Rockets and Tubes: Why not?
June 27, 2013 9:45 AM   Subscribe

Why do we not launch rockets out of tubes? If the most expensive bits of launching a rocket are the first few feet off the ground, why don't we put the rockets into a tube to make launching more efficient?

July 4th is upon us, and with that, fireworks. Specifically, those fireworks that shoot out of tubes. It's a little ball that you put in the tube, light the fuse, boom: up it goes, a hundred feet in the air and explodes. Key element: if you don't put it in the tube before lighting the fuse, the mortar only goes about 10 feet in the air and then falls to the ground and explodes on the ground. This is largely because the tube itself compresses the air and focuses the lift of the initial explosion that propels the mortar.

Minuteman missiles did this. Missiles launched from submarines do this.

Why didn't the Saturn V do this?
Why do we not launch rockets out of tubes?
posted by Freen to Science & Nature (13 answers total) 3 users marked this as a favorite
 
I believe, most likely because you can't contain a Saturn V's power inside a tube with all the engines going, at least not anything that would be re-useable. The difference in thrust power is simply staggering.

Minuteman missiles and sub launched missiles are ridiculously lighter than the payloads lifted by the Saturn V series and the like. 600 lbs vs something like 63000lbs.

Essentially the only way you keep anything still barely rebuildable when you launch a saturn V is to dump the equivalent of, oh i dunno, 10 olympic sized swimming pools of water on the launch structure every second.

Behold the power of the Saturn V. Tremble at it's might.
posted by iamabot at 9:53 AM on June 27, 2013 [2 favorites]


To avoid cooking the rocket.

Inertia being what it is, a rocket doesn't get up and leave the instant the propellant is ignited. Thrust has to build up to the point where it counteracts the weight and inertia of the vehicle, and during that time, the hot gases must be shunted somewhere. Minuteman missiles were not really launched from tight tubes, but from silos, which incorporated vast duct work to channel hot gases away from the vehicle to ground-level vents.

Likewise, submarine-launched missiles are expelled from the tubes by compressed air, and are clear of the tube before the rocket motors ignite.

Air-launched ground attack missiles are indeed launched from tubes, but the rear of the tube is open, allowing an escape path for the gases.
posted by dinger at 9:55 AM on June 27, 2013 [14 favorites]


Minuteman missiles were launched from underground bunkers for protection from enemy attack. Sub launchers have tubes because it's a convenient way to store the missiles, I imagine. The Saturn rockets did not have either of these needs.

Look at that photo of the Minuteman you linked. See all that empty space around the missile? That launch facility is not providing much if any extra thrust by putting the missile in a tube. Applications where the launch tube actually helps - your mortar, in this case, or a rifle barrel - have the projectile fit much more snugly against the walls of the barrel to minimize pressure losses. What does the silo for the Minuteman gain you in terms of efficiency, versus the cost it took to build the thing in the first place?

Imagine the logistics of building a 400 ft. silo for a Saturn V rocket, and compare that to how much fuel you could save. Now consider that the Minuteman is essentially smooth on the outside, while the Saturn V has small fins at the bottom that need to be accounted for. It would be even harder for something irregularly shaped like the Space Shuttle.

As with all engineering problems, it came down to cost and time in the end. Why build a huge tube for the rocket when you can much more easily put a few extra tons of fuel on board?
posted by backseatpilot at 9:56 AM on June 27, 2013 [1 favorite]


The fireworks you describe are not rockets, they are mortars. Mortars are shot from a barrel, like a bullet from a gun. The propellant used to loft the mortar is not part of the projectile, although they may be packaged together (again like gunpowder packaged together with a bullet in an ammunition cartridge).

An important aspect of designing a gun/mortar barrel is that the propellant must finish burning before the projectile leaves the barrel, or else the energy is lost. Consider how long the Saturn V burns. That would make a very long barrel indeed.

You can shorten the barrel by using faster reacting propellants, at the cost of increased acceleration and in all likelihood, insurmountable chemical and engineering difficulties. But even if you could do it, you really can't accelerate your spacecraft like a bullet if there are delicate instruments (or people) inside.

Also, the two ICBM examples you listed are proper rockets, not mortars. They are not shot out of a tube like a fireworks mortar. They are simply housed in a tube-shaped container.
posted by ryanrs at 9:57 AM on June 27, 2013 [8 favorites]


Even above-ground launches involve tunnel systems to channel exhaust gases away from the vehicle.

Watch the video linked to by iamabot above. At 35 seconds, you will begin to see the smoke and flame get sucked down below the vehicle. If you want to see where they're going, watch the right side of the frame. There you will see one of the outlets for the tunnel network that channeled the combustion products away from the rocket. It will gradually light up as the thrust is directed through it and away from the pad.
posted by dinger at 10:31 AM on June 27, 2013


The Peacekeeper (MX) missile was originally a mortar launch but as you can see in the video not much altitude or velocity is gained. If I remember right, the method was devised to simplify the MX's hide & seek mobile launcher for the reasons dinger mentions.

The leftover Peacekeepers have been reincarnated as Minotaur IV, a commercial launch vehicle. At least for this flight it has a traditional launch.
posted by tinker at 10:37 AM on June 27, 2013


There's a lot of odd discussion going on here about containing gases. The answer is actually much simpler:

Rockets go up because the force created by the kinetic energy of the gases immediately exiting the engine exceeds the weight of the rocket. The mass of the gas times the acceleration the molecules undergo at the exit is the force generated, regardless of what happens to the gas molecules in the next 10 meters.

It doesn't matter one bit if those gases are contained or not; the rocket's velocity is unaffected by that.

As others have noted, sub missiles are gas-propelled, not propelled by expelled mass. IOW: pressure, not kinetic energy.
posted by IAmBroom at 11:02 AM on June 27, 2013


On a somewhat relevant point, there are plans for a 'rail' launcher that would accomplish pretty much what Freen is implying about launching from a tube.
posted by bartonlong at 11:12 AM on June 27, 2013


The acceleration needed to make a tube launch worthwhile would kill the passengers. There are people trying to do tube launch for items like fuel and food and electronics. The video is worth watching.
posted by Sophont at 11:35 AM on June 27, 2013 [1 favorite]


I've had a conversation with a weapons designer about this! In addition to what other folks have said, if you use a very long tube, there's a bunch of air in front of the projectile that has to be moved out of the way. There are solutions, sure (vacuum, venting), but nothing has worked very well yet, from what I understand.
posted by MrMoonPie at 12:28 PM on June 27, 2013


IAmaBroom makes a good point, but a tube would certainly accelerate a rocket faster. The expanding gasses would create a bullet or cannonball effect that would work in addition to the pure thrust of the rocket engine.

But I kind of doubt it's possible to contain that kind of pressure in a way that would be useful. There would need to be a relatively tight seal between the rocket and tube assembly, and that would veer dangerously close to accidentally creating a really large pipe bomb.
posted by gjc at 3:03 PM on June 27, 2013


MX was ejected with CO2, as I recall, then ignited. You don't want ignition in a storage tube. exhaust components are nasty, hot, and have unburned/burning particles in the mix. nasty for finishes and unpredictable. there are moving parts on many rockets, too, like active airfoils that steer the beast. big mothers like Titans use really nasty propellants and the amount of fuel consumed would take your breath away. getting rid of it is hard enough, but bathing a thin exterior, painted skin in such evil soup is just a recipe for trouble. folks who have never touched a missile or launch vehicle imaging them to be cast iron or something. Titans are sheet metal. Others are aluminum tubes. Paint is the normal finish. Sometimes anodizing. It's not pure unobtanium from the planet Gorgonzola. The surfaces that interact with propellants are exotic, though. Miracles of precision machining, material science, coatings and processes. The stuff doesn't have to last long, but the demands over a few minutes (or seconds) are intense.

Vertical launch Sea Sparrow conventional missiles are on tubes shipboard, and I am not sure what they do with the exhaust. A bunch of tactical airborne rocket types are tube launched, and the exhaust is vented to ambient behind the motor.

A common misconception (for many folks) is that rockets work like bullets and push against the air somehow to get going or are propelled forward by the air pressure behind. (Usually, this goes away when you explain that this would mean you could not use rocket motors in space... where there is no air to "push" against.) It's simply action/reaction.

my experience in aerospace (and missiles/rockets in particular) suggests that when you don't see something in the field, it's because there are really good reasons. designs are exercises in optimization, and any advantage conferred by your concept is certainly offset by some overwhelming disadvantage, which explains why it's not out there. kinda.
posted by FauxScot at 3:10 AM on June 28, 2013


Thanks everyone!
posted by Freen at 11:03 AM on June 28, 2013


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