On Fri, Dec 15, 2017 at 04:30:41PM -0500, Henry Spencer wrote:
On Fri, 15 Dec 2017, Norman Yarvin wrote:
I think the idea is that pogo arises from rocket acceleration producing
increased pressure at the pump inlet. In big rockets, one has a long
column of fluid, which yields a proportionately large pressure increase
from acceleration. This pressure increase translates to increased
pressure at the pump outlet, which translates to more thrust. By
itself, this would just be a mechanism for runaway, but the control
system senses it and reduces thrust; thus it becomes an oscillation.
No, the control system normally isn't involved. There is enough phase lag
in the flow and the combustion to give you an oscillation when things are
otherwise right -- in particular, when the natural frequency of the
combined feed+engine system happens to line up with a structural resonance
of the rocket. And of course, the structural resonances sweep over wide
ranges as the tanks empty, so it's not uncommon for pogo to occur only at
specific times during the burn.
Of course this explanation makes it sound like instead of adding an
extra device to stop pogo one could put more cleverness into the design
of the control system.
It would need quite fast response -- pogo oscillation is often at 10Hz or
more -- and I believe turbine control on even relatively modest engines
typically doesn't respond all that quickly.
