Friday, November 6, 2009

Thesis Procrastination - VASIMR Rockets

Since I am in my final year of an engineering physics program I am required to do a thesis based on an applied physics topic. It is a challenging project but so far I am learning a lot and find it quite interesting. In order to avoid actually doing the work I figured I would share some of the more interesting parts with the Internet.

Over the summer I was wasting my time one evening endlessly clicking the Stumble! button in my browser and I came across a NASA press release on a new type of rocket technology being developed which sparked my interest. It was called the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). Instead of using the combustion of either a solid or liquid fuel this upcoming technology uses electricity to generate a plasma and send it out the rear of the engine at tremendous speeds.

Plasma is the fourth state of matter and the most common form of visible matter in the universe. Whenever a gas gets hot enough the electrons, that were happily orbiting the nucleus get enough energy that they are no longer bound to the atom. The gas then becomes ionized with free electrons (negative charge) coexisting with positive ions. This gives the substance some interesting properties. Since it consists of charged particles a magnetic field can act on it and contain it. This is important because the temperatures required to create a plasma are too hot to be contained by any known material.

So the basic design of the engine works like this. A gas is sent into a chamber. A helicon source then turns the gas into a plasma. Superconducting magnets then contain this plasma and bring it into the second stage. Another process called Ion Cyclotron Resonance Heating then takes over and turns this incoming "cold" plasma (ie. low energy) into a hotter plasma. Both the helicon source and the ICRH are powered by electricity and are forms of RF heating. In the final stage the hot plasma moving down the engine rotating in a shape similar to a helix. The magnetic field is then decreased in a "magnetic nozzle" and this spinning motion is converted to linear motion and fired out the rear of the engine at high speed. The advantage of this high exit velocity is that less propellant mass (the ionized gas) can be used to achieve greater changes in momentum and therefore velocity.

Chemical rockets in space are effectively using the same technology the Chinese developed in the 9th century. You combine a explosive material with an oxygen source and ignite it. It works but it is wasteful because the exhaust gases leaving have an upper limit on their velocity. The chemical energy stored in the fuel just can't push the exhaust infinitely fast. In automotive terms the fuel economy of chemical rockets is low. They throw out a lot of mass and fairly low speeds. Plasma rockets shoot out less material but at much higher speeds, increasing their fuel economy. (In technical terms a chemical rocket has a low specific impulse while the plasma rocket has one of the highest compared to other methods of space travel.)

Basic layout of the VASIMR - Photo: NASA

Imagine sitting on an office chair in the middle of the room. You are carrying a box of BB's, a box of shotgun shells and a shotgun. If you were to grab a handful of BB's and throw them you would start moving slightly in the other direction. Here the BB's are the exhaust gases of the rocket and your muscles that throw the BB's represents the stored chemical energy of the fuel. Now load the shotgun and fire it towards the wall. You will feel a big kick and your chair will start rolling backwards. The contents of the shell might only have the equivalent mass of 30 BB's, far less than handful you just threw yet the chair moved much faster. Clearly the shell had more momentum in the forward direction, therefore it changed your momentum in the backwards direction by a larger amount. In this analogy the chemical energy contained in the shotgun shell is represented by the VASIMR system.

In a chemical rocket the propellant and the energy source are combined into the rocket fuel, while in plasma rockets the energy source and propellant are two different entities. A gas such as hydrogen or argon acts as the propellant and the energy is supplied by either a nuclear reactor or solar panels.

So that is the basic physics behind the idea of a plasma rocket but as I said before this was an applied physics thesis. The applications of this promising technology include faster interplanetary missions (39 day trips to Mars have been estimated!) and efficient orbital boosting of the ISS. Both are very cool and I hope we will see these ideas come to fruition.

I had a somewhat different idea and was interested to see if the technology could be applied to deflect incoming asteroids and comets. They are a real threat and for the first time in the history of our planet there is a species that can actually do something about it. Very little actual work has been done on what to do if an impact is suspected. It is a low probability yet high consequence event. Current proposed methods include lasers, gravity tugs, kinetic impactors and a volley of nuclear weapons. The techniques then require you to change the orbit very slightly. Because distances are so vast in space a small change in speed (~1cm/s) should alter the orbit enough so that a collision is avoided. I hope that the VASIMR thruster will be able to change the velocity of the incoming asteroids in a shorter period than competing ideas. Just like every other method however it is crucial to discover these potentially dangerous rocks early, years in advance.

My basic goal of this thesis is to design a system that can land on an incoming asteroid, deploy a nuclear power station and a plasma thruster system and then fire the engine for months at a time slowly changing the asteroid's orbit. It has been interesting so far, and I am curious how feasible it turns out to be. Any questions or genius ideas to help? Let me know.