4 ms·

The major issue is the energy storage, not the power plant itself. The penalty of the battery mass scales far worse than a tank holding very energy dense rocket

The major issue is the energy storage, not the power plant itself. The penalty of the battery mass scales far worse than a tank holding very energy dense rocket propellant.

Robotbeat 5y ago
Again, the energy storage scales just fine: linearly. Rocket propellant tends to scale better than linear, but it is not in every case better than lithium ion batteries! For example, the R7/Soyuz rocket family is the most-launched orbital rocket ever, and it uses a hydrogen peroxide gas generator to drive the turbopump. That has a concentration of 82.5% peroxide. Pure peroxide has a heat of decomposition of 2.84MJ/kg, and turbine that is typically 30% efficient (actually, it might be much less than that… I think the V-2 turbine was only like 10% efficient, maybe worse… so 30% is optimistic) gives you a usable energy density of only 700kJ/kg, or about 194Wh/kg. The best lithium batteries available are about twice that, up to 400-500Wh/kg (with those in the lab even better still), and electric motors can have 90-95% efficiency. And gas generators using main propellants are better, certainly, but less than you might think because they have to haul all their oxidizer with them (unlike aircraft) and are also usually run very far from stoichiometric (maybe just 0.3 O:F ratio compared to a stoichiometric 3.4) to keep the temperature down. So unless you have a pretty high temperature turbine, you might not beat peroxide by much! So the easiest gas generators have worse energy density (keep in mind RocketLab does stage off batteries if necessary…), and the next easiest, while better, aren’t MASSIVELY better without careful efficiency improvements. The real efficiencies come when you use like an expander cycle or a staged combustion cycle or you feed the gas generator exhaust back into the nozzle like Merlin Vacuum or F-1. And those are all much more complicated. A level of complication that is not worth it for small rockets but is for larger. So it’s really not about electric scaling poorly (electric scales just fine) but about the greater complexity of better engine cycles being worth it at larger scales.
- avmich 5y ago
  > The real efficiencies come when you use like an expander cycle or a staged combustion cycle or you feed the gas generator exhaust back into the nozzle like Merlin Vacuum or F-1. If I remember correctly, with F-1 the gas generator exhaust was sent to the nozzle to cool the nozzle, not to add efficiency to the engine main cycle. We're still trying to get more efficiency from isochoric combustion, but the expected wins aren't too big. It's good that full-flow combustion becomes more of a norm.
  - Robotbeat 5y ago
    That as well. But the extra mass accelerated by entrainment (instead of dumped overboard at low velocity) is still helpful for increasing efficiency.
- deleted 5y ago
  [deleted]

The major issue is the energy storage, not the power plant itself. The penalty of the battery mass scales far worse than a tank holding very energy dense rocket propellant.

Robotbeat 5y ago

Again, the energy storage scales just fine: linearly. Rocket propellant tends to scale better than linear, but it is not in every case better than lithium ion batteries! For example, the R7/Soyuz rocket family is the most-launched orbital rocket ever, and it uses a hydrogen peroxide gas generator to drive the turbopump. That has a concentration of 82.5% peroxide. Pure peroxide has a heat of decomposition of 2.84MJ/kg, and turbine that is typically 30% efficient (actually, it might be much less than that… I think the V-2 turbine was only like 10% efficient, maybe worse… so 30% is optimistic) gives you a usable energy density of only 700kJ/kg, or about 194Wh/kg. The best lithium batteries available are about twice that, up to 400-500Wh/kg (with those in the lab even better still), and electric motors can have 90-95% efficiency. And gas generators using main propellants are better, certainly, but less than you might think because they have to haul all their oxidizer with them (unlike aircraft) and are also usually run very far from stoichiometric (maybe just 0.3 O:F ratio compared to a stoichiometric 3.4) to keep the temperature down. So unless you have a pretty high temperature turbine, you might not beat peroxide by much! So the easiest gas generators have worse energy density (keep in mind RocketLab does stage off batteries if necessary…), and the next easiest, while better, aren’t MASSIVELY better without careful efficiency improvements. The real efficiencies come when you use like an expander cycle or a staged combustion cycle or you feed the gas generator exhaust back into the nozzle like Merlin Vacuum or F-1. And those are all much more complicated. A level of complication that is not worth it for small rockets but is for larger. So it’s really not about electric scaling poorly (electric scales just fine) but about the greater complexity of better engine cycles being worth it at larger scales.

avmich 5y ago
> The real efficiencies come when you use like an expander cycle or a staged combustion cycle or you feed the gas generator exhaust back into the nozzle like Merlin Vacuum or F-1. If I remember correctly, with F-1 the gas generator exhaust was sent to the nozzle to cool the nozzle, not to add efficiency to the engine main cycle. We're still trying to get more efficiency from isochoric combustion, but the expected wins aren't too big. It's good that full-flow combustion becomes more of a norm.
- Robotbeat 5y ago
  That as well. But the extra mass accelerated by entrainment (instead of dumped overboard at low velocity) is still helpful for increasing efficiency.
deleted 5y ago
[deleted]