4 ms·

> 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

> 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.

hinkley 8y ago
Bigger tank does mean less payload because of the rocket equation (you now have to lift the bigger tank and the extra fuel, which takes more fuel, or less payload). I wonder what the physics looks like for landing a linear spike. One of the problems that SpaceX mentions in their voiceover work for landings is that the thrust for the rockets is so high that even at the lowest thrust it's almost too much to land. With a linear spike I wonder if they could turn off most of the pumps. Might give them more pitch control if only the middle ones were running?
- ThenAsNow 8y ago
  > Bigger tank does mean less payload because of the rocket equation (you now have to lift the bigger tank and the extra fuel, which takes more fuel, or less payload). For fixed gross mass. The point is, for a given payload mass fraction, you can increase the gross mass of the system to increase payload. And indeed for larger systems, you get a subsystem mass amortization effect that tends to decrease the dry mass fraction. > I wonder what the physics looks like for landing a linear spike. One of the problems that SpaceX mentions in their voiceover work for landings is that the thrust for the rockets is so high that even at the lowest thrust it's almost too much to land. The difference in nozzle performance shouldn't change this much at all. To make this problem more tractable, you need more net throttling capability. The problem is the gross mass at liftoff vs. at landing. When you size the engine(s) for liftoff mass, it's difficult to throttle them down enough to keep Thrust / Weight (T/W) low at landing when the system is not much more than first stage dry mass. This is especially true when a propellant is liquid as it leaves the main injector (such as with the Merlin, injecting liquid RP). Gas/gas injection (such as with an expander cycle) can help to some extent. See the CECE testbed engine for an example. As per my comment about the prospective AFRL project, one of the ideas that tends to go along with contemporary aerospike engines is the idea of modular thrust chambers of smaller size. Depending on how granular the turbomachinery is and the chosen cycle, this approach could potentially permit deeper throttling. The XRS-2200 engine for the X-33 was designed with this modular approach. One system-level benefit for the aerospike is the ability to use an "easy" cycle, like gas generator, and use the gas generator exhaust to improve the aerospike performance, such as by plug base pressurization. This is more constructive than what is done in conventional bell nozzle systems with gas generator exhaust. Linear spikes exacerbate the mass inefficiency problem, BTW, and are not likely the way any operational aerospike engine will be designed in the future. Also, when you think of throttling modular thrust chambers for steering, realize that you are either reducing the net thrust coming from the engine, which is not what you want to do, or you are jacking up chamber pressure on some modules while reducing it on others. Also not something you want to do, as if you have the capacity for higher chamber pressure, you want to use it for the whole flight, not just leave that mass margin there for steering. A number of studies have been done on steering using fluidic or hinging/flapped aerospike configurations, and again, it's hard to beat old-fashioned gimbaling. Yet another system-level reason why bells continue to be the status quo.

hinkley 8y ago

Bigger tank does mean less payload because of the rocket equation (you now have to lift the bigger tank and the extra fuel, which takes more fuel, or less payload). I wonder what the physics looks like for landing a linear spike. One of the problems that SpaceX mentions in their voiceover work for landings is that the thrust for the rockets is so high that even at the lowest thrust it's almost too much to land. With a linear spike I wonder if they could turn off most of the pumps. Might give them more pitch control if only the middle ones were running?

ThenAsNow 8y ago
> Bigger tank does mean less payload because of the rocket equation (you now have to lift the bigger tank and the extra fuel, which takes more fuel, or less payload). For fixed gross mass. The point is, for a given payload mass fraction, you can increase the gross mass of the system to increase payload. And indeed for larger systems, you get a subsystem mass amortization effect that tends to decrease the dry mass fraction. > I wonder what the physics looks like for landing a linear spike. One of the problems that SpaceX mentions in their voiceover work for landings is that the thrust for the rockets is so high that even at the lowest thrust it's almost too much to land. The difference in nozzle performance shouldn't change this much at all. To make this problem more tractable, you need more net throttling capability. The problem is the gross mass at liftoff vs. at landing. When you size the engine(s) for liftoff mass, it's difficult to throttle them down enough to keep Thrust / Weight (T/W) low at landing when the system is not much more than first stage dry mass. This is especially true when a propellant is liquid as it leaves the main injector (such as with the Merlin, injecting liquid RP). Gas/gas injection (such as with an expander cycle) can help to some extent. See the CECE testbed engine for an example. As per my comment about the prospective AFRL project, one of the ideas that tends to go along with contemporary aerospike engines is the idea of modular thrust chambers of smaller size. Depending on how granular the turbomachinery is and the chosen cycle, this approach could potentially permit deeper throttling. The XRS-2200 engine for the X-33 was designed with this modular approach. One system-level benefit for the aerospike is the ability to use an "easy" cycle, like gas generator, and use the gas generator exhaust to improve the aerospike performance, such as by plug base pressurization. This is more constructive than what is done in conventional bell nozzle systems with gas generator exhaust. Linear spikes exacerbate the mass inefficiency problem, BTW, and are not likely the way any operational aerospike engine will be designed in the future. Also, when you think of throttling modular thrust chambers for steering, realize that you are either reducing the net thrust coming from the engine, which is not what you want to do, or you are jacking up chamber pressure on some modules while reducing it on others. Also not something you want to do, as if you have the capacity for higher chamber pressure, you want to use it for the whole flight, not just leave that mass margin there for steering. A number of studies have been done on steering using fluidic or hinging/flapped aerospike configurations, and again, it's hard to beat old-fashioned gimbaling. Yet another system-level reason why bells continue to be the status quo.