Vacuum launch tube?

[AllDigital]

My son and I have been toying with a launch idea, but I figured someone must have considered it long before us. A thorough search of the internet yielded nothing, so I'm opening up here for dialog.

Here is the hypothetical setup: Take 20 feet of 1.5" PVC and mount it vertical as a launch tube. Cover each end in lightweight mylar from a balloon. Make a rocket that is exactly the same diameter as the ID of the launch tube (e.g. 1.25") and equip it with spring loaded fins. Place the rocket and a remote ignitor in the bottom of the tube, before securing the bottom mylar. Then pull a vacuum on the launch tube and turn a valve to close it off. Stand back and light up the ignitor.

When the engine lights up it will pop the bottom mylar, thus breaking the vacuum. Air will enter the bottom of the tube at 500mph giving the rocket lift, while the engine thrust will also push it up with zero drag, until it hits the top mylar, punches through, and hits solid air at 21 feet, but going XXX mph. I think, freakishly faster than a normal launch.

This is based on a the same concept as a ping pong cannon we built in our garage, but a ping pong ball is obviously more conducive to air "thrust". Here is a video of the ping pong experiments. The ball gets going 400-500 mph when it exits the tube, enough to go through a coke can. only using a vacuum in a ten foot tube.

[video=vimeo;127385173]https://vimeo.com/127385173[/video]

If we could get this to work with a model rocket what would the equation look like for the benefit? It seems to me we'd get increased thrust newtons from the air coming in and we'd have decreased drag for the first 20 feet.

thoughts?

 

[AllDigital]

Additional searching yielded this thread...

https://www.rocketryforum.com/showthread.php?39490-Vacuum-Assisted-Takeoff

I knew someone had to have discussed this before. It still looks like an interesting science project to see if the results hold to the math with a very small rocket.

 

[bobkrech]

The force on the bottom of a 1.25" diameter object generated by a 14.7 psi pressure difference is ~80 N.

So the question becomes how light can you make your rocket? I seriously doubt you can't make a rocket for this launcher weighing less than 50 grams (0.5 N) and more likely not less than 100 grams (1 N). The former would be accelerated at a = T/W = 80N/0.5N = 160 G and the latter at 80N/1N ~ 80 G.

The barrel time would be t = sqrt(2d/a) = sqrt(2x20/(160x32.2)) ~ 0.09 s and v = at = 160 x 32.2 x 0.09 ~ 463 fps (M~0.4) and more likely t = sqrt(2d/a) = sqrt(2x20/(80x32.2)) ~ 0.4 s and v = at = 80 x 32.2 x 0.12 ~ 309 fps (M~0.3).

The drag forces on a rocket at low velocity are not high and is proportional to Mach number cubed. At M = 0.4 the drag on a 1.25" rocket will be only ~2.5 N, and at M = 0.3 ~ 1 N.

Ping pong balls weight 2.7 grams (0.026N) and would theoretically accelerate much faster (80N/0.026N=3076 G) and to a much higher theoretical velocity (~2000 fps o rM~1.8) than any rocket you might be able to construct for your launcher, but note that due to residual air drag in the barrel due to the leakage around the ball, the ping pong ball will not come close to this velocity and can just exceed Mach 1.

While the calculated muzzle velocity for the proposed rocket is rather high, the proposed launcher would be very difficult to transport and operate. The rocket would also have to form a tight seal around it to transfer the pressure energy to rocket which would add friction and not be easy to make. A 50 to 100 gram would likely have not more than a C-impulse motor, and the pad weight of a 1.3" Estes rocket with a C motor can be less than 100 g and it can reach the same Mach number, but at a higher altitude. Any descent 29 mm rocket on a mid-power APCP motor, or a high power 24 mm or 29 mm motor will seriously outperform the gun launched rocket in terms of both altitude and velocity.

The reason why the ping pong gun works is that the ball is extremely light, so it accelerates very fast. The downside is that it decelerates very fast as well. While it may actually get a ping pong ball slightly over Mach 1 under the best condition, the ball will decelerate rapidly once in the atmosphere so you only get the high velocity very close to the muzzle of the air cannon.

Bob

 

[stealth6]

I'm not sure what you mean by "a remote igniter", so you maybe have thought of this already. If not.....

Wouldn't there be a problem getting an igniter to work in a vacuum?

s6

 

[Bat-mite]

Sounds like a fun science experiment, if you have the time to spend on it; but I could never see this catching on as a new way to launch rockets, even if it works.

 

[bobkrech]

I'm not sure what you mean by "a remote igniter", so you maybe have thought of this already. If not.....

Wouldn't there be a problem getting an igniter to work in a vacuum?

s6

Rocket motors work in a vacuum as do igniters because rocket motors and igniter pyrogen both contain an oxidizer. The only question is the ignition delay of the motor due to the vacuum so the igniter might need to be boosted with extra pyrogen.

Bob

 

[Winston]

My son and I have been toying with a launch idea, but I figured someone must have considered it long before us. A thorough search of the internet yielded nothing, so I'm opening up here for dialog.

Here is the hypothetical setup: Take 20 feet of 1.5" PVC and mount it vertical as a launch tube. Cover each end in lightweight mylar from a balloon. Make a rocket that is exactly the same diameter as the ID of the launch tube (e.g. 1.25") and equip it with spring loaded fins. Place the rocket and a remote ignitor in the bottom of the tube, before securing the bottom mylar. Then pull a vacuum on the launch tube and turn a valve to close it off. Stand back and light up the ignitor.

When the engine lights up it will pop the bottom mylar, thus breaking the vacuum. Air will enter the bottom of the tube at 500mph giving the rocket lift, while the engine thrust will also push it up with zero drag, until it hits the top mylar, punches through, and hits solid air at 21 feet, but going XXX mph. I think, freakishly faster than a normal launch.

This is based on a the same concept as a ping pong cannon we built in our garage, but a ping pong ball is obviously more conducive to air "thrust". Here is a video of the ping pong experiments. The ball gets going 400-500 mph when it exits the tube, enough to go through a coke can. only using a vacuum in a ten foot tube.

[video=vimeo;127385173]https://vimeo.com/127385173[/video]

If we could get this to work with a model rocket what would the equation look like for the benefit? It seems to me we'd get increased thrust newtons from the air coming in and we'd have decreased drag for the first 20 feet.

thoughts?

Besides the other factors already mentioned in detail, I think you may have some high temperature exhaust related issues with the internal surface which might at least roughen it up. Also, although commercial motors are designed for nozzle or end plug failure to prevent a burst casing, you can't count on that 100% and PVC shatters into fragments which is why HDPE is used instead for fireworks mortars.