Laser Launch (idea)

Laser Launch is an idea for launching vehicles into space, an example is lightcraft, but the idea is broader than this. Laser Launch offers the possibility of lowering the cost of going into space from $10,000/kg down to around $100/kg; and might be achieved surprisingly soon, within the next 5-10 years. If successful the cost of putting someone into orbit could be about $20,000; currently the market price is about $10 million. Bringing the price this low means that the number of customers for it would be enormous.

The basic idea is that you take a large laser (1MW-1GW) and point it at a specially designed rocket. The rocket takes the laser energy and uses it to heat some propellent to a high temperature. The hot gas then pushes the vehicle into orbit.

The advantage of this technique is that the generator- and the laser- doesn't have to be carried with the vehicle, so it can be reused over and over again, and also the propellent can end up hotter than it would otherwise be, which gives better performance.

The rule of thumb of the laser/energy source is that you need 1 megawatt of power per kg of payload you wish to put into orbit. For example, a person would probably require 200kg or so, so a 200 megawatt laser is needed.

One difficulty with laser launch, is focusing the beam down enough when the vehicle is near to LEO- about 100-300km from the launch point. This is limited by the atmosphere- just like the way stars twinkle in the sky, a laser may tend to miss the launch vehicle entirely unless the aiming system can compensate for atmosphere distortions.

4P

The simplest laser launch design is colloquially called 4P- Payload, Propellent, Photons, Period. It would consist of a payload attached to a block of propellent. You shine the laser on the base, and part of the propellent vapourises and pushes on the base. You can steer the vehicle remotely by illuminating different parts of the propellent.

In principle, the scheme is very simple and elegant, however some problems crop up. First, normally rockets use DeLaval nozzles; this focuses the thrust into a straight line and turns the hot exhaust into a cooler, fast moving stream; greatly increasing the thrust. The 4P scheme lacks a nozzle, this would reduce the efficiency by almost 3x. However, if the fuel gets hot enough, this will tend to compensate, but then the laser needed is much bigger, and at some point the extra electricity starts to cost too much.

The end result is that 4P systems suffer from either poor performance and/or poor efficiency.

lightcraft

Then there are lightcraft. This design differs mainly in that they have a nozzle. Lightcraft hold the current record for laser launch- just over 100 ft.

The main downsides of the lightcraft system is that the amount of propellent (a plastic called Delrin) carried is small relative to the vehicle size, in fact too little to make orbit, the vehicle tends to melt due to high temperatures, and the laser needed is a pulsed laser- these kind of lasers tend to be extremely expensive, perhaps $10-$100/watt, and this has limited the tests so far; the lasers used were SDI project lasers, and funding for that has been reduced after the fall of the Soviet Union.

There is little sign that pulsed lasers will become cheap at the moment, so this approach is less favoured currently.

heat exchanger

Instead a new architecture is being looked at. The vehicle is slightly more conventional looking- a big tank full of liquid hydrogen (no oxygen however, unlike a conventional vehicle), a conventional nozzle to raise the efficiency, but the vehicle also sports a heat exchanger that the laser shines upon.

The laser heated hydrogen has an ISP of between 600 and 900 seconds (which compares very favourably with the Space Shuttle's ISP of 453 seconds).

The laser system doesn't need to be pulsed; therefore the laser turns out much cheaper; semiconductor laser diodes might be used with a cost under $1/watt.

The big upside of this scheme is that the propellent doesn't need to get too hot to give good ISP; just 1000C. This means that low cost construction can be used; and no regenerative cooling is required; as the metal will not melt.

The big downside of this scheme is that the hydrogen tank is heavy, as liquid hydrogen needs a large amount of insulation to keep it cool, and this pushes up the tank mass, making the rocket fairly heavy. This means that SSTO is barely achieveable, and probably not worth it (due to low payload). However, a TSTO rocket could probably be made and would have a decent payload, due to the high ISP.

The laser is the most difficult aspect however. To construct it, multiple laser diodes can be ganged up behind a telescope like device; which is straightforward, but the focusing and aiming are complex, as with all laser launch systems.