 What is new in Space Propulsion?
We are trying to develop efficient methods to move spacecraft once they leave the Earth's atmosphere. Traditional rockets based on chemical reactions have relatively low specific impulses. The goal is to design and develop new engines with performance levels higher than those of chemical rockets.
Fine, but what is "specific impulse" anyway?
It is a definition engineers use to measure how well a propulsion engine performs. In mathematical language is just how much thrust the engine produces divided by the amount of mass flow rate contained in the exhaust . The higher the thrust, the better for ground-to-orbit rockets, this however comes with a correspondent increase in mass flow rate (more propellant is needed to make the rocket move against the gravity pull), in most cases this sets a limit on specific impulse. The specific impulse bears a direct relationship with the kinetic energy contained in the rocket exhaust, or, in other words, with the velocity at which this exhaust leaves the engine. For chemical rockets the limit is set by the amount of energy produced by the reaction. Unfortunately for rockets, this energy is not high enough and by now the limit has been practically achieved. Chemical rockets cannot have specific impulses higher than about 500 seconds.
Why seconds?
Well, it is a convention. You could well multiply by earth's gravitional constant (g = 9.8 m/s^2) and have the specific impulse (or Isp, as rocket scientists love to call it) units converted into m/s, or speed. A chemical rocket (extremely good, by the way) with an Isp of 500 sec produces an exhaust which moves at approximately 5000 m/s.
How high is the Isp for the newest non-chemical propulsion systems?
As high as the energy you can put into the exhaust, and that depends almost uniquely in the technology level.
Is this a Star-Trek vision?
It was some time ago. Not anymore. New propulsion systems are being tested all the time and some of them have already flown. Electric propulsion seems to be the baseline for future developments of spacecraft engines.
Electric Propulsion?
The exhaust of the rocket is made out of charged particles, which can feel and react to electric and magnetic fields. In electric propulsion we want to use these fields to accelerate charged material to very high velocities. In principle one could make these particles accelerate to relativistic speeds, obtaining Isp's of hundreds of thousands of seconds!! Great! So why don't we build a rocket and go straight to Mars in a single day?
Because we need energy, a LOT of energy to accelerate the particles and to create them in the first place. In some cases we could get this energy from nuclear reactors, unfortunately, even though nuclear energy in space is perhaps the best use of this technology, the question of its applicability is more political in nature. For the time being we are stuck with solar arrays, which limit the amount of power to a few tens of kilowatts, allowing us to get thrusts of the order of 0.01 - 1 Newton, but with the advantage of very low mass flow rates, thus yielding specific impulses in the range of 1,500 - 20,000 sec. Absolutely higher than chemical Isp's!
What kind of missions can be accomplished?
With current electric propulsion systems we can get considerable savings in propellant quantities for most missions we already do, like orbital maintenance and transfers. These savings reduce costs and increase performance. The only drawback, perhaps, is the time to complete mission . Low power electric propulsion is efficient but slow: a chemical rocket can put a given payload in geosynchronous orbit (high altitude orbit, about 36,000 km) after one engine burn in a low altitude orbit (say, 250 km away) to force a transfer to the high altitude orbit and finally a second burn to make the orbit circular. The time to complete this mission can be measured in hours. For low power electric propulsion, the same mission may take weeks with a trajectory that spirals up from the low altitude orbit to the high altitude orbit. The good thing is that instead of carrying one ton of fuel you need a fraction of that. Weight savings are related to launching savings. Such a change in propulsion systems can save the satellite operator millions of dollars in launching costs. The bad thing is that the satellite would receive strong doses of radiation as it spends considerable time crossing Earth's radiation belts. Some trajectory optimizations, however, are possible and this time can be minimized for low thrust missions.
There are missions (deep space planetary exploration) that simply cannot be attempted using chemical rockets given the very high changes in velocity required (for example deltaV=30 km/s for JIMO, the Jupiter Icy Moon Orbiter). We could say that the biggest market for advanced propulsion concepts is that of exploration and discovery.
MPD thruster test firing at MIT's Astrovac |
