Propep and Burnsim

The Rocketry Forum

Help Support The Rocketry Forum:

JackO

MIT AeroAstro Undergrad -- HSRPL Founder
Joined
Mar 13, 2016
Messages
109
Reaction score
0
Location
Los Angeles
Hi guys,

Ok, I have spent far too long running numbers without any results, and no matter who I ask or what I read... I'm going in circles. Can someone who uses or knows both propep and burnsim please explain this nonsense to me.

When I plug the lsp* number from propep (Isp* to my understanding refers to characteristic ISP which means the specific impulse of the propellant at 1000 psi without the benefit of an expansion cone getting this from Prof. McCreary) into Burnsim I get great results since the Isp* is 195. High, but with high solids and Al, not unreasonable.

However, Propep also gives me a C* value, but a rather low one one- 5104. When I plug in 195 for the Isp* in burnsim it auto-fills the C* value to 6270...when I plug in the C* value of 5104, it auto-fills the Isp* value to 159.. which seems low.

So, which value would you trust, I'm leaning towards Isp* right now because even if the value is way too high, at least the motor won't CATO as a result, but I'm a perfectionist, and I want to understand why the programs are being so non-compliant.

We have great thrust data, and decent pressure data from our characterization tests, something as simple as the simulation program should not be holding us up.

If this post violates the limits of the non-research category by all means take it down, but first, please DM me and tell me what's the problem.

Thanks,
Jack
 

tuxxi

Well-Known Member
Joined
Nov 4, 2012
Messages
86
Reaction score
0
Always use C*

5104 f/s for C* is not low at all. SRB propellant (very near the practical limit for AP/Al propellants afaik) has a C* of 5180 f/s. This translates to an Isp* of 160.8 seconds, since Isp* should be C*/g as you noted. I'm not sure what PEP is doing with an Isp* of 195.
 

JackO

MIT AeroAstro Undergrad -- HSRPL Founder
Joined
Mar 13, 2016
Messages
109
Reaction score
0
Location
Los Angeles
Always use C*

5104 f/s for C* is not low at all. SRB propellant (very near the practical limit for AP/Al propellants afaik) has a C* of 5180 f/s. This translates to an Isp* of 160.8 seconds, since Isp* should be C*/g as you noted. I'm not sure what PEP is doing with an Isp* of 195.
Tuxxi, while I agree that 195 is too high, would you feel safe designing a motor to that low an ISP*? I figure I don't have much of a choice but to use 195 for sims now because it'd be irresponsible to design with a lower number by accident and lead to some nasty erosive burning and a CATO.

We got an Isp* of 188 with an average chamber pressure of only around 280, so it seems unlikely to me that the propellant would drop all the way down to 156 without an expansion nozzle but at 1000 psi.

-Jack
 

tuxxi

Well-Known Member
Joined
Nov 4, 2012
Messages
86
Reaction score
0
You probably mean delivered Isp of 188 at Pc 280. Isp* in BurnSim is just C*/g. BurnSim's use of Isp* as an input parameter is extremely confusing, so I recommend using only C*.

Use the pressure trace you got from the test burn to calculate C*, and use that in BurnSim.

where At is throat area, Mp is propellant mass
 
Last edited:

Handeman

Well-Known Member
TRF Supporter
Joined
Jan 18, 2009
Messages
7,868
Reaction score
433
Location
Stafford, VA
Just a question, if you "We have great thrust data, and decent pressure data from our characterization tests" why are you not calculating the a and n number and plugging those into Burnsim? I've found those to work very well.

Also, if you are using an Al mix, are you doing your test burns with 75mm or larger motors? AL doesn't work well with motors much smaller then 75mm. At least the characterization numbers from the small motors don't translate to larger motors very well.
 

JackO

MIT AeroAstro Undergrad -- HSRPL Founder
Joined
Mar 13, 2016
Messages
109
Reaction score
0
Location
Los Angeles
You probably mean delivered Isp of 188 at Pc 280. Isp* in BurnSim is just C*/g. BurnSim's use of Isp* as an input parameter is extremely confusing, so I recommend using only C*.

Use the pressure trace you got from the test burn to calculate C*, and use that in BurnSim.

where At is throat area, Mp is propellant mass
Tuxxi, that is great advice; thank you. I'll try this method and see what I come out with.
 

JackO

MIT AeroAstro Undergrad -- HSRPL Founder
Joined
Mar 13, 2016
Messages
109
Reaction score
0
Location
Los Angeles
Just a question, if you "We have great thrust data, and decent pressure data from our characterization tests" why are you not calculating the a and n number and plugging those into Burnsim? I've found those to work very well.

Also, if you are using an Al mix, are you doing your test burns with 75mm or larger motors? AL doesn't work well with motors much smaller then 75mm. At least the characterization numbers from the small motors don't translate to larger motors very well.

Handeman,

I am plugging the calculated a and n into burnsim, and now that I have established 157 as the Isp*, I can finally check the calculations to the actual tests with only the a and n variables to worry about. Strangely, I have found that the a and n from the Propel Ballistic spreadsheet (weird numbers .0422 and .266) are not giving the delivered performance we found. I might at this point ditch the spreadsheet and guess and check with the a and n until I get the right performance for both tests.

Not to worry, the tests and this larger motor are 5" OD (Only 4.13" grains as a result of all the insulation but still large enough to burn all/most of the Al).
 
Top