One caution - I'm pretty sure from context that 296 seconds theoretical
max for cold-gas hydrogen I saw quoted is a vacuum figure. (No mention
of how much expansion though.)
If you're getting results in the 320-340 second range, before trusting
the program too much you might want to look carefully for better
real-world data to check against than my hasty google search.
Henry
On 3/13/2017 8:14 AM, Galejs, Robert - 1007 - MITLL wrote:
Henry -
The 218 s figure is what Propep 3.0 displays on the primary screen. Looking
at the tabulated results, that number appears to be for an expansion ratio
of 1.0. The sea level optimum expansion ratio is about 8 or so that yields
an ISP of about 280. The vacuum ISP gets well into the 300s, 320-340
depending on how big an expansion you care to consider.
Thanks for verifying that these ISPs are reasonable.
- Robert
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Henry Vanderbilt
Sent: Monday, March 13, 2017 10:31 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: WProPep question
That would be in effect a hydrogen cold-gas thruster, and the Isp sounds
reasonable. Maybe even a bit low - I find theoretical Isp for an H2
cold-gas thruster listed at 296 seconds in a couple of places.
Coming at it another way, nitrogen cold-gas thrusters get practical Isp
around 70 seconds with a molecular weight of 28. H2 is 1/14th of that,
square root of 14 is around 3.7, times 70 gives around 260 seconds, rough
ballpark.
Are you under-expanding the H2 to get only 218 seconds?
Henry
On 3/13/2017 7:03 AM, Galejs, Robert - 1007 - MITLL wrote:
I got Propep 3.0 downloaded and running and can match the results fromMcCreary
a few Propep example files, so I think I understand how this works.
However, I found a case that looks odd to me. If you run it with
gaseous hydrogen as the only propellant, it gives an ISP of 218. Is
this really the case, or are there some other assumptions getting in
the way like maintaining a 1000 psi chamber pressure that adds energy
or something.
Thanks,
Bob
*From:*arocket-bounce@xxxxxxxxxxxxx
[mailto:arocket-bounce@xxxxxxxxxxxxx] *On Behalf Of *Terry McCreary
*Sent:* Wednesday, March 08, 2017 4:15 PM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Re: WProPep question
Propep 3.0 is available free at http://tclogger.com/ There's also a
version that runs under Excel but I don't have it handy.
On 3/8/2017 2:52 PM, Galejs, Robert - 1007 - MITLL wrote:
I'm trying to get WPropep to run on my computer but am not having
any luck. I'm guessing that wpropep.exe calls propep.exe at some
point, but I can't get propepe.exe to run by itself. I get an error
message to check how many bits my windows operating system (32 or
64) is and then ask for the appropriate version of propep.exe but I
don't see any indication of what operating system is assumed for
Propep. Any suggestions?
Thanks,
Robert
*From:*arocket-bounce@xxxxxxxxxxxxx
<mailto:arocket-bounce@xxxxxxxxxxxxx>
[mailto:arocket-bounce@xxxxxxxxxxxxx] *On Behalf Of *Terry McCreary
*Sent:* Wednesday, March 08, 2017 1:12 PM
*To:* arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
*Subject:* [AR] Re: ISP vs thrust?
Propep will calculate Isp at different pressures.
On 3/8/2017 9:49 AM, Galejs, Robert - 1007 - MITLL wrote:
Thanks Terry, that's the kind of rules of thumb/potential issues
that I was looking for.
I don't suppose there would be some sort of general ISP vs
pressure, would there.?
- Robert
*From:*arocket-bounce@xxxxxxxxxxxxx
<mailto:arocket-bounce@xxxxxxxxxxxxx>
[mailto:arocket-bounce@xxxxxxxxxxxxx] *On Behalf Of *Terry
*Sent:* Wednesday, March 08, 2017 8:48 AM
*To:* arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
*Subject:* [AR] Re: ISP vs thrust?
Hello Robert,
There will be some practical difficulties constructing a 1000 Ns
motor with very low thrust. For example, to keep Isp at similar
levels for all motors will require them to operate at similar
pressures. A motor operating at 600 psi requires a nozzle
throat of slightly over 1/16" for 10 N of thrust; about 1/50"
diameter for 1 N thrust! Even the smallest amount of solid
residue at the nozzle throat would cause a significant if not
dramatic increase in chamber pressure.
Best,
Terry
On 3/7/2017 8:12 AM, Galejs, Robert - 1007 - MITLL wrote:
Is there any general relationship between ISP and thrust for
solid rocket motors? In particular, I'm wondering about
making something like a 1000 Ns motor in several variants:
1000 N, 100 N, 10 N and 1 N thrust levels. Maybe there is
some minimum practical thrust? Pointers to any good
references on this are appreciated!
Thanks,
Robert
--
Dr. Terry McCreary
Professor of Chemistry
Murray State University
Murray KY 42071
--
Dr. Terry McCreary
Professor of Chemistry
Murray State University
Murray KY 42071
--
Dr. Terry McCreary
Professor of Chemistry
Murray State University
Murray KY 42071
