One of the more annoying things that I find in commentary on space policy is the assumption that there is One True Way to get off the planet, and that working on anything else (particularly chemical rockets) is a waste of time and money. Often it’s space elevators, but here’s another case in point: an Orion fan (the original Orion, not the current Apollo crew module on steroids):
Nuclear power is still the only thing that’s going to allow us to get large amounts of mass into Earth orbit and beyond. Nothing else has enough specific impulse to do the job.
While nuclear-pulse propulsion may be an interesting technology for in-space transportation, where the radiation level is pretty high to start with, it was never going to be used for earth-to-orbit transportation. One does not have to be a luddite to believe this. I’m all in favor of getting access to orbit as low cost as possible, as soon as possible, but I think that the notion of using Orion for this is nuts (and not just for the radiation and atmospheric contamination issues–consider the EMP…). I highly respect Professor Dyson and Jerry Pournelle as well, but that doesn’t mean that there aren’t some major technical issues in getting such a system practical and operational. If such a system is ever built and tested, it will be built and tested in space, after we’ve come up with other ways of getting large amounts of mass into orbit, affordably. And I’m quite confident that if and when we do this, it will (at least initially) be with chemical rockets.
Part of the misunderstanding is revealed in the second sentence. The assumption is made that the reason costs of getting into space are high is due to performance, and particularly a specific performance parameter–specific impulse. For those unaware, this is basically a measure of a rocket’s fuel economy. The higher the Isp, the less propellant is required to provide a given amount of thrust over a given time period.
But there is no equation in vehicle design or operations that correlates cost with Isp. If Isp were the problem, one would expect propellant costs to be a high percentage of launch costs. But they’re not. Typically, propellant costs are on the order of a percent of the total launch costs. Yes, requiring fewer pounds of propellant means that the vehicle can be smaller, which reduces manufacturing and operations costs, but it still doesn’t account for the high costs.
Chemical rockets are perfectly adequate for affordable launch–their specific impulse is not a problem. As an example of why there’s a lot more to rocket science than Isp, consider that some of the more promising concepts (LOX/hydrocarbon) actually have lower specific impulse than so-called “high performance” propellants (LOX/LH2). Why? Because liquid hydrogen is so fluffy (the opposite of “dense”) that the tank sizes get large, increasing vehicle dry mass and atmospheric drag. For instance, the Shuttle external tank carries six pounds of LOX for each pound of hydrogen, but the LOX is all carried in a little tank at the top, and most of the ET that you see contains liquid hydrogen.
As I’ve noted many times before, there are two key elements to affordable launch using chemical rockets. Fly a lot, and don’t throw the vehicle away. Despite the mythology about the Shuttle, we’ve never actually done this in a program. It seems unlikely that NASA ever will, but fortunately, private enterprise is finally stepping up to the plate.