One of the limiting factors of electric pumped rockets is that there
is an upper limit to how quickly you can drain batteries. (And no,
capacitors are not the answer.) So for first stages where the burn is
shorter than that time period can't stage batteries, whereas an upper
stage like Rocketlab's that does the majority of the delta-v and burn
time of the system can stage batteries. It is more complex, certainly,
but it makes the cycle a little bit closer to gas generator where the
mass you are extracting pump power from is continuously discarded.
Generally I think that 'stage and a half' approaches are excessively
discounted. If it makes sense to drop boosters, it makes sense to drop
engines or batteries. I don't think it should be looked down on as
showing a limitation of the system, but rather an obvious bit of
performance optimization.
With TAN (patent expiring soon) or the Volvo super simple altitude
compensating nozzle (patent expired already,
https://patents.google.com/patent/US6176077B1/en , thanks to Jeff
Greason for pointing it out on Twitter) a modern remake of Atlas could
be the lowest cost launch on the market. One set of avionics, one size
of tank to design and set up tooling for, one engine program. Even if
Elon figures out that running people ragged leads them to making dumb
mistakes and losing great big rockets over and over, there's still
going to be demand for one-off launches. Especially in other
countries.
On Fri, Apr 3, 2020 at 10:21 PM Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
On Fri, 3 Apr 2020, ken mason wrote:
I would love to see a scaling analysis of modern electric driven pumps
and where their advantage is lost to GG cycle,kind of like the expander
cycle is ~ 20KLbF,i.e. the RL-10...
Not everybody thinks the expander cycle poops out at 20klbf -- P&W's bid
for the J-2 (200klbf) was an expander, as was a much later proposal for a
600klbf engine. It's not a well-explored part of the design space.
As for the electrics, note that Rocketlab found it important to jettison
spent batteries from the second stage. The need for that added complexity
suggests that straightforward electric isn't currently performing as well
as GG even at Electron size. The advantage is simpler hardware and easier
development. (Which is to say, an analysis showing that advantage would
be economic more than technical, and would have to make a bunch of
debatable assumptions.)
That said, even a purely technical scaling analysis of electric pumps
definitely *would* be interesting reading.
Henry
