1
$\begingroup$

Project Orion was a project to construct a spacecraft that rode the shockwaves of nuclear blasts as a form of propulsion, but the project was abandoned because of budget cuts, environmental concerns, and the end of the space race.

In my world, a new space race starts between the U.S. and China over lunar recourses; due to China using "pseudo-orion" spacecrafts powered by conventional explosives to "cheaply" go to and from the moon, extracting lunar resources to substantial economic gain. This starts a great space race between two states with similar technological and economic might, causing it to go much , MUCH farther and out of hand than the first one ever did.

My Question:

What practical issues might prevent such a craft from being made?


Next Question

$\endgroup$
  • 4
    $\begingroup$ Any one of these questions is probably fine. All three together feels to me like it is too broad. $\endgroup$ – a CVn Aug 27 '17 at 10:04
  • $\begingroup$ The space race didn't end in 1963. The Partial Test Ban Treaty is usually considered the reason for shutting down the Project Orion study. It never reached the developmental stage. For the history & background see here: en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) $\endgroup$ – a4android Aug 27 '17 at 11:48
  • 1
    $\begingroup$ @nzaman An internal combustion engine doesn't work by explosions, it works by deflagration. Compare en.wikipedia.org/wiki/Deflagration#Applications. $\endgroup$ – a CVn Aug 27 '17 at 12:20
  • 2
    $\begingroup$ The article I mentioned on unconventional explosives is "Boom or Bust" by David Hambling in New Scientist, Volume 235, no. 3136, 29 Jul 2017, pages 36-39. Their proposed uses include better rocket propulsion. $\endgroup$ – a4android Aug 27 '17 at 12:23
  • 4
    $\begingroup$ Using conventional explosives, what you'd get would be just a very inefficient rocket. Current rockets ARE one continuous, controlled explosion, using propellants that are much more energetic than conventional explosives - which are limited by storability and safety considerations, among other things. $\endgroup$ – jamesqf Aug 27 '17 at 17:35
5
$\begingroup$

Your bomb powered rocket just might be able to reach space. It has no hope of reaching orbit.

First, energy. From Wikipedia:

Energy density of TNT: 4.6Mj/Kg. Energy density of H2: 142Mj/Kg. Unfortunately, this doesn't count the weight of the oxygen, that's 8x the weight of the hydrogen. This gives 15.7Mj/Kg for the total energy of your rocket fuel. You have 3.5x the energy in the rocket fuel (and in practice the different is greater as the rocket exhaust is lighter molecules which translates into more push for the energy.) Furthermore, when you set off that charge the energy that goes down does nothing about pushing your spacecraft. Getting 50% out of it would be very optimistic. Now we have a 7x difference.

Given an equal mass ratio that means the bomb-powered rocket can only do 1/7th the velocity of the LH2/LOX powered rocket. Since the rocket equation is exponential and already climbing very steeply by the time you reach orbit your bomb rocket simply can't be made big enough.

The reason behind Orion is the 100,000x greater energy from fission than from LH2/LOX. Lose even 90% of the energy and you're still have 10,000x the energy.

If you want a cheap rocket for your story I suggest looking at the Sea Dragon. Crude in comparison to anything that's gone to space but it's so much simpler that it might be cheaper anyway. (Remember, the fuel costs of a Falcon 9 launch are probably under 1% of the cost of the booster.) Just take a big pressure tank with a hole in the bottom that feeds into the engine belll. No big fancy pumps or the like, just a valve. The "Sea" part of it's name is that it was so big that it could only launch from the ocean rather than from a pad.

$\endgroup$
  • $\begingroup$ I'm pretty sure your math is off for "100,000x". LH2/LOX mix approximates an Re Factor of 1. From my answer, about the best you can do with fission is 100:1, so a 100x increase in power, not 100 000. $\endgroup$ – jdunlop Aug 28 '17 at 18:29
  • $\begingroup$ Also, having lighter molecules in your rocket exhaust isn't that desirable - you want to fling heavy things away from the ship. Having faster but lighter exhaust is no better than slower but heavier. $\endgroup$ – jdunlop Aug 28 '17 at 22:30
  • 1
    $\begingroup$ @jdunlop Your link is looking at actual bombs--plenty of extra stuff on them besides the boom. Those weren't designs engineered for the minimum possible weight. Also, he said nothing about the rocket size, you can't discount boosted-fission or even fusion designs. And light stuff certainly does help--we are throwing the same mass in each case, thus it's just faster or slower. $\endgroup$ – Loren Pechtel Aug 29 '17 at 0:20
  • $\begingroup$ You are completely right about the numbers, don't mind me. The numbers in my answer are off by three orders of magnitude. $\endgroup$ – jdunlop Aug 29 '17 at 0:32
3
$\begingroup$

There are, technically, no problems with building a pseudo-Orion using conventional explosives - that is, in fact, what they did to test the concept. The practical problem, as alluded to by Joe Bloggs' answer, is the difference in energy density between conventional and nuclear explosives.

Part of the reason that, say, automotive gasoline and oxygen aren't (usually) used as rocket fuel (they prefer an ultra-refined kerosene) - is that in addition to fouling problems, the specific impulse of gasoline isn't spectacular. (This isn't to say it can't be done, though.)

However, all of this assumes that you're using a rocket nozzle to optimize the speed of your propellant stream. Orion didn't do that; it used a pusher plate, and (this is actually pretty cool to be able to say) a shaped nuclear charge (diagrammed here). The pusher plate (an enormous, metres-thick slab of steel connected to the ship proper by equally-enormous shock-absorbers) had the fast-moving propellant, now a plasma thanks to the nuclear detonation, hit it, absorbed the energy and let Newton do the rest. This allowed for an enormous amount of thrust, but wasn't nearly as efficient as a rocket engine. And here's where we get to the math bit.

The theoretical maximum achievable efficiency of a pure-fission device (because you wouldn't want to be using fusion weapons with a Project Orion unless it was really big) is about 0.1kt per kg of mass (source). But this means that you get the explosive force of 100 000kg of TNT for every 1kg of bomb weight. Assuming that packaging and propellant are a constant and can be disregarded, that's still a lot of bang for your buck, which is extremely important in rocketry. The most powerful known chemical explosive is Octanitrocubane, which has an RE factor of 2.38, making it 2.38 times as powerful as TNT per unit mass. Pretty good, but you'd still need forty thousand times as much mass of fuel to achieve the same amount of thrust, and rocketry doesn't allow for that The more mass you require to achieve the same amount of thrust, the less you can lift with it, and it's non-linear (since you have to bring the mass with you).

The Tsiolkovsky rocket equation applies here, though it's difficult (unless you're a physicist specialising in bomb design) to determine the exact specific impulse of a shaped nuclear charge's plasma propellant impacting a pusher plate. Basically, the higher the specific impulse (which is probably associated, when using explosives, more or less linearly with the explosive force generated by the bomb), the greater the initial mass of the rocket can be (assuming you're starting from a gravity well). If your efficiency (explosive yield per unit mass) is too low, you can't lift as much.

So, to sum this up, if you're not using nukes, you can't use the Orion design to make a useful rocket, let alone a less expensive option.

Sidenotes:

  1. You could probably get a little better yield-per-mass from your nuclear weapons with boosted fission designs, but I stuck with pure fission weapons because that's what they were working with in 1966 when they were designing Orion.
  2. Just because nukes were off the table didn't mean that rocketry scientists weren't up to trying everything; if a conventional-explosive bomb-driven rocket were actually in any way more efficient or cheaper, you can bet that they'd already have them in action. They aren't... so they aren't.
  3. Pursuant to #2, I recommend giving Ignition! by John D. Clark a read. When I say rocket scientists (and rocket chemists in particular) were up to trying everything, I mean everything. It was nuts. And it's a great read!

Edit: Loren Pechtel's figures are right, I was mistaking kt for t. Tens of thousands of kg of TNT per kg of bomb, rather than hundreds. (This actually makes the conventional-explosive variety of Orion less plausible.)

$\endgroup$
  • $\begingroup$ For those wondering: my answer was this, but a lot shorter and with no examples. $\endgroup$ – Joe Bloggs Aug 28 '17 at 10:44
  • $\begingroup$ I can't help but wonder about replacing the fission fuel with inertial confinement fusion. $\endgroup$ – jamesqf Aug 29 '17 at 17:48
  • $\begingroup$ @jamesqf - that was the idea behind Project Daedalus. Since ICF doesn't yet work properly, it's not something that could reasonably be constructed, while Orion is feasible, if environmentally disastrous. $\endgroup$ – jdunlop Aug 29 '17 at 17:53
0
$\begingroup$

As others pointed out, using on-board conventional explosives would simply be too inefficient an use of chemical energy compared to rockets. Project Orion could work because nuclear explosives have a such massively higher energy density to compensate.

So if you want to use chemical external pulse propulsion, the vehicle itself cannot carry enough explosives. Which means that you need to send the pulse units at it, for example by firing guided HE shells from strategically placed howitzers on the ground.

It is important to note that this would only work in atmosphere, as the explosive would use air as a medium to transmit its energy. Once out of atmosphere, chemical explosives are simply too weak to be worth it. Orion used extremely small pulse units in atmosphere for this reason, as the power of its space pulses would have destroyed the craft.

This way, you can have a high-performance first stage as it doesn't have to carry its own fuel or reaction mass. It is used to accelerate as much as possible before leaving atmosphere, then releases a conventional rocket second stage. Ideally, the first stage comes back to be reused.

It is easy to see where the challenges would lie: you need high-precision projectiles, fired with perfect timing, that will explode right behind the craft as it is passing by. Any precision error may damage the craft, or fail to propel it enough, leading it to fall from its trajectory. If acceleration is high enough, the craft may end up going faster than the projectiles are fired from the guns, so they would have to be fired before, and manoeuvre to put themselves right behind the craft as it is passing by before exploding.

Also lobbing high explosive shells in the atmosphere may make neighbours a bit nervous, particularly in tense regions like the South China Sea.

This could be the key for sending big payloads in orbit for a reasonable cost, and the Chinese may indeed just go for it, but they (or anyone else) would have to work very hard to achieve it.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy