What benefits can be gained from launching below ground?

Question

ICBMs are commonly launched from below ground. How much fuel could be saved if a missile is launched below ground instead of above? If the tube was deeper with modified launch by a separate chemical launch mechanism to create a back pressure on a piston not the rocket achieving much of the velocity to achieve orbit be more efficient? This way rockets of many sizes could use the extra push and may be a reusable first stage launch.

This piston is powered by rockets that are efficient whilst in compression. The lift is created by pressure build up like in internal combustion chamber with a constant burn not from rocketing out. The maximum Gs up and out. What prevents the use of electric engines to assist with spacecraft launches? (Launch Booster Supplement)

https://metro.co.uk/2018/03/30/russia-tests-4000mph-hypersonic-missile-near-impossible-to-shoot-down-7428694/

What is the deepest we have ever gone into the Earth?

Bonus questions: Will a 3 dimensional magnetic bearing rail set up be strong enough to keep the ship or piston from coming in contact with the rail/wall? Would composing the ship of graphite and bismuth keep it center? Will there be a static charge and how to absorb it?

Answer 1

This is essentially piston launch; in principle a deep enough hole could let you get the piston up to speeds approaching the speed of sound in the expanding gas driving the piston.

Assuming you're limited to 6g by the strength of the rocket structure, and ordinary air for the driving medium, you need a piston a kilometer deep, and you accelerate to mach 1 (~340 m/s) in 6 seconds. That saves you about 4% of the total delta-v you need to reach LEO, or about 10% of a typical first-stage contribution.

It's not done because engineering a rocket-sized, kilometer-deep piston is a fairly gigantic task, and it's easier to just make the first stage of your launcher slightly bigger.

As Henry Spencer notes in this post on yarchive:

Many novel launch schemes need some amount of help from rockets. What kills a lot of them is doing a tradeoff study of just enlarging the rocket part and getting rid of the non-rocket part. Surprisingly often, that works out to be better and cheaper.

Answer 2

Note that for silo launched missiles the exhaust is typically vented out of the silo instead of serving to pressurize the silo.

Why are they vented?

An unvented silo would vastly increase the ambient pressure the engines are exhausting into with associated thrust loss, possible flow separation, etc. You do not want to run your engine in a pressurized chamber. See this answer for the equations: How much efficiency is lost from a fixed De Laval nozzle when modified for atmospheric use?

There was no appreciable fuel savings from silo launch. It was done to protect the missile from enemy attack.

Answer 3

What benefits can be gained?

Energy-wise, not so much. You're basically talking about a variant of a first stage, but one that only helps for the first few seconds.

A real benefit however might be the increased ability to launch in relatively bad weather. Winds are more of a problem at slow speeds. So if you breach the surface level going sufficiently fast, you can launch at higher wind speeds.

It's not really the wind itself that makes a launch problematic. The problem is that the surface of the earth slows down the wind. This means you get an altitude-dependent wind profile. The higher up, the more wind you encounter. This topples over the rocket.

Answer 4

Benefits:

1. less fuel needed to reach orbit

Problems:

1. To get enough speed to matter, you need a long track = a deep shaft. 3 km is about the practical limit.
2. To get the rocket (let's say this weighs 550 tons, the starting weight of a Falcon 9) to a usable speed in only 3 km of track, you need lots of power. The F9 uses 9 Merlin engines to accelerate off the pad, so the amount of power you need: Falcon 9 FT: 7,607 kN (1,710,000 lbf) thrust at liftoff.

The RR Trent 800 jet engine is available as an aircraft engine or as a powerplant. It produces 36 MW of shaft power or 80 klbf/360 kN of thrust. Using that conversion as a shortcut, you need 21 of these engines to provide as much power as the F9 at liftoff, or 760 MW to lift the rocket with its platform at the same rate of acceleration as an F9 coming off the pad. Linear motors capable of handling 760 MW are going to be huge (count on a few hundred tons), not to mention the power lines needed to carry that much power. 760 MW is equivalent to 30 TGV trains.

3. You have to brake the platform from halfway up the shaft to avoid it shooting out (at a few hundred tons, there's no way it can make a soft landing). That halves the speed you can reach.

4. you lose the ability to start the engines and test them before launch. Any malfunction when you start the engines will lead to a crash and loss of mission. You can't start the engines while you're still in the tunnel. You don't want the rocket exhaust blasting up the sides of the rocket.

If you want to move the piston via compressed gas, how much gas do you need?

• let's assume 550 tons of rocket plus 50 tons of piston
• piston diameter 5 m, area 19.6 m³
• shaft 1 km deep (borrowing from Russell's answer)
• pressure needed to lift this off the ground: 600,000/19600 = 30 kg/dm² (30 bar)
• amount of gas needed to fill a shaft 1 km deep to 30 bar: specific weight of air is 1.2 g/m³, shaft contains 19600 m³ x 30 bar is 705 tons of air.

So if you want to do this, you need 705 tons of rocket exhaust at 30 bar to fill the shaft. That's more than the F9 first stage uses for its entire burn, so instead of saving fuel you're using more. You've also traded making the rocket 10% smaller (a negligible saving, you're just saving one barrel section of aluminum by shortening the tanks a bit) for having to build and maintain a shaft 1 km deep with a gas-tight seal between the piston and the shaft.

And this calculation ignores some problems:

• When you use a rocket to generate this gas, the gas cools as it leaves the rocket engine, lowering its pressure.
• Rocket engines become inefficient when they have to run against a high exhaust pressure.
• I've ignored that the gas needs to accelerate the piston and rocket, the pressure I calculated just holds the piston off the ground without accelerating it.

So both electric and pneumatic power for moving the piston are not feasible.

Answer 5

I would not make it on a chemical basis but rather on an electromagnetical like the Electromagnetic Aircraft Launch System (EALS) that is currently introduced on the new aircraft carriers of the Gerald R. Ford-class. This is quite exactly what you'd want.

Two issues regarding this technique come to my mind:

• First you'd need a gigantic storage of electrical energy that could deliver it's stored electricity in fractions of a second. This would be a huge task escpecially as rockets are significantly heavier than fighter planes.
• Even if you overcome the engineering problem, you would still be limited by the stability of the rocket and the drag in the lower atmosphere. If you launch too fast the rocket will disintegrate or burn up in the atmosphere. As Russell Borogove already stated in his answer, you will be limited to about 6g which will not give a lot of advantage inrespect to delta-v. You could make the rocket sturdier, but this in return would increase weight and therefore counteract your intention.

Answer 6

Since nobody else has included this in their answer yet:

What benefits can be gained from launching below ground?

Secrecy about exactly what rocket is there, what it has on top, and its launch readiness state. Maintenance can also be done without easy observational detection. This is important for ICBMs, whose deterrent effect depends somewhat upon the panopticon principle, that an adversary believes they could be ready for launch at any time without actually knowing whether or not they are launch-ready at any particular moment. When the rockets are not launch-ready, but underground, this weakness in the system is not necessarily exposed in a way that an observer could identify and take advantage of. This allows the maintainer to be a bit more flexible in scheduling repairs etc.

Further, if the rocket commander is trying out installation of some new/upgraded payload, the underground silo allows them to be strategic about if/when to reveal this information to a potential adversary.