Vacuum Assisted Takeoff

[Timothy Allen]

I saw an article about vacuum powered ping pong ball canons and thought it might be fun to replace the ping pong ball with a rocket. In a vacuum canon a ping pong ball is placed in the end of a PVC pipe, the ends are sealed with foil and the air is evacuated with a pump. The end closest to the ball is opened and atmospheric pressure accelerates the ball out the other end. With good technique the ball can hit a bit below mach 1. I thought with a rocket I might be able to break mach 1. I haven't seen this done with a rocket and I'm not sure how NAR would feel about it, but it could be fun.

[nh4clo4]

It works well with a ping pong ball because it is round and fits tight inside the tube. If you want to try something like this with a rocket you will have to fill the area between the fins or add some kind of piston under the rocket in order to get some lift.

What happens when you reach the other end of the tube?

[SwingWing]

Some local physics professors used to do a demonstration of the ping pong cannon at a high school physics night. I believe the tube was 1 1/2" pvc drain pipe and the ends were simply covered in a couple of layers of clear packaging tape. They simply touched the back end with an exacto knife, and shot the pp ball through 2 empty pop cans.
I don't think the pp ball fit very tight in the tube, and it simply tore through the packing tape on the exit end.
With a rocket, I would think a 3 or 4 petal foam piston would work nicely. you could use a second ignitor to rupture the tape seal on the back end.
I do not think any of this would be NAR approved stuff.

[Timothy Allen]

The rocket would require a chapeau (probably a paper cone that would go from around the engine out to the walls of the pipe). Both ends of the pipe are covered with aluminum foil which is strong enough to hold the vacuum, but will rupture easily when contacted by the nosecone of the rocket. I am hoping that the ignition of the engine will be enough to rupture the foil on the rocket's end of the pipe, but that will need to be tested.

[CarVac]

Do black powder motors ignite in a high vacuum? If not then this would be hard to do.

I wonder if you can use them for high altitude deployment.....I imagine that if the motor ignites it will stay lit due to internal pressure.

[Timothy Allen]

I'm not sure that it would work at all, I know that composite motors can ignite in vacuum, so i might need to use one instead. I own a vacuum pump and PVC is cheap, so I figure I'll probably give it a try.

[luke strawwalker]

Do black powder motors ignite in a high vacuum? If not then this would be hard to do.

I wonder if you can use them for high altitude deployment.....I imagine that if the motor ignites it will stay lit due to internal pressure.

No reason I know of why they wouldn't... after all the oxidizer and fuel are pre-mixed and uses no oxygen from the atmosphere...

Later! OL JR

[Rex R]

a low tech version uses a shop vac, two pieces of cardboard, and a length of pvc pipe with a tee fitting at one end.
rex

[G_T]

Realize that as one scales this up, the forces involved scale as the area, or square of the size. The energy storage scales as the volume... These things can get quite dangerous quite quickly. A vacuum chamber is the reverse of a pressure chamber and needs to be engineered for the forces involved. The "cone" at the back of the rocket may need to support and transmit quite high forces to the rocket - potentially forces exceeding what normal thrust would generate for that same rocket. The window at the end of the tube needs to get much stronger as the size increases. Run some numbers and do a bit of engineering please before attempting this with a rocket.

The piston rocket approach is likely much safer and easier to engineer. Something like an ARCAS launch, though probably on a much smaller scale. But even so it needs to be engineered.

0.02c

Gerald

[Timothy Allen]

I agree about the scale issue. I don't plan to exceed the 1.5 inch pipe that the ping pong ball uses (I figure I can put an A engined rocket into the tube easily and that way I'm not using too much mass. This was the reason that I thought I could use a paper chapeau it could be supported by the swept fins and if the forces are too much I would prefer that the paper ruptured rather than anything else.

[CarVac]

A rocket will not get nearly the boost a ping-pong ball does. If you compare the aft surface area of a rocket (even with a sabot) to its weight, it's going to be far, far less than that of a ping-pong ball, so it won't be accelerated anywhere as fast by the incoming air as a ping-pong ball will.

[[POW]Eagle159]

Wouldnt a really long tube/ pipe work, so it could accelerate more?

This sounds like a cool idea, I never heard bout it till today.

[BEAR]

In response to the question about black powder rocket motors burning in a vacuum, I beleive they should. We launched a rocket in the swimming pool at the Holiday Inn at NARAM 14, (I think) and it was a successful launch. If the motor will burn under water, it should burn in a vacuum and be a true rocket motor, supplying it own fuel and oxidizer. I think for this vacuum launcher, you will need, as G_T and CarVac have already mentioned, you probably would be more successful with sabots like those used in the breech loaded launcher of the ARCAS. (As for the underwater launch, management at the motel was less than thrilled when we turned all of the water black from the exhaust. They threatened to evict us all from the motel, except that we were a money maker for them and the place was sold out, along with the rest of the motels in Nassau Bay, where the Manned Spacecraft Center was. They did not get over it as quickly as we thought they should have either. They were really pissed.)

[G_T]

Lighting a motor underwater is a different proposition to lighting one in a vacuum. For a rocket motor, it is not the presence of oxygen which matters. It is the ambient pressure. The lower the pressure starts, the greater the difficulty the motor has in coming up to pressure. Underwater is at higher than atmospheric pressure.

Gerald

[bobkrech]

I saw an article about vacuum powered ping pong ball canons and thought it might be fun to replace the ping pong ball with a rocket. In a vacuum canon a ping pong ball is placed in the end of a PVC pipe, the ends are sealed with foil and the air is evacuated with a pump. The end closest to the ball is opened and atmospheric pressure accelerates the ball out the other end. With good technique the ball can hit a bit below mach 1. I thought with a rocket I might be able to break mach 1. I haven't seen this done with a rocket and I'm not sure how NAR would feel about it, but it could be fun.

You need to breakdown you question into several separate ones in order to find a solution.

1. Why and how vacuum cannon works. The vacuum cannon works because the pipe is evacuated to a low pressure which allows a 1 atmoshere pressure differnce to accellerate a light ping pong ball to about Mach 0.8 in a relative short barrel.

The maximum force that can accelerate the ball is PA = 14.7 psi x pi x 0.75"^2 sq.in. = 14.7 psi x 1.77 sq.in. ~ 26 pounds. The ping pong ball weighs about 0.005 pounds, so the initial thrust to weight ratio is 26/0.005 ~ 5200. The initial acceleration in units of G is T/W-1 or ~5199 G or ~ 166.4 kft/s/s which is a very high acceleration rate. This rate can not be sustained due to leakage around the ball which lowers the pressure differenctial and conduction limitations to allow air to flow into the tube. These items and a few more complicated physical phenomon limit the ping pong ball velocity to ~ Mach 0.8 regardless of the tube length and if the tube is much longer than 2 or 3 M the muzzle velocity will actually be lower due to internal drag. The time in the barrel is not more than 0.01 seconds in this length barrel.

2. A sabot is required to make a circular seal around a rocket. The weight of the sabot and the rocket is likely to be 50 to 100 times that of the ping pong ball which will reduce the acceleration by a factor of 50 to 100 or to ~50 to 100 G. Residence time in the barrel increases to 0.06 to 0.1 seconds, with a final velocity in the Mach 0.14 to Mach 0.2 range. This is significantly slower than the ping pong ball and comparable to what the same rocket would obtain on a standard launch rod.

3. By definition, a rocket motor operates fine in a vacuum as well as the air as the propellant contains a fuel and oxidizer. The problem is not operation but ignition. A BP hobby rocket motor needs air to transfer energy from the igniter to the propellant grain. At pressures equivalent to 20 kft or higher, there iis not enough air to transfer the heat from a standard model rocket igniter to the propellant grain surface and the motor will not ignite. You need to have a hermatic seal at the nozzle to the motor remains pressurized near atmospheric pressure to ignite a BP motor in a vacuum.

4. Most of the impulse from a BP motor would be delivered after the rocket exits the barrel. The rocket would be subjected to atmospheric drag outside the barrel so the rocket performance would not be significantly enhanced by a vacuum cannon.

Bob

[JordanT]

I believe the vacuum argument is very close to the same one Robert Goddard had with his contemporaries - that a self-contained propellant would, indeed, provide thrust in the absence of an atmosphere. As for black powder burning, the flame front / thermal energy simply needs a mechanism to sustain the deflagration, which is why loose black powder will not effectively ignite in a vacuum, but a confined charge (packed with air, or as a single grain) will. (http://projects.fit.edu/jamstar/html...d_chamber.html)

[Salvage-1]

The needed sabot would also sap energy due to the friction inside the barrel when the item is being blasted forward.

If the rocket motors exhaust was sealed with tape, this would trap a little air inside. If the motor were a CTI which has a plug of pyrodex, ignition should not be that much of a problem.

If then you used the ignition and motor pressure of the rocket to open then rear seal of the vacuum chamber, the should motor would be running as the projectile ran the barrel.

Although something in the back of my brain is telling me that there is / was a military application of this (or it was researched and dropped).

[JStarStar]

Some local physics professors used to do a demonstration of the ping pong cannon at a high school physics night. I believe the tube was 1 1/2" pvc drain pipe and the ends were simply covered in a couple of layers of clear packaging tape. They simply touched the back end with an exacto knife, and shot the pp ball through 2 empty pop cans.
I don't think the pp ball fit very tight in the tube, and it simply tore through the packing tape on the exit end.
With a rocket, I would think a 3 or 4 petal foam piston would work nicely. you could use a second ignitor to rupture the tape seal on the back end.
I do not think any of this would be NAR approved stuff.

It certainly would not be legal for NAR competition, since the rocket would be gaining launch momentum from a source other than the motor.

However, I don't think there would be any reason it would not be legal for "sport" or "research" flying, as long as the launch device was designed so as to provide guidance for the initial direction of launch.

[bobkrech]

I believe the vacuum argument is very close to the same one Robert Goddard had with his contemporaries - that a self-contained propellant would, indeed, provide thrust in the absence of an atmosphere. As for black powder burning, the flame front / thermal energy simply needs a mechanism to sustain the deflagration, which is why loose black powder will not effectively ignite in a vacuum, but a confined charge (packed with air, or as a single grain) will. (http://projects.fit.edu/jamstar/html...d_chamber.html)

If you check out the JAMSTAR website you'll see me referred to as "Dr. Bob" in the midterm and final reports. http://projects.fit.edu/jamstar/JAMS...erm_Report.pdf and http://projects.fit.edu/jamstar/JAMS...NAL_REPORT.pdf

I helped them with high altitude deployment, electronic, and ground test procedures. I had them contact Adept and purchase the apogee deployment electronics and told them how to do the "hazard chamber" testing.

FYI - Since rocket motors carry their own oxidizer, there's never a question if they will operate in vacuum. The issue is will they ignite in vacuum, and the simple answer is not with a hobby rockt motor igniter unless you pressure seal the motor so that heat is transferred from the igniter to the propellant grain. The problem is the initial pressurization of a vented motor casing. A simple hobby rocket motor igniter does not produce enough gas to pressurize a vented casing above 1 atmosphere for a long enough time for the grain to deflagrate. Read all about how need to design an igniter for high altitude applications in NASA SP-8051.

Bob

[billspad]

The issue is will they ignite in vacuum, and the simple answer is not with a hobby rockt motor igniter unless you pressure seal the motor so that heat is transferred from the igniter to the propellant grain.

Getting back to what the original poster proposed, I can tell you for a fact that you can ignite a black powder motor in the kind of vacuum you can pull with an air conditioner compressor. I did it over 40 years ago when I was in high school. I don't remember how deep the vacuum was but I do remember running the compressor for a long time. The igniters were whatever Estes was selling at the time held in with a piece of wadding.

[bobkrech]

Getting back to what the original poster proposed, I can tell you for a fact that you can ignite a black powder motor in the kind of vacuum you can pull with an air conditioner compressor. I did it over 40 years ago when I was in high school. I don't remember how deep the vacuum was but I do remember running the compressor for a long time. The igniters were whatever Estes was selling at the time held in with a piece of wadding.

I wasn't thinking about an old pure nichrome wire igniter in firm contact with the BP grain. That might work, but for BP motors only. APCP won't ignite below about 0.4 atm. without a really honking igniter.

However in this application, the rocket has a substantial mass, much greater than a 2.4 g ping pong ball.

Bob

[billspad]

I wasn't thinking about an old pure nichrome wire igniter in firm contact with the BP grain.

I believe the Estes igniters at the time were the ones that came as a strip of three with pyrogen coating the nichrome. It's also possible that I used bare nichrome. Either way it was definitely in firm contact with the propellant.

That might work, but for BP motors only. APCP won't ignite below about 0.4 atm. without a really honking igniter.

There have been days when I couldn't get APCP to light at 1 atm.

However in this application, the rocket has a substantial mass, much greater than a 2.4 g ping pong ball.

I agree that the vacuum probably won't give him that big a push but if he wants to try it the motor will fire.