For an electric motor torque is proportional to amps, voltage to RPM and
heating is amps ^ 2.
In short, the higher the RPM the more power you can get out of a given
sized motor.
If you were looking to optimise motor mass, it'd potentially be
interesting (and solve vacuum operation issues) if you stuck it in your
working fluid.
On 16/12/16 12:04, Henry Vanderbilt wrote:
"Yes, they [piston pumps] generally run at lower RPM than turbopumps, although making the cylinder volume larger at a given maximum pressure costs mass, so there tends to be an optimum."
So there IS the possibility of that optimum involving trading off increased piston-pump mass for reduced NPSH which could allow reduced tank+press-system mass.
An entirely separate question for the crowd: How does electric motor RPM relate to maximum motor power-to-weight? If, to pull numbers out of the air, the optimum piston-pump implied above wants to operate at a few thousand RPM (rather than the tens of thousands mentioned for turbine pumps) does that make the electric motor heavier? Lighter? No difference?
Henry
On 12/15/2016 3:55 PM, Jordin Kare wrote:
That’s correct. Oversimplifying, piston pumps are inherently constant-pressure, vs. turbopumps which are inherently constant flow
(they optimize fairly sharply for a specific flow rate) and they are efficient all the way to zero flow. Piston pumps tend to win over turbopumps at small sizes
(John Whitehead, who did a lot of work on piston pumps at LLNL in the late 80s, put the crossover at 5000 lbf thrust) and/or intermittent flow or widely-varying
flow rates. Yes, they gnerally run at lower RPM than turbopumps, although making the cylinder volume larger
at a given maximum pressure costs mass, so there tends to be an optimum.
On Dec 15, 2016, at 12:32 PM, Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx> wrote:
My impression is that piston pumps don't lose efficiency at lower RPMs to the same degree turbine pumps do. This might imply that for given performance requirements a physically larger, slower-rotating piston pump might tend to have lower minimum required feed pressure without impractically low efficiency. Maybe.
This opinion and two bucks will get you a cup of coffee...
On 12/15/2016 1:20 PM, Pierce Nichols wrote:
The piston cylinder volume still needs to be filled from tank pressure,
so there's a relation there between tank pressure and maximum pump
speed. IOW, it becomes another knob for the designer to twiddle.
-p
On Thu, Dec 15, 2016 at 12:03 PM, Brian Feeney <alaiadesign@xxxxxxxxx
<mailto:alaiadesign@xxxxxxxxx>> wrote:
Would a piston pump reduce or eliminate the problem of cavitation
one can get with a turbine? If yes, this would lead to lower tank
pressure, lighter weight tanks - buys back some of the weight
increase of the piston vs turbopump??
Cheers
Brian Feeney
On Dec 15, 2016 2:40 PM, "Henry Vanderbilt"
<hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
On 12/15/2016 9:15 AM, Dave McMillan wrote:
On 12/15/2016 11:01 AM, Thomas McNeill wrote:
I have been curious about different pumping mechanisms.
Instead of a
turbo pump what about positive displacement pumps, like
gear or lobe?
Has there been any attempt at using one of these types
of pumps?
I have to admit to being curious why I've never heard of any
amateurs trying a variant on XCOR's piston(less) pumps --
those seem
like a much lower bar to get over than any kind of
turbomachinery. Some
subtle difficulty I'm not seeing, there?
XCOR's pumps are not pistonless; that's Flometrics you're
thinking of.
As for the root question, I see no reason why developing an
electric-drive piston propellant pump should be particularly
challenging.
Off the top of my head...
- You need to pay attention to compatible piston-seal materials
(and to flow-path materials compatibility in general, of course).
- For cryos, you need to pay attention to thermal dimensional
issues, seal & lube temperatures, and flow-path conditioning.
- You need to pay attention to output pressure variations, to
the extent your motor/application may be sensitive to them.
None of those involve bleeding-edge new tech development. Just
a matter of known detail engineering. And there's this: There's
a HUGE existing parts-base out there for piston machinery
components, both in pumps for other applications
(pressure-washers anyone?) and in the small IC engine field.
I'd lean toward the explanation that people haven't looked at
piston over turbine pumps more out of established habit than for
fundamental practical reasons.
After all, if you're using a turbine for pump power, making the
pump a turbine wheel on the same shaft uses design/manufacturing
resources you'll need regardless.
Going to electric pump power, that no longer holds true. You
can now skip the (apparently) considerable investment in
high-performance turbine design/fab capabilities.
Henry
