Two D15 grains makes a ???

I just cant see how the burn area is doubled...whats the ends got to do with it? They are butted up against one another...I dont doubt what you are saying...I just can visualize the burn area being doubled.

Dont bother going into detail...my brain is overloaded with IT studies at the moment....I canna absorb no more captain!!! :-(

In WA we have the 320N rule and the 1500g rule...civil aviation safety authority enforce thoes rules. I don't believe the others apply - at least they are not enforced by anyone. I think I have seen the 62.5g propellant rule, but no one pays any attention to that, including my supplier of motors who are fairly strict about the supply of motors.

We use the NAR rules as a guide, but we are not forced by anyone to adhere to them...only thoes specificaly stated by people like CASA.

Well that would be no big deal. If I lived in the states I'd join NAR and get my level 3 but would fly mostly on F-H reloads. I doubt I would cluster (It costs me a fortune to launch my viper 3 - and thats only 24mm!).

Probably becase of the regressive thrust-pressure of the c-slot design.

It is also used in SU as a "phenolic saver".

Speaking of which I saw what I believe to be a "new" AT G80 this weekend. They sure are "pretty".

Jerry

So in other words, optimal.

:)

Which is why the rules need to be changed to a firm 160ns, or

125g-240ns, threshold.

Alcoa did :)

"Optimal".

"Optimal".

This should be in the FAQ.

Even though the ends are butted up against each other, the flames have no trouble penetrating the crack between them. Each grain burns over its entire exposed surface, including both ends and the inside of the slot. When you add a second, identical grain, it burns from all of the same surfaces as the first grain, doubling the surface area.

In an F24 motor, the grain is a single unit, about twice as long. This has less surface area than your 2 grains, since the two "middle" ends don't exist. The lower surface area, combined with a much larger nozzle throat area in the F24, means that the F24 has a chamber pressure *much* lower than your doubled D15.

To "safely" (albeit in violation of the NAR "safety" code) use two D15 grains in an Aerotech RMS 24/40 casing, you would probably want to use an even *larger* nozzle than that used by the F24. The nozzle from an F39 would probably work just fine, or you could drill out a D15 nozzle you already have to the same diameter as an F39 nozzle. The combination would still be uncertified, and therefore not allowed at any NAR or TRA sanctioned launch, but it would be much less likely to overpressurize and damage your casing, and would probably (in all likelyhood, but use a tool like BurnSim to get a more accurate prediction) work just fine.

Keeping the chamber pressure in the same ballpark as the "stock" motors is a good idea, and not just from a conservative "that's what the casing was designed for" standpoint, either. The burn rate of a given solid propellant is an *exponential* function of pressure. Relatively small increases in pressure can lead to much larger increases in burn rate, leading to still larger increases in pressure, in a "runaway" fashion.

At some point, the pressure *will* be released. If it's all released through the nozzle, then your rocket gets a nice ride. If it comes out through some other part of the casing, then you've got problems.... As you've demostrated, the 24/40 casing does have a safety margin above its normal operating pressure. How close you ame to exceeding that margin is a matter of guessing, at this point.

It's an interesting experiment, in any case.

- Rick "except in *my* 24/40 case" Dickinson

Using the a & n numbers Gary provided: 0.104" nozzle (D15 nozzle, per David Erbas-White's info) 1" x 0.5" grain 0.1" x 0.25" slot Max Pc = 195.06

0.104" nozzle 2 1" x 0.5" grains 0.1" x 0.25" slot Max Pc = 598.78 0.104" nozzle 2 1" x 0.5" grains with inhibited ends 0.1" x 0.25" slot Max Pc = 538.24 0.141" nozzle 2 1" x 0.5" grains 0.1" x 0.25" slot Max Pc = 223.61

That 0.037" nozzle throat size difference makes a tremendous difference in case pressure, as you can see.

Note that Gary's numbers may well disagree with mine as he is likely using the actual grain dimensions and has more details on the propellant than I have.

-Kevin

Let me hasten to add that these numbers are those freely available from the Aerotech Resources web page -- I didn't just pull them out of the air. I simply made myself a little Excel spreadsheet to better 'comprehend' the makeup of the various mid/high power engines that I use.

David Erbas-White

Kevin Trojanowski wrote in news:qtMne.27086$iU.5852@lakeread05:

Well,I KNEW the nozzle size had to be enlarged if adding a second slug. I was looking for data on what difference the *two extra end areas* made in case pressure,compared to a single grain.(with the larger nozzle.)

So,if you could calculate for a .141" nozzle and single grain length equal to two slugs,that should do it.

In article qtMne.27086$iU.5852@lakeread05, Kevin Trojanowski at snipped-for-privacy@nospam.cox.net wrote on 6/2/05 5:18 PM:

The D15 grain dimensions are .750" propellant O.D., .802" grain length, .125" slot width, .375" slot depth.

Plus there is some minor contribution by the delay grain to the average operating pressure.

Gary

How about with the F39 nozzle: .166" I hink that might be a better match.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Thanks; I'll rerun with those numbers.

Agreed, although I'm ignoring that.

Rerun with updated numbers:

1 grain, 0.104" nozzle Max Pc 408.35 2 grains, 0.104" nozzle Max Pc 1253.52 1 grain, 1.604" long, 0.104" nozzle Max Pc 773.37 1 grain, 0.141" nozzle Max Pc 288.8 2 grains, 0.141" nozzle Max Pc 886.55 1 grain, 1.604" long, 0.141" nozzle Max Pc 468.11 1 grain, 0.166" nozzle Max Pc 170.29 2 grains, 0.166" nozzle Max Pc 522.74 1 grain, 1.604" long, 0.166" nozzle Max Pc 276.02

Whew!

I need to have Greg rewrite this so I can script it. 8-}

-Kevin

I usually treat the delay face as a propellant face since it burns at a higher rate during burn.

In article 541oe.27395$iU.5850@lakeread05, Kevin Trojanowski at snipped-for-privacy@nospam.cox.net wrote on 6/3/05 12:12 PM:

Cool. I'm glad we had a chance to improve the S/N ratio around here, even if just a bit. :-)

Gary

Thanks for providing the numbers, Gary -- I have to admit, this was an entertaining exercise, and I may do some additional playing.

-Kevin