How to build a static test stand to measure induction/GDS stability?

[BigMacDaddy]

So I have been exploring the induction stability of the William the Bullet model and wondering if a bigger engine will create greater induction flow and thus more stability inducing forces. Seems intuitive that it would but I am not a rocket scientist.

Someone mentioned wind tunnel testing and I realized that would not be particularly useful with a model that uses induction stability. However, it got me thinking -- How could a person build a test stand that would measure the stabilizing forces created by induction / GDS in a particular model?

For example, could you use a static motor test stand on a pivot mounted at CG location. Use a load cell, pressure sensor, strain gauge and either push or pull the rocket measuring the countervailing force experienced (I suppose first pushing / pulling it without the engine firing to get a baseline).

Useful? Possible?

Again, I am not a scientist.

 

[aerostadt]

IMHO, if this could be done to some degree, this might make a good high school science project and the information could be useful to the rocket community. Almost 60 years ago I did a science project on ambient temperature effects on model rocket performance. The project worked out pretty good. I used the recording static test stand that I bought from Rocket Development Corp. Unfortunately, these stands are no longer available and researcher would have to build one on his own. That said, I do not have any ideas off the top of my head for using such a test stand in GDS research.

To make a long story short I do not believe that a straight parallel tube surrounding a solid rocket motor will produce any thrust augmentation. I pointed out on a TRF thread that the commercial augmentation tube available 60 years ago for Jetex motors for small air planes did work.

With all that having been said, there might be a simple way to show that "induction" is occurring in a static model rocket motor GDS. So, set up a static model rocket motor stand with a gas induction tube and then attach a auto vacuum gauge. Anyone can Google this. Cost will be roughly in the range of $20 to $50 and can measure up to 30" of mercury vacuum. Years ago I used a similar device on a 5-gallon vacuum chamber that housed a crude static test stand. To make a long story short it worked. This needs to be attached perhaps by rubber tubing upstream of the rocket motor on the induction tube to see how many inches of vacuum are being pulled by the induction effect. I don't think a home made manometer using water will work. Normal atmospheric pressure will support a column of water over 30 feet high. Mercury is not good to work with either.

 

[aerostadt]

For example, could you use a static motor test stand on a pivot mounted at CG location. Use a load cell, pressure sensor, strain gauge and either push or pull the rocket measuring the countervailing force experienced

This might work. I am sure it would involve a lot of work and might be expensive. A better baseline would be having a model without the induction tube firing at an angle. Of course, having external flow as some people may have suggested with a wind tunnel adds even more complication to the testing, but would be better.

 

[shockie]

" I pointed out on a TRF thread that the commercial augmentation tube available 60 years ago for Jetex motors for small air planes did work."

Url?

 

[manixFan]

" I pointed out on a TRF thread that the commercial augmentation tube available 60 years ago for Jetex motors for small air planes did work."

Url?

https://www.rocketryforum.com/threa...u-are-my-only-hope.126772/page-5#post-1494654
Maybe starts with that post? Aerostadt can verify.


Tony

 

[BigMacDaddy]

Here is an old link to the actual product...

https://web.archive.org/web/20090228021830/https://jetex.org/motors/motors-accessories.html

 

[aerostadt]

Here is an old link to the actual product...

Jetex.org: Motors - Accessories

Yes, that is the augmenter tube. I never saw that turbo-prop before. That is another digression..
In any case I remember posting my thoughts on the Jetex augmenter tube in several places. One of them listed by manixFan. Here is my hand-sketch for the Jetex augmenter tube. There are 2 sources of thrust from the Jetex motor/ augmenter tube. 1) the aspiration of ambient air into the tube will have a flow with a static pressure less than ambient. The difference in the static pressure on one side and the ambient pressure on the other side will give a net thrust. 2) The Jetex motor only has a hole for a throat and no divergent nozzle. Thus, the static pressure at the exit is based on choked flow (sonic M=1) and will be about half of the chamber pressure or fairly high. This can happen because the flow is sonic and will adjust to ambient as the flow goes outside of the motor walls. In the process some of the high static pressure could exert a force on the rear dome of the Jetex engine.

In any case I don't think a straight tube like standard GDS conception is going to have augmentation. Another classic example is the Estes Lil' Augie sold years ago. It was just a straight tube. (Edit: Actually, Lil' Augie was an Estes plan, not a kit.)