CTI 38mm 5 grain and above VMAX?

[Threemorewishes]

Where are you? Are you too energetic for a plastic threaded closure? Inquiring minds want to know.

 

[Jeroen_at_CTI]

Where are you? Are you too energetic for a plastic threaded closure? Inquiring minds want to know.

The current grain core size does not allow adding another grain without running into serious problems (trust me: it has been tried ;-)). To make a 5G Vmax motor we would have to open up the cores, resulting in the same total impulse as the 4G motor. Only difference would be 25% more thrust.
Jeroen

 

[nh4clo4]

A geometry problem.

Fast burning propellants require large nozzles. The longer the motor, and the faster the burning rate of the propellant, the larger the nozzle.

If the nozzle is not large enough, pressure will build up and :y:.

Now, the core of the grains have to be larger than the nozzle throat. If not, the last grain of the motor (the one closer to the nozzle) will become an impromptu nozzle, which leads to very erosive burning, which means :y:.

Therefore, those very fast burning motors are usually limited to 2-4 grains. There are exceptions, of course, like the N-10000. The N-10000 has a large enough diameter to allow for both a large grain core and a large nozzle. The drawback is poor volumetric loading. I'm sure someone has a nice picture of an N-10000 grain to post here, but have a look at the total impulse of the N-10000 Vs other reloads for the 98-6G casing. It is much lower, simply because there is not a whole lot of propellant in there. If we look at the propellant weight:

6778 grams of propellant for the N-2500 (Classic) in the 98-6G casing
5200 grams of propellant for the N-10000 in the same casing

For a 38 5G motor there would probably not be enough room for a significant amount of propellant...

But who knows, maybe Dr J. will find a way.

 

[nh4clo4]

Doh!...

Dr J. beat me to it and replied first.

 

[Reinhard]

(trust me: it has been tried ;-))

You just ignored the prime directive of TRF: Post pictures!

Reinhard

 

[Rocketman248]

Therefore, those very fast burning motors are usually limited to 2-4 grains. There are exceptions, of course, like the N-10000. The N-10000 has a large enough diameter to allow for both a large grain core and a large nozzle. The drawback is poor volumetric loading. I'm sure someone has a nice picture of an N-10000 grain to post here, but have a look at the total impulse of the N-10000 Vs other reloads for the 98-6G casing.

https://www.rocketryforum.com/showpost.php?p=129412&postcount=91

Yeah, the cores are pretty big on the N10000. Almost comically so.

 

[Threemorewishes]

The current grain core size does not allow adding another grain without running into serious problems (trust me: it has been tried ;-)). To make a 5G Vmax motor we would have to open up the cores, resulting in the same total impulse as the 4G motor. Only difference would be 25% more thrust.
Jeroen

So it would be a 419I1000 with a 0.419 sec burn? Sounds like the perfect motor for my Big Daddy.

 

[bobkrech]

You mean your No Longer a Daddy?

 

[Le Aeronautical]

If the nozzle is not large enough, pressure will build up and :y:.

Psh, you just need to use a thicker motor case to handle the pressure

https://www.rocketryforum.com/showpost.php?p=129412&postcount=91

Yeah, the cores are pretty big on the N10000. Almost comically so.

Man, there's a lot of iron oxide in that propellant...

 

[Aksrockets]

Psh, you just need to use a thicker motor case to handle the pressure.

Then they would have to make the case wider and thicker.
If they made it wider everyone could not use it because it doesnt fit their motor mount diameter and no one would buy it. Plus one case that fits just 1 engine wont sell well either.

 

[Jeroen_at_CTI]

You just ignored the prime directive of TRF: Post pictures!

Here ya go.

View attachment 02161103_5G_Test.pdf

 

[troj]

Here ya go.

I'm gonna guess that the funky spike in the middle is when it spat a grain?

-Kevin

 

[bobkrech]

Where are you? Are you too energetic for a plastic threaded closure? Inquiring minds want to know.

All Vmax propellants have the same energy. Energetic means something entirely different.

The gas generation rate is proportional to the burning surface area x the the linear burn rate of a given propellant type. For CTI 38 mm propellants the burn rates vary by a factor of 4.

CTI PropellantRel Burn RateClassic™1.0White™1.2Red Lightning™1.2Skidmark™1.5Blue Streak™1.5Smoky Sam™1.7White Thunder™2.2Vmax™4.0

Vmax burns ~4 x faster than Classic so it generates about 4x the amount of gas for an equivalent burning area during any time period during the burn. To maintain equivalent chamber pressure, the Vmax nozzle must be 4x larger in area (twice the diameter) to keep the same pressure. IN addition, the propellant grain bore needs to be somewhat bigger than nozzle throat. In smaller diameter cases, what Jeroen said is that you have to increase the bore so much that you would have the ~same weight of propellant in a 5G or 6G 38 casing as you have in a 4G casing. Since you need to make 1 or two more grains, the cost of a 5G or 6G 38 mm Vmax motor would be higher than the 4G and not provide any extra performance, so CTI doesn't offer it.

No amount of wishing is going to change the physics. Jeroen's thrust curve from the 5G static fire test is interesting as the initial thrust is higher than expected by just adding an additional grain. It looks like there is erosive burning on the lower grain and at the anomoly, maybe a casting liner was spit, the thrust to burnout of the upper 4 grains is at lower thrust level due to the now oversized nozzle.

Bob

 

[Le Aeronautical]

Then they would have to make the case wider and thicker.
If they made it wider everyone could not use it because it doesnt fit their motor mount diameter and no one would buy it. Plus one case that fits just 1 engine wont sell well either.

Sure, but the only time I ever make rockets to these "standard" dimensions are when I use commercial motors. Besides, it's called sand paper.

 

[Reinhard]

Here ya go.

Thanks for the openness. I'd love to see the results of "edgy" tests more often (this is in no way specific to rocketry), but unfortunately too many companies prefer to hide this kind of data. Just like the soviet space program...

BTW: I'd certainly fly that motor, as long as this behavior is reproducible.

Reinhard