[Tech] HPR Tech Article - Thrust Coefficient Losses from Straight-Cut Throats

.....Continued from Previous Post.

3) Continued....

Smaller, shorter motors using aluminized or metallized propellants will have lower residence times in the motor, thus reduced combustion efficiency, and hence producing a lower c* Efficiency Factor. Larger motors are more efficient than smaller motors, due to increased residence times, proportionally less heat transfer into the motor case and nozzle, and proportionally smaller boundary layers for lower boundary layer losses. Thus for any given propellant it is important to track the variation of the c* Efficiency Factor with motor size.

Based on representative data presented in the tech article, as the motor size is increased from 1 lb propellant weight the delivered specific impulse can be increased by 5% for 50 lb propellant weight motors, and 7.5% for 350 lb propellant weight motors.

4) For the first time to my knowledge, thrust coefficient losses from using straight-cut throats for high power and experimental/amateur solid rocket motors have been quantified. The thrust coefficient losses were much higher than most high power and experimental/amateur rocketeers probably would have expected. While most high power and experimental/amateur rocketeers have performed thrust coefficient calculations assuming no losses beyond divergence losses (equivalent to CF Efficiency Factor = 1.0), the actual thrust coefficient loss from using a straight-cut throat was found to be approximately 10% (CF Efficiency Factor = 0.90) for the straight-cut throat lengths used on most high power and experimental/amateur rocket motors. While the results were primarily based on historical Thiokol solid rocket motor nozzle thrust coefficient data, static firings with chamber pressure measurements for determining experimental values for thrust coefficient were also performed for two large high power rocket motors, the test results from which confirm the trends from the historical Thiokol data.

5) Most high power and experimental/amateur rocketeers, beyond expecting thrust coefficient losses from straight-cut throats to be small, probably expected a somewhat linear variation of thrust coefficient loss with straight-cut throat length. A surprise finding from the CF Efficiency Factor results was a definite jump in the CF Efficiency Factor, and hence the nozzle thrust coefficient as the straight-cut throat L/D was reduced to less than 0.45. Figure 28 (CF Efficiency Factor plotted versus straight-cut throat L/D) is the key figure, and graphically shows the jump in CF Efficiency Factor when the straight-cut throat L/D is less than 0.45.

The CF Efficiency Factor models I've developed from the historical Thiokol data and the recent experimental data from static firings, both of which are plotted in Figure 28, are as follows:

For Conical Nozzles with Straight-Cut Throats:

For Throat L/D < 0.45

High Performance:

CF Efficiency Factor = 0.99 - ( 0.0333 * L/D )

Low Performance:

CF Efficiency Factor = 0.95 - ( 0.0333 * L/D )

For Throat L/D >= 0.45

CF Efficiency Factor = 0.905

For Conical Nozzles with Rounded Throats:

CF Efficiency Factor = 0.99 (high performance) to 0.95 (low performance)

6) From the above models it's clear that if the nozzle designer can get the throat L/D of the straight-cut throat under 0.45, there will be a large (3.5% to 8%) jump in performance. Based on this result, I recommend the following throat design criteria for conical nozzles with straight-cut throats:

Throat Design Criteria for Conical Nozzles with Straight-Cut Throats:

Throat L/D

Post a drawing to a website (email it to me for posting if you do not have that capability) of a typical (say 75mm) HPR nozzle mold blank with the proposed shape change installed.

Tech Post Jerry

I posted my ONE request to the prior post.

Jerry

I checked out jpegs of your nozzles for your motors on your web site. Up through the 98mm motors there are frankly a lot of clones of Aerotech and Kosdon nozzles, nozzles designs I'm quite familiar with which do not have straight-cut throat L/D's less than 0.50. From the outside nozzle envelopes and knowing the convergent/divergent angles you favor it looks like a lot of your nozzles have straight-cut throat L/D's between 0.50 and 1.0. I'd be surprised if the straight-cut throat L/D's were under 0.50, although some nozzles might be just above

0.50.

Jerry, you just had to make the straight-cut throats just a LITTLE shorter, (to a throat L/D of 0.45, 0.40 for margin) and you would have improved the performance of your motors 3.5%-8%!

Most of the very large motors, where there are actual photos of the motors, have good looking short throats, although there are a few motors where the straight-cut throat L/D looks a little marginal on being below 0.50 or 0.40.

The above is similar to the nozzle survey results I included in the tech article, where 76% of E-N motors surveyed and 67% of H-N motors surveyed had straight-cut throat L/D's > 0.40, and which would get a

3.5%-8% increase in performance by reducing the straight-cut throat L/D's to less than 0.40.

Frankly Jerry, I'd be willing to bet you haven't even been tracking throat L/D on your nozzle designs. You've probably been figuring "keep the throat L/D somewhere between 0.50 and 1.0, 0.50 is probably better than 1.0". I'd be willing to bet you've never backed-out the thrust coefficient and compared it to the ideal/predicted thrust coefficient for any of your nozzles. The tech article points out techniques on how to optimize nozzle design and thrust coefficient performance using only load cell thrust-time curve data with no chamber pressure instrumentation required, but I'm guessing you didn't do a systematic study using those or any other techniques and basically have been guessing at what throat L/D to use.

It took some very detailed data to find the "magic" throat L/D of 0.45 (with margin 0.40) for the big jump in thrust coefficient performance.

0.45 or 0.40 are pretty specific numbers, not something you'd guess for a throat L/D (like 0.25, 0.30, 0.50, etc.) In fact another conclusion you can reach from the CF Efficiency Factor versus Throat L/D data plotted in Figure 28 in the tech article is that once you get the throat L/D down to 0.40, there is little additional benefit from further throat L/D reductions or even from replacing the straight-cut throat with a rounded throat. A straight-cut throat L/D of 0.40 has a performance close enough to a rounded throat, that there is no need to use a rounded throat.

Jerry Irv>

Well, if your nozzle designs are already so great, why do you need a copy of the Optimal and Universal Nozzle designs? :)

The Optimal Nozzle design is presented in the HPR tech article as Figure 29, the Universal Nozzle design is presented as Figure 30. Those interested can order the two back issues of HPR. The geometry for each nozzle is defined in the nozzle drawings based on being scaled from the throat radius/diameter. Larger or smaller throat radius/diameter, scale the nozzle geometry up or down. Included with the Optimal and Universal nozzle designs are the predicted CF Efficiency Factor based on the CF Efficiency Factor models presented in a previous post, and the calculated Divergence Correction Factor.

A nozzle pretty close to the Optimal Nozzle design was tested and the test data is presented in the tech article (Figures 22-27). It delivered a CF Efficiency Factor of 0.99. I'm claiming a CF Efficiency Factor of 0.98 taking into consideration the other data included in the CF Efficiency Factor models.

Chuck Rogers snipped-for-privacy@aol.com

You would not know this because the motors that are at issue were "lost" under your tootleidge at TRA, but all those SU motors had 0.3:1 or so. The divergent depth and exit area ratio varied a bit, but the throat was in this long known, but now characterized for HPR spec.

Too bad HPR has been killed (largely by your efforts) "just in time", eh?

I have both of course.

0.3. How close was I?

Now I am not saying I did that 100% of the time, but as a product moved into mass-production it became reasonable to emphasize details. Like targeting exit ratios of 6.5:1 if possible, but 4:1 at miniumum.

Unfortunately any foray into Tripoli as a motor sales increaser failed, and any emphasis on either unaffiliated launches (Fest) or lone rangers, was a grand success.

Hence why all the stuff you have been doing for a decade is a royal buttfuck supreme to everyone.

Consumer cerets made mandatory

Bad cert policies making larger motors be "born regulated" and inviting ATF to the party unnecessarily.

Banning every motor supplier who was bold enough to live the ATF exempt lifestyle. Several.

YOU personally started it.

Jerry

I agree. Congrats.

Jerry -

Do you always have to be such an asshole? Hijacking every tech thread into a thread against TRA or those that don't share your opinions.

Has that guy from the ATF contacted you yet?

A thread is only "hijacked" if everybody is more interested in the tangent than the original discourse...

Note that Jerry also made positive comments, but you focus on the negative...

-dave w

He started with accusatory remarks. I just cited fact in reply.

I assume you do not agree, as you WANT more regulation AND you have no frst hand information on anything related to this.

And added in his preference that ATF exceed its scope and "go after me".

For what, I do not know.

Jerry

What positive comments?

Chuck started with a tech thread. you, jerry irvine attempted to hijack and subvert the thread into yet another jerry "wackathon".

Has the BATFE contacted you, yet?

Past sins and recent deceptions, so I have heard.. (:-)

Well, based on my nozzle survey in the tech article, and a random survey of EX nozzles referred to in the tech article, apparently not.

Looks like new info to me, published for the first time with extensive professional data and recent high power/EX test data to back it up.

Because I posted 0.30 as one of the numbers that would have been a good guess in the post right before?

And all the above has what to do with nozzle thrust coefficients?

Chuck Rogers snipped-for-privacy@aol.com

How do you know what Jerry's primary interest is? I think that due to the consistency of his regurgitating the same old shit, his only objective here is as I stated. The rest is a diversion.

Did you notice how Jerry first came out with how he has always done things as Chuck's artile suggests. Then he asks for pictures of the nozzels. Based on this my only conclusion is that Jerry is running his mouth and has no clue of what the entire article says.

Mommy - Jerry said regulation.

If I wanted ATF to go after you, I'd do more than bust your chops about it. I think what you say on public forums makes you worthy of a closer look by ATF. I doubt if my opionon on this matters much since your friends at DOT have put in a few good words for you.

David, this was clearly a tech thread. It was labeled as a tech thread. On the 6th post Jerry was already working on derailing the tech thread, which he apparently has succeeded at.

Jerry's strategy; If the thread is not about him, or his Wackathon issues, derail it.

It seems that it's something like 4 or 5 to 1 that the majority of the participants in this thread would like to see it stay a tech thread, not a Jerry Wackathon thread.

Jerry Irv>>Hence why all the stuff you [Chuck Rogers] have been doing for a decade is a royal

buttf*&k supreme to everyone.

Dave; That was a real positive comment. Were there any others I missed?

Chuck Rogers snipped-for-privacy@aol.com

As I have repetitiously stated, you have been the leader of banning ATF exempt willing motor suppliers since at least 1992.

That is your legacy.

As I said I agree with that and to repeat myself, kudos.

:)

Jerry