As Henry said, your qualification metrics should be driven by your system
requirements for your engine. So if it is intending to be a flight engine,
things like; runtime, system weight, thrust, achieved specific impulse,
magnitude of pressure oscillations in steady state operation, etc. These
metrics should be set in a way to help control scope creep, and to drive
the project to attaining the minimum viable product (So don't set real high
isp requirements, runtime, or stability). So if the hope for the project is
to get something to leave the ground and maybe fly to an altitude, the
qualification criteria should be set to achieve that end.
I would highly suggest dividing up the requirements into at least two
categories. "Mandatory" Which would be the barest minimum requirements you
want to achieve that would let you call the project successful and
"Desired" which would be goals you would like to achieve, but if you ran
out of time/money/test engines and you didn't get to them it wouldn't cause
the project to fail. In my opinion, for the first, second, or even maybe
third engine design a team has done, "performance" should be reallly low on
the list of "Desired" objectives. Reason being, performance almost is
always coming at a cost of design effort, reliability, or cost; which are
very finite quantities on a student project. It is a really common trap for
student liquid teams to fall into. Trying to make the engine too fancy on
the first go around, and accidentally introducing more problems than they
need, or even making the project unviable as the development time got too
long/expensive.
On Thu, Jan 28, 2021 at 5:11 AM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
I suspect that most of those engines in the old lit were developed for
specific vehicles flying specific missions, so "acceptable operation" went
largely without saying as whatever would allow a given vehicle to fly a
given mission with acceptable reliability. ("Acceptable reliability" could
of course vary widely, from as low as 70-80% for expendable weapons to 99%+
for human spaceflight.)
So I think if you define a notional vehicle and mission, you can then
start to derive answers to your questions. Given the real mission is
teaching, I would expect that the way the derivation varies with variations
in vehicle/missions could be more instructive than the specifics of any one
notional vehicle/mission...
(I am not a teacher, nor do I play one on TV.)
Henry
On 1/28/2021 12:20 AM, Nathaniel Van Rumpt wrote:
My student team (12kN, LOX/Kero) is trying to determine what flight engine
qualification criteria should look like. Most old literature merely contain
phases such as "the engine shall be tested to confirm acceptable
operation," yet they often don't define what "acceptable operation" looks
like.
We had a few questions in particular about:
- What would be an acceptable level of chamber pressure fluctuations?
- How would one measure and determine an acceptable vibration level?
- How quickly should steady state operation of the engine be
achievable?
- How long should one test the actual flight engine before assembling
into the rest of the rocket?
Furthermore, what other engine acceptance criteria should we be
considering?
Cheers,
Nathan