NASA's Marshall Space Flight Center is the central location for development of alternative space propulsion. Its focus is on developing and testing new technologies that will enable interplanetary and interstellar flight.
Unlike a car that has the road to push against, or an airplane that has the air to push against, rockets don’t have roads or air in space. Today’s spacecraft use rockets and rockets use large quantities of propellant. As propellant blasts out of the rocket in one direction, it pushes the spacecraft in the other direction.
NASA is currently developing a RLV (reusable launch vehicle). The Space Shuttle currently costs $10,000/pound to put something into space. NASA's goal is to develop a craft capable of putting something into space at a cost of $1000/pound. The X-33 (scheduled for launch in July 2000) is the first RLV since the shuttle. Its looks like a Stealth bomber. It is an unpiloted vehicle that will take off vertically like a rocket and land horizontally like an airplane. Two aerospike engines power it.
The X-34 currently under development at NASA is designed to be carried on the underbelly of a L-1011. The first engine developed since the shuttle, called Fast Trac, will power it.
This chart highlights two critical features of a rocket, Thrust and Specific Impulse. Thrust is how much push a rocket can give. The higher up on the chart, the greater the push. Specific Impulse can be thought of as a kind of fuel efficiency for rocket engines, analogous to the miles-per-gallon for cars. Specific Impulse is the time that it takes to burn one pound of fuel to produce one pound of thrust. The farther right on the chart, the less propellant you’ll need. It really has to do with how fast the fuel blasts out of the rocket. What you should notice is the red region. This is the range of rocket performance we can conceivably create with what we know today. And what we need for interstellar travel is in that desired region or even more fuel-efficient.
Chemical propulsion is advantageous for earth orbits as it has high thrust to weight ratio. The space shuttle has an Isp of 500 sec. There is not enough mass in the universe to send the shuttle to our nearest star (allowing about 900 years for it to make this journey).
NASA is currently developing a nuclear thermal propulsion system proposed for interplanetary use. Testing began in 1969. It has an Isp of 1000 sec. At rip to Mars would take about 300 days using this fission system. It would take a billion supertankers full of fuel to use this fission system to send our "shuttle" to our nearest stellar neighbor.
NASA is also developing an antimatter propulsion system having a predicted Isp of 1,000,000 sec. It uses antiprotons to produce gamma rays. A trip to Mars using this fusion system would take 30 days. It would take a thousand supertankers of antiprotons to make the round trip to our closest neighbor. Fermi Labs currently produces nanograms of antiprotons per year at a cost of $62.5 trillion/gram.
We’d really like to have a form of propulsion that doesn’t need any propellant! This implies the need to find some way to modify gravitational or inertial forces or to find some means to push against the very structure of spacetime itself. Our third big challenge is energy. Even if we had a nonrocket space drive that could convert energy directly into motion without propellant, it would still require a lot of energy. Sending a Shuttle-sized vehicle on a 50 year one-way trip to visit our nearest neighboring star (subrelativistic speed) would take over 7 x 10^19 Joules of energy. This is roughly the same amount of energy that the Space Shuttle’s engines would use if they ran continuously for the same duration of 50 years. To overcome this difficulty, we need either a breakthrough where we can take advantage of the energy in the space vacuum, a breakthrough in energy production physics, or a breakthrough where the laws of kinetic energy don’t apply.
To enable practical interstellar travel, here are THE 3 breakthroughs that we’ll need.
Discover new propulsion methods that eliminate or dramatically reduce the need for propellant. This implies discovering fundamentally new ways to create motion, presumably by manipulating gravity or inertia or by manipulating any other interactions between matter and spacetime.
Discover how to attain the ultimate achievable transit speeds to dramatically reduce deep space travel times. This implies discovering a means to move a vehicle at or near the actual maximum speed limit for motion through space or through the modification of spacetime itself.
Discover fundamentally new on-board energy production methods to power propulsion devices. This third goal is included in the program since the first two goals could require breakthroughs in energy generation to power them and since the physics underlying the propulsion goals is closely linked to energy physics.