# The Naked Scientists Forum

### Author Topic: How many lbs of thrust is 10 megatons of TNT?  (Read 4260 times)

#### realmswalker

• Sr. Member
• Posts: 205
##### How many lbs of thrust is 10 megatons of TNT?
« on: 20/06/2005 20:27:41 »
Im curious because im contemplating the idea of using a nuclear explosion to propel a space ship...
If the blast was focused backwards how many lbs (or tons if its easier!) of thrust could it possibly provide? Could this propel say a 100-500 ton space ship into orbit? Once in space could you use reccurrent explosions to increase the speed of the ship? To what speed could you possibly get? it?

I think this would be a brilliant way of gettin into orbit (we have all the nukes were dismantling, why not used em eh eh?
If you took 7 of our 10 megaton nukes and lined em up in toobs that were HEAVILY sheilded so that the thrust went complently out 1 end could how much could you lift?

#### gsmollin

• Hero Member
• Posts: 749
##### Re: How many lbs of thrust is 10 megatons of TNT?
« Reply #1 on: 21/06/2005 02:24:49 »
This sounds really wacky, but it was actually evaluated by the DOE years ago. There were many nuclear propulsion schemes, but I don't remember them all, or what DOE called them. There was a Nerva rocket engine, but I don't think it was the h-bomb method. I read about this ~30 years ago in a microfilm archive, so my memory is pretty fragmentary.

OBTW, a megaton is a measure of energy, not force.

#### realmswalker

• Sr. Member
• Posts: 205
##### Re: How many lbs of thrust is 10 megatons of TNT?
« Reply #2 on: 21/06/2005 06:31:51 »
i just did some research on it and that nuclear propolsion ship was called the 'orion'.
It used thousands of nuclear small nuclear explosions to propel it...

A 10 megaton nuclear bomb = 10 megatons of tnt...what i was asking is if 1lb of tnt went off how much explosive power would it have? If it was under a rock how heavy of a rock could it blow upwards basically...

#### daveshorts

• Moderator
• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Re: How many lbs of thrust is 10 megatons of TNT?
« Reply #3 on: 21/06/2005 14:48:05 »
1kg of TNT has about a quarter the energy of the equivalent amount of sugar (as it has to include its own oxygen) or about 4.7MJ/kg so with perfect conversion you could acelerate 1kg to 400m/s with it however you are not going to get anything like perfect energy conversion in the nuclear rocket.

This is because the energy efficiency of your rocket is going to be related to the amount of mass you throw out the back, if it is much heavier than the rocket (like when you push off the earth) it is very efficient, but makes for a very heavy rocket, however if you want to throw soemthing light out the back your rocket gets very inefficient. Eg if you are throwing photons out the back the 1kg of TNT will produce enough momentum change to accellerate 1kg by about 1/100 m/s.

I think most of the nuclear rocket designs get more of their thrust by ablating (boiling off) a heat shield and the bomb itself, but the efficiency is not going to be that high...

#### gsmollin

• Hero Member
• Posts: 749
##### Re: How many lbs of thrust is 10 megatons of TNT?
« Reply #4 on: 21/06/2005 19:45:27 »
My reference* says 4.2 MJ/kg, nuclear equivalent of TNT. That may represent the difference between an "official" conversion factor, and an actual yield of TNT.

In any event, even a small nuclear device is going to release kilotons of energy, in a few microseconds. In a vacuum, the energy is released as soft X-rays and neutrons. Converting that to usable thrust is not easy. I imagine the "Orion" heated a shield with the X-rays, and propelled vaporized heatshield through the nozzle of the engine. As I remember, the nuclear device in the Orion was detonated a long way behind the spacecraft, for obvious reasons. Even at that, the spacecraft had to be subjected to intense prompt radiation, and the resulting SGEMP would have been pretty bad.

There were many "peaceful" uses for atomic energy proposed in the fifties, Project Plowshare was going to dig a sea-level canal between the Atlantic and Pacific oceans. There was a nuclear airplane, rocket engines, and plenty more. We did manage to get the steam nuclear cycle for electric powerplants, submarines, and navy ships. The Russians have launched nuclear-reactor-powered spacecraft that used thermionic converters. Periodically the military or NASA proposes nuclear powered spacecraft for outer planet exploration. I worked on one of those programs (SP-100) back in the eighties. It basically went nowhere, although we did develop some good thermoelectric materials and high temperature insulators. One of the fundamental problems with anything nuclear is that even 0.99999 reliabilty is not good enough.

*Mechtly, E. A., "The International System of Units", NASA SP-7012, 1973.
« Last Edit: 21/06/2005 19:47:15 by gsmollin »

#### chimera

• Sr. Member
• Posts: 475
##### Re: How many lbs of thrust is 10 megatons of TNT?
« Reply #5 on: 21/06/2005 23:44:45 »
Project Orion was a plot feature in a novel by Larry Niven and Jerry Pournelle (Footfall). They used a decomissioned aircraft carrier for starters and built some enormous shield under it and off they went. Impressive project, that. The political greens among us would be somewhat taken aback, I think...

Time is a great teacher, but unfortunately it kills all its pupils - Hector Louis Berlioz

#### The Naked Scientists Forum

##### Re: How many lbs of thrust is 10 megatons of TNT?
« Reply #5 on: 21/06/2005 23:44:45 »