SpaceX put themselves through the USAF launch certification process. Don't underestimate the impact of the legacy mission assurance mindset. The US has a major Federally-Funded Research and Development Center, The Aerospace Corporation where likely the largest component of its income is being involved in DoD mission assurance.

ThenAsNow 8y ago

Not that I'm expecting any replies given the thread's age, but for future reference of any interested party... > At sea level it has vurtually no advantage, it’s not until Tye rocket ge s to thin air that the Aerospike efficiency is higher. There's more to it than this. Almost all practical rocket engines will have nozzle area ratios sized for altitude rather than sea level. That is to say, actual design nozzle area ratios will be significantly higher than the optimal ratio for ground level. This implies overexpansion losses at low altitude. The higher the area ratio, the worse this effect is. Rockets are designed this way because when you consider the mission-average Isp, it's better to reduce the underexpansion losses at altitude through higher area ratio and eat the overexpansion losses at low altitude than to optimize for liftoff conditions. An ideal aerospike lets you run a very high geometric area ratio with reduced overexpansion penalties. So it's conditional as to whether the aerospike "has an advantage" at sea level. If you are running a large enough area ratio, then compared to an equivalent area ratio bell, the aerospike will minimize the overexpansion losses at lower altitude, and additionally it will have reduced unsteady pressure loads due to avoidance of internal shock-induced separation, the way you would with a bell. The limiting area ratio for a bell nozzle is usually this internal shock separation criterion, whereas that constraint is removed for the aerospike. So at high enough area ratio, the aerospike could indeed have a low altitude benefit over an equivalent area ratio bell. As you point out, in practice, this effect doesn't tend to buy you a large net difference in mission-averaged Isp, particularly with staged rockets. Also as you correctly indicate, the largest mission-average Isp benefit for an aerospike is when the engine fires over a large enough range of altitude (e.g., SSTO) and when you can maximize the geometric area ratio. Such a case would also include the Shuttle, where an aerospike could have made a meaningful mission-average Isp difference, though for the same cycle parameters as the RS-25/SSME it likely would have increased engine weight. Also note that assuming you truncate the spike (as all practical aerospike engines would be designed), a high area ratio aerospike configuration would be shorter than the equivalent area ratio bell.

valuearb 8y ago
Good stuff, thanks for the insight.