Zero Friction launch tube?

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Well-Known Member
Apr 8, 2004
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I was watching a show on the discovery channel last night about transportation spanning the pacific underwater using incredibly high speed trains.
Of course you think high speed is probably around 200-300 mph? right?
Well someone came up with the idea of removing all the air from the tunnels.
What effect does this have speed wise?
Well with no air there is no FLUID friction this caused incredible increases in speed with test models in labs. Speeds upwards of 2000 mph as i remember.
But then they thought well we still have the friction between the trucks on the train and the rails...well and i know this isn't really anything knew b/c i think this has been experimented with in the UK for sometime. But they put the thing on huge magnets! This means there is zero friction.
Of course there would need to be some type of acceleration period so that the passengers on board wouldn't dissenigrate from the incredible g-forces.

But I got to thinking today.
And I'm not sure if this has been tried so don't poke fun if it has.
But what if there was someway to make a tube launcher and remove all the air from it. Now I know this would cause a problem with the engine igniting. but here me out.

Make a tube with sometype of lid that is airtight but can be triggered or pushed through by the rocket, then remove all the air. Of course this would have to be porportionaly larger in height than the roc to have an effect.
Then have some type of piston or spring inside of the tube to push the roc to higher speeds. Then as soon as the rocket leaves the tube some type of eletronic system would ignite the engine.

I think I explained it well enough?
Does it seem like a logical try? or even a start of something in some direction...of course this would probably be realitvely expensive but this would be a great competition type thing.

post all your comments.

i don't know if its practical, or if it would really help you gain much if any altitude, but it is interesting:cool:

you said that it would be a problem w/not beeing able to have the engine ignite in an air-less tube, but i hafta disagree... even BP motors don't need the oxygen in the air to burn; they have their own oxydizerz :p this would make the design allot more simple. of course, similiar to the train situation, you will still have the problem of needing some sort of guidence system that would cuase friction (rod, tower, piston, etc).

verry cool idea, be interesting to see what others think.
Originally posted by moocrew

Make a tube with sometype of lid that is airtight but can be triggered or pushed through by the rocket, then remove all the air. Of course this would have to be porportionaly larger in height than the roc to have an effect.
Then have some type of piston or spring inside of the tube to push the roc to higher speeds. Then as soon as the rocket leaves the tube some type of eletronic system would ignite the engine.

It's an interesting idea... here are my random thoughts.

As noted, you'd need a rail launch guide, but that wouldn't be a big deal, other than your vacuum tube has to be large enough to accomodate the fins.

You'd have to put a vent hole in the rocket... otherwise when you depressurize the launch chamber, the higher pressure inside the rocket dislodge the nose cone. I wonder if suddenly entering atmosphere would cause the rocket to collapse... if the hole is big enough to avoid collapse, is it big enough to cause too much loss of ejection pressure?

Think about this... what happens when point the rocket breaks through the vacuum seal? Air rushes in to fill the vacuum, and where is the rocket? Trying to escape the mouth of the tube where all that air is rushing in. Will the rocket be going fast enough to escape before inrushing air becomes a problem? Because of the next issue, the mouth of the tube will be open before the rocket even gets there. The rocket's going to be trying to escape a vortex of inrushing air.

Note that you'll have to have some mechanical device to open the tube and it's going to have to be very fast. Zero atmosphere inside the tube means it will take a lot of force to remove the seal on the end... it's not like you can use tissue the rocket can tear through. The rocket isn't going to be able to force its way out. (Okay, I'm thinking low-power. If you want to get into high power, you could probably force your way out, but that would certainly be a lot of "friction", the avoidance of which is the point of this exercise.)

If you're really going to near zero atmosphere, what happens to the relatively open cell structure of balsa? Is there enough air inside the wood to cause the balsa to expand? If you put a marshmallow inside a vacuum bell, it grows to giantic proportions... will your nose cone do the same thing?

In short, it's an interesting idea, but I don't see how you could make it work.

If you're looking for more speed/height, better to reverse your pump and attach it to the bottom of a piston launcher and pre-pressurize the piston before igniting the motor. Doesn't solve the friction problem, but adds energy to the launch.
It almost sounds like you'd want to pressurize the tube, so that the OUTrushing of air matches the rocket's speed - zero relative speed, zero friction.

But at the point you leave the launch rod you're only going, what, 30-40 MPH? How much friction would you expect at those speeds, compared to air friction during flight?
yeah I was sure there were plenty of things i hadn't thought of. I did however think of the "vacum" effect the tube would have on the rocket. Can't really think of any good way to deal with that.
Doesn't seem like this would work well or even at all if there was a need to switch between the two pressures. But whos gonna build a guide tube to the outer atmosphere.. :eek:

I wonder that if there was a way to make it so that after the roc launched and started moving through the tube that right before it got to the top "lid" the bottom of the tube opened...causing the tbue to fill in from the bottom. This might help decrease the amount of vacum coming from the top and mabey the in rushing air would push a little on the roc.

Of course im no math wizard or rocket scientist so i have no idea if the two would equal out and cause nothing and the whole thing would be a waste but it seems like if effort was put into it this "Vacum Launcher" it might give small increases in performance. or it might just blow your roc apart :D

its time like this i wish i had Albert Einstiens brain...........

The only reason it works for the train is that the train is inside the tube for its entire trip.
Originally posted by jflis
inside the tube for its entire trip.

Aww man...simple solution.


:D :D :D


Unless the tube is very long, you would not gain a significant advantage.

Aerodynamic drag is proportional to 1/2*Cd*A*Rho*V^2 where Cd is the drag coefficient, A is the cross-sectional area, Rho is the atmospheric density, and V is the velocity. The density is quite low and the velocity is low in the initial launch phase of a rocket flight and the motor thrust dominates the velocity.

Let's compare 2 cases and see what difference a vacuum might make for a model rocket and a high performance high power rocket.

A model rocket like an Estes Alpha 3 on a C6 motor at sea level has a velocity of 312 ft/sec after 1 second and has traveled 168 ft. Take the air out and it reaches 351 ft/sec after 1 second and has traveled 178.5 ft. Taking the air away increased the velocity by only 12.5% and increased the distance traveled by 10 ft. or ~ 6% increase.

A higher performance rocket like a PML Cirrus Dart at sea level on a J350 will have a velocity of 1441 ft/s after 1 second and will have traveled 831 ft. Take away the air and the rocket would have a velocity of 1571 ft/sec after 1 second and would have traveled 865 ft. That only a 9% increase in velocity and a 4% greater distance.

From these examples we can see that drag forces due to air resistance are small compared to the thrust of the motor, and the greater the thrust to weight ratio, the less enhancement you get, so unless you go to a very long tube you won't gain much and it's not worth the effort.

The coast phase is where all the differences arise. Play with wRASP and run the same rocket at 29.92 " of Hg pressure and 0.01 " pressure and note the results.

A ram accelerator, on the other hand, is a projectile that flies through a high pressure combustable atmosphere and gains energy by the combustion of the fuel just like a ram jet. This is potentially practical and is theoretically capable of generating orbital velocity projectiles in a tube.

Check out
for the real information and for some fantasy.

Bob Krech
hmm..yeah sounds pretty useless, anyone wanna build a tube to the moon with me? :D
how about a magnetic rail with railbuttons that has two magnets with opposing polarity?
that would have been my next question.......what about weight? I don't know much in the way of magnets. but i do know that they are somewhat heavy (dunno if they are heavy enough to matter). I think this seems like the more logical thing to do.
anyother opinions on this?