Thrust curve differences

It appears that both the TRA and the NAR have tested the H220. Would
someone please explain why the two results are so dramatically
different. For instance, in the NAR test, the peak thrust is 275N but
in the TRA test it is 496N.
Bob
Reply to
baDBob
Loading thread data ...
Also post as many of the initial conditions as you can find so this anomoly is more fully documented on usenet for permanant future reference.
Please.
Jerry
Reply to
Jerry Irvine
Bob,
If you drag up the thrust/time curves from each organization you see a significant difference. The NAR curve builds to ~250N and stays pretty steady until burnout. The TRA curve rises to almost 110 lbf (~500N) and then exhibits a damped oscillation at about 30Hz converging on about 60 lbf. (~270N).
The TRA test shows a lightly shorter burn time and higher average thrust than the NAR test. It also exhibited a nasty case of combustion instability. Which could be an artifact from the test setup. A motor with that sort of thrust curve is on the edge of a CATO. I wouldn't certify it if I had the choice, since it is an accident waiting to happen.
I looked at some other test data to see if I could see any trends. It is hard to see anything because the published thrust curves have been processed in different ways and it is difficult to tell if the processing (and test conditions) are similar. So I am mainly looking for documents that have the same format. The test data for the H238 also exhibits the same damped oscillation of about 30 Hz. I find this odd because the two motors are of different lengths and I would expect the frequency of this oscillation to change with length/volume. I also see this oscillation in the I435 and I370 data. The I370 data is a "Kosdon by Aerotech" and the physical configuration is quite different (and the propellant formulation as well, I assume) so I would expect a different frequency of oscillation but it is the same. The damping factor is also quite similar across the different motors.
I suspect that this oscillation is the result of a mechanical response from the test cell and not a true measure of the motors performance.
However, I am not an expert on rocket motors and what I know comes from Rocket Propulsion Elements. :-)
baDBob wrote:
Reply to
David Schultz
I have used about 28 H220's and have seen this spike off the pad. I have attributed it to the use of a large heavily dipped ignightor. Never seen it with a small head ignightor. I think the propelant can flash resulting in a preasure spike with a large ignightor. Also when there is a off the pad spike, the motor screams. Same rocket and altimiter always have flights + = 50 ft no matter how the motor starts out. Gary Deaver
Reply to
Deaver
I believe the issue is differences in TEST EQUIPMENT and procedures not in the motors. NAR has "better equipment" made by MIT graduaites :)
TRA has equipment thrown together by hicks to give them the power base they need by selectively testing motors, and the actual results are unimportant as evidenced by the fact sometimes they don't even bother to test at all before listing a motor as certified.
At NAR if it gets listed it has followed the procedure TO THE LETTER.
Jerry
Reply to
Jerry Irvine
Another possibility that I forgot to mention is that the test stand data might have been processed with a poorly designed low pass filter with much too high a value for 'Q'. A good 'Q' for this application would be 0.7.
The more I think about it, the more likely this seems. Other data on the TMT pages exhibit lots of high frequency energy and this might be the result of an attempt to remove it.
Reply to
David Schultz
Maybe But I have observed variabilty in actual flights.
Reply to
Gary Deaver
It's been a while since I saw it, but back when TMT published the gray TMT binder, it had a D12 test included. Compare the two and see the differences. IIRC TMT rated a D12 a lot higher than NAR S&T did. And we all know that D12sd are well under 20NS. Well most of us do.
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!
Reply to
Bob Kaplow
"Gary Deaver" wrote in news: snipped-for-privacy@comcast.com:
Where can I get a set of those force calibrated eyeballs?
len.
Reply to
Leonard Fehskens
ave 2k
Reply to
Jerry Irvine
It maybe because of the load cell being used. A higher rated loadcell may not be as accurate than one rate for 100lbs or less of thrust Which in turn might give different figures. Along with the already mentioned environmental conditions to learn more try this link:
formatting link

JD
differences.
Reply to
JDcluster
Bob, Thanks, I had completely forgotten that TRA sent me a copy years ago. My copy is dated June 1996.
The "D11" data is flagged as being from NAR S&T and shows a total impulse of 17.49 N*s.
Curiously, the H238 thrust time curve in the 1996 book does not show the oscillation present in the data on the TRA web site now.
Bob Kaplow wrote:
be 0.7.
gray TMT
differences.
formatting link
formatting link
Reply to
David Schultz
Hmmm, maybe I'm remembering the wrong motor, or it was published in an old TR instead of the gray book, but I do remember some NAR certified motor that TMT tested and got different numbers for. One of these days I'll have to dig through my archives...
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!
Reply to
Bob Kaplow

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.