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Comment by Patient0 | Hacker News Reader

Can anyone explain the flaw in Bickerton's reasoning? It's not obvious to me where the mistake is:

"For a projectile entirely to escape the gravitation of earth, it needs a velocity of 7 miles a second. The thermal energy of a gramme at this speed is 15,180 calories... The energy of our most violent explosive--nitroglycerine--is less than 1,500 calories per gramme. Consequently, even had the explosive nothing to carry, it has only one-tenth of the energy necessary to escape the earth... Hence the proposition appears to be basically impossible."

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He is correct. He is saying that a gram of fuel doesn't have enough energy to get itself into orbit. That is a true statement. But he ignores the fact that rockets spend many many grams of fuel to get a gram of non-fuel into orbit.

I don't think that right. He's saying you can expend 10 grams of nitroglycerine, but the 10 grams isn't enough to get the 10 grams up into the orbit how can it possible carry more.

Because the 10 grams don't enter orbit. They're expended and stay on Earth. You'll still need more than the 10 grams because you will be lifting the some portion of the 10 grams up into the atmosphere because you can't use or get the energy in an instant. The explosion and propulsion happens over a period of time.

Projectile vs. rocket.

The calculation is entirely correct -- we cannot shoot a projectile into space. There is no sensible way to get something to go fast enough to make it to space from Earth with no further propulsion. A better argument, actually, is about acceleration. In order to not be utterly crushed, a projectile would need a massive vertical track to be shot into space. Earth's escape velocity is around 10km/s. g=10m/s^2. If we assume maximum acceleration of 10g to not crush the pilot, to get out, we'd need to accelerate at 10g for 1,000 seconds. During that time, we'd travel a few thousand kilometers. Accelerating faster -- as with a nuclear explosion -- only exasperates the problem.

We solved the problem by having propulsion the whole way up. We can fire a rocket into space, and it can take it's sweet time to get up there.

Also fun to note this argument from first principals could be levied against the Earth if it were just a bit bigger:

"If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."

http://www.nasa.gov/mission_pages/station/expeditions/expedi...

One wonders if it were the case, if humanity would come up with a solution by necessity or we'd still be grounded. My guess is we'd have nuclear powered rockets.

He just described the energy requirements: each gram of payload needs 15k of energy put in to escape orbit. He didn't think of fuel which exists simply to move the rest of the fuel (the rocket equation).

Imagine 50 sticks of dynamite separated by metal plates. The bottom stick explodes, lifting the pile a bit (energy split 50 ways). Then the bottommost explodes, lifting more (split 49 ways). This continues until the second stick propels the first into orbit.

You can play with the numbers so

explosion energy * (1/50 + 1/49 + ...) = escape energy

You quickly realize it gets asymptotic, and extra fuel contributes relatively little to the payload.

I think he is imagining detonating all the explosives at once, like a gun, the bullet being the payload you want to put into orbit - like in the Jules Vernes books - instead of progressively consuming a fuel like kerosene (which itself has, from wikipedia, an energy density of about 10 kcal/g), leaving more and more mass behind as you ascend.

The gun idea seems to be, instinctively, very inefficient because of the need to fight the air density in the lower atmosphere all in one shot. I may be wrong there - mostly imagining what may be wrong with the approach (beyond reducing a live payload to a pink mist) based on kerbal experience :-p

Just guessing here, but he assumes that the thermal energy of that gramme traveling at 7 miles/second needs to be applied instantaneously or nearly-instantaneously, when it can instead be given to that gramme over the course of several minutes via, say, a Saturn V rocket system.

> The energy of our most violent explosive--nitroglycerine--is less than 1,500 calories per gramme.

Well, part of it is that you don't really want "violent explosives", you want a controlled combustion. Once you take that limitation out of the way, turns out that explosives don't have very high energy densities, they are actually, for the most part, less energy-dense than food. Take a look at this table: http://physics.info/energy-chemical/

(Funnily enough, was discussing this over lunch just now).

What about project orion? https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propu...

I think exploding nuclear bombs as a propellant qualifies as "violent explosives". Then again it was the 50's and those guys loved nuclear things.

I believe the flaw was the assumption that the only way to get something into space was via detonating explosives.

Rockets have multiple stages. Most of the fuel doesn't have to be accelerated nearly that fast.

(Also, I don't know how that compares to rocket fuel.)

Correct me if I am wrong, but we created more powerful fuel than he had when he made that statement.

I've always thought about it as the difference between "throwing something out of Earth's gravity" (launching a projectile with initial force only) versus "climbing [a ladder?] out of Earth's gravity" (continually applying force just over gravity's influence.)

I think Bickerton has confused the two concepts.

Consider that gravity is the weakest of all forces. You overcome it every time you jump, run (with both feet off the ground) or climb a ladder. You don't overcome it for long enough to get into orbit, but you still overcome gravity. Birds and airplanes regularly overcome gravity.

That statement considers a projectile shot with a cannon. Gravimetric energy density of e.g. gasoline is much higher than nitroglycerine (6.4 MJ/kg vs. 46 MJ/kg).

He ignores staging and conservation of momentum. The explosives stay on Earth, and transfer their energy to a smaller mass which departs it. Since that smaller mass can in turn be explosives, you can repeat the process infinitely to achieve any desired net thrust-to-weight ratio for the final stage.

Also, nitroglycerine is pretty terrible fuel. You can get 7 times more energy from burning a gram of ethanol than from exploding one gram of TNT.

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