A "diesel electric" turbopump?

[nicp]

Electron has demonstrated fairly reliable launch performance, the propellant pumps being driven electrically from batteries.
The batteries add otherwise dead mass, and are not at all energy dense.
But the concept behind Electron is I think good. Instant startup and easy precision control of the engines. But the energy available to drive the pumps is very limited.

Ok, so you know where I am going given the subject line - a small turbine driving one or more alternators that electrically drive pumps for the engines.

Yes,  I know, there would be loads of loss in conversion from propellant to pumping, but the batteries you aren't carrying don't go flat and you have the advantage of the precision electronic control of the rocket engines themselves as demonstrated thoroughly by Electron.
(I know a gas turbine skips complexity, etc, I do know how conventional rocket engines work.)

The propellant pumps - however you drive them - are a relatively small power expenditure in comparison to the propellant carried, so perhaps some loss there is not a big deal.

It's probably a bad idea - have I missed a square/cube relation here that makes this utterly unworkable beyond a small scale, in the same way battery powered pumps don't  scale for Electron?

By the way -  I only use the word 'diesel' in the subject line to make this a familiar concept, as used in diesel electric railway  locomotives.

[Tommyboy]

Modern lithium batteries can output insane ammounts of power for their weight. Multiple kilowatts per kilogram of battery is nothing special. To get to this same ammount of power from a generator will probably add one or two orders of magnitude more mass.
Oh, and you'd still need a battery as a buffer because your generator can not respond quickly enough to varying demand feom your pumps.

[nicp]

Modern lithium batteries can output insane ammounts of power for their weight. Multiple kilowatts per kilogram of battery is nothing special. To get to this same ammount of power from a generator will probably add one or two orders of magnitude more mass.
Oh, and you'd still need a battery as a buffer because your generator can not respond quickly enough to varying demand feom your pumps.

The problem is energy density not power, batteries have a terrible energy density, however much power they might produce for a short time. (A firecracker produces loads of power. For a VERY short time. There is a reason internal combustion engines were king for a very long time). EDIT: Liquid fuels and oxidizers are very energy dense.
However, your comment may have kicked a big hole in my idea. The alternator (generator) would have to be on a similar scale to the motors that drive the propellant pumps, so there is a doubling in mass right away - I will think on this, but I think you have shot me down.

Have a good day.
Nic

[redneck]

At least you would expending the batteries as reaction mass same as in a gas generator cycle.

[Robotbeat]

The problem with a “cheap” fuel option like diesel is you still need to bring your oxygen with you. And you can’t run it stoichiometric because that will burn out your engine. Most engines are designed to run on air. Diesel isn’t that different from kerosene so I’ll use kerosene numbers. Stoichiometric o:f ratio for air:kerosene mass is 15:1. So while the specific energy of the fuel might be I dunno 43MJ/kg, including air mass that’s 2.69MJ/kg. Small Diesel engines are only like I dunno 15-30% efficient, so at best you have a usable specific power of 0.8MJ/kg

…which isn’t any better than batteries. Diesel engines are heavy, too. And high specific power small combustion turbine engines are crazy inefficient. I think the small hobby turboprop engines are only like 5% efficient.

Electro pumps are under rated.

[edzieba]

Prior discussion of this concept here.