Rocket Science

If the ship is indeed the star of the story – at least, the star of the story as the story first played itself out to me – then in order to understand the odd configuration, you have to understand the ship’s backstory.

This is not the vessel they intended to build.

The movie opens with a catastrophic explosion of a gas-core nuclear rocket engine, one with a really remarkable Isp of about 3,000 seconds. Isp (I sub sp) represents specific impulse, and specific impulse is to space exploration what lift coefficient is to aviation.

Specific Impulse is a measure not of thrust, but of efficiency. Isp is a measure of how much force each pound of propellant produces, and is measured in seconds. It is, essentially, propellant flow rate per unit of thrust. In other words, if you had a rocket that produced a set amount of thrust, and you carried a set amount of propellant, then a rocket with an Isp of 200 would produce that level of thrust for 200 seconds before burning through the propellant, but one with an Isp of 3,000 would take 3,000 seconds – at the same thrust level! – to burn through that same amount of propellant. Rocket A burns for 3.3 minutes before running out of propellant; Rocket B produces the same thrust level for fifty minutes!

And Specific Impulse does not equal Thrust. Thrust is a measure of raw power, and in order to get into orbit you will sacrifice efficiency for thrust to get you out of the gravity well. For example, each of the five Rocketdyne F-1 engines on the Saturn V first stage produced a little over one million pounds of thrust each, but their efficiency – their specific impulse – was about 263 seconds (the actual burn time for the Saturn V main stage was only 150 seconds).

On the other hand, the Ion engines on the Dawn probe, have tremendously high efficiencies – 3,100 seconds – but produce very little thrust – less than an ounce! But an Ion engine can burn for months, and as long as the probe is small, and you are not in much of a hurry, then you can in fact steer that probe from one target to another once it is free of the earth’s gravity.

So obviously, what we need is something with enough thrust to move something about the size of the International Space Station, at a reasonable enough velocity to get to Jupiter in a year or so, but yet efficient enough to do it without the entire vehicle being nothing but propellant. Because not only do you have to carry the propellant to get you there, you need to carry the propellant to stop you. And you need to carry enough propellant to push the deceleration propellant. And that’s assuming you have no plans to come back!

So you can see that Specific Impulse – engine efficiency – is the Holy Grail, because the more efficient the engine, the less propellant you have to carry and the more ship you can afford under the total mass.

So, back to the story…

The initial engines for the Aurora vehicle are experimental – just barely on the engineering horizon for today. The Saturn V main engines had an Isp of 263. The Space Shuttle main engines were significantly better: about 450. But that’s about the limit, because when it is all said an done this is about expanding hot gases coming out of one end of a chamber, and not the other – and a chemical reaction can only get so hot.

A gas-core nuclear reactor, on the other hand, can get very, very hot since the energy is contained not in a metal structure, which would melt, but rather in a magnetic field torus. Since it gets so incredibly hot, the expansion rate of the propellant (in this case liquid hydrogen) is very high, and you get both high thrust and high efficiency.

But that engine blows up. I blew it up for story purposes. I blew it up so that they would have to go Back to the Future, and dust-off something we have already done, back in the days of the Glory of Gondor and the Kings of Numenor: Jackass Flats, Nevada in 1970.

(to be continued…) 

Published Nov 23, 2013