New batteries for electric pump-fed engines?

[Sarigolepas]

The first stage of a rocket fires for 2:30 and the second stage fires for 6:30 so you need at least 24C for the first stage and at least 9C for the second stage, if you have more you can split them into smaller batteries and jettison them to save weight.

BYD has just unveiled a battery capable of 10C charging and CATL has unveiled a battery with 12C charging, and they can probably do 3 times that when discharging.

That should be enough for a rocket engine, with around 50% more specific energy than li-po batteries used by electron.

[Robotbeat]

You can get LiPos that claim to do 50C, 100C or more. (With ridiculous voltage drop, but still.)

[Twark_Main]

You can get LiPos that claim to do 50C, 100C or more. (With ridiculous voltage drop, but still.)

Note that the power output of a battery is maximized when 50% of the energy dissipation (and voltage drop) is in the battery + wiring and 50% goes into the useful load. After that point increasing the C rate further has become futile.

This general rule is useful sizing for napkin math. To find the optimum size battery, start at 50% internal dissipation and work the C rate downward until the engineering closes.

[Sarigolepas]

You can get LiPos that claim to do 50C, 100C or more. (With ridiculous voltage drop, but still.)

Yeah, electron currently only jettison the battery on the second stage, if they had higher C rate they could also jettison them on the first stage.

Big issue with batteries is not specific energy, which is actually just as good as the energy you can extract from your fuel using a turbine, the real issue is that they are dry mass, with a higher C rate they could drop them as they fly and they would count as propellant, not dry mass.

Batteries could get you the same performance as a full flow engine if you could drop them as you fly.

[InterestedEngineer]

Big issue with batteries is not specific energy, which is actually just as good as the energy you can extract from your fuel using a turbine

wut.

Methalox:  50MJ/kg
Experimental Lithium Sulphide Batteries: 3MJ/kg
Common lithium batteries: 1.8MJ/kg

There is no useful turbine that is only 6% efficient, so your assertion is off by an entire order of magnitude

[Robotbeat]

That’s not true. Small turbine engines are incredibly inefficient (including air breathing, such as the hobby turbine engines used by that one jet pack guy in the UK), especially as you can’t operate them near stoichiometric. Gas generator turbines are incredibly fuel rich.

Batteries are about as energy dense as the peroxide used for Soyuz gas generators when taking into account mechanical output.


Also, your methalox value is off because you only counted the energy of the fuel, ignoring the oxygen. If you include oxygen, it’s more like 10-11MJ/kg, not counting efficiency or the need to run off-stoichiometric.

[InterestedEngineer]

That’s not true. Small turbine engines are incredibly inefficient (including air breathing, such as the hobby turbine engines used by that one jet pack guy in the UK), especially as you can’t operate them near stoichiometric. Gas generator turbines are incredibly fuel rich.

Batteries are about as energy dense as the peroxide used for Soyuz gas generators when taking into account mechanical output.

At the low end (5kW) turbines are 10% efficient at converting chemical to mechanical energy.    So that's still 50*.1 = 5MJ/kg for methalox.

peroxide is <3MJ/kg by itself, so it's a poor comparison.  Who even uses that anymore?

[Twark_Main]

peroxide used for Soyuz gas generators

Who even uses that anymore?

[Robotbeat]

That’s not true. Small turbine engines are incredibly inefficient (including air breathing, such as the hobby turbine engines used by that one jet pack guy in the UK), especially as you can’t operate them near stoichiometric. Gas generator turbines are incredibly fuel rich.

Batteries are about as energy dense as the peroxide used for Soyuz gas generators when taking into account mechanical output.

At the low end (5kW) turbines are 10% efficient at converting chemical to mechanical energy.    So that's still 50*.1 = 5MJ/kg for methalox.

peroxide is <3MJ/kg by itself, so it's a poor comparison.  Who even uses that anymore?

You ignored the mass of the oxygen. WHY

[Robotbeat]

https://www.chiefaircraft.com/jc-spt10.html

Additionally, this small turbine generates 9kW mechanical power with .33kg/minute of Jet A, which has a HHV of 43MJ/kg, so 236.5kW of fuel energy input, giving an efficiency of 3.8%. Small turbines are just incredibly inefficient. And if you need to carry the oxygen with you…

[Sarigolepas]

Big issue with batteries is not specific energy, which is actually just as good as the energy you can extract from your fuel using a turbine

wut.

Methalox:  50MJ/kg
Experimental Lithium Sulphide Batteries: 3MJ/kg
Common lithium batteries: 1.8MJ/kg

There is no useful turbine that is only 6% efficient, so your assertion is off by an entire order of magnitude

Even on raptor the turbine only extracts 100,000 out of 5,000,000 hp so around 2% of the energy.
That's only 1 MJ/kg of propellant that is exctracted and used to run the pumps.

"Tiny" amount of energy

But still, if you can't jettison the batteries during flight like a propellant they end up as dry mass which is way worse, that's why C rate matters more than specific energy unless you want to reuse the battery.

[Sarigolepas]

That’s not true. Small turbine engines are incredibly inefficient (including air breathing, such as the hobby turbine engines used by that one jet pack guy in the UK), especially as you can’t operate them near stoichiometric. Gas generator turbines are incredibly fuel rich.

Batteries are about as energy dense as the peroxide used for Soyuz gas generators when taking into account mechanical output.

At the low end (5kW) turbines are 10% efficient at converting chemical to mechanical energy.    So that's still 50*.1 = 5MJ/kg for methalox.

peroxide is <3MJ/kg by itself, so it's a poor comparison.  Who even uses that anymore?

Yeah, but we are talking about a rocket engine, not a turboshaft engine, the turbine is not optimised to take as much energy as possible, it's optimised for maximum exhaust pressure and if you take too much energy you lose pressure.