Frequently Asked Questions
1. What type of propellant do VASIMR® engines use?
2. Would it be possible to mount one of your VASIMR® motors on a high altitude airship?
3. What is the top speed of a VASIMR® engine?
4. Can VASIMR® vehicle take off from the ground like the Space Shuttle?
5. How does rocket efficiency relate to speed?
6. How much money did USA government spend on VASIMR® technology?
7. What was the highest efficiency demonstrated by the VASIMR®?
8. While the plasma in the VASIMR® engine gets to be close to the temperature of the sun's surface, how is the engine effectively cooled so its hardware does not melt?
1. What type of propellant do VASIMR® engines use? [top]
The VASIMR® engines are capable of using almost any gas or substance with a high vapor pressure as a propellant. However, some gases are better suited to forming plasmas than others, and the gas’ mass also plays an important role in rocket performance. For missions near Low Earth Orbit and the moon, argon gas has the highest performance for a 200 kW VASIMR®. However, for missions that are much farther away, e.g. Mars or Jupiter, lighter gases are more desirable and give VASIMR® an optimum performance in these cases.
2. Would it be possible to mount one of your VASIMR® motors on a high altitude airship? [top]
VASIMR® engines are not well suited for atmospheric flight, even at high altitudes characteristic of an airship. The high specific impulse of the engine makes it a much better choice for missions in the vacuum of space.
3. What is the top speed of a VASIMR® engine? [top]
VASIMR® does not have a maximum speed. Any rocket engine will continue to accelerate its spacecraft until it runs out of fuel. On Earth, an airplane has a top speed because at some velocity the drag from air friction matches the maximum force that the airplane engines can apply. Space is a vacuum and hence there is no air friction to slow spacecraft down. VASIMR® thruster is very efficient with its fuel, so it can attain very high velocities before it runs out of fuel. For example, a 200 kW VASIMR® spacecraft designed to sling-shot a payload to Jupiter will attain a velocity relative to the Sun of ~50 km/s before releasing its payload and returning to Earth orbit.
4. Can VASIMR® vehicle take off from the ground like the Space Shuttle? [top]
No. Plasma rockets are very fuel efficient but produce much less thrust than their chemical cousins. So, for the foreseeable future, chemical rockets will remain the most effective way to get cargo and humans to and from low Earth orbit (and to land and take off from other planets.) Plasma rockets such as VASIMR® spacecraft on the other hand, are far more suitable to enable a very high speed interplanetary cruise. They would be launched into space by a conventional chemically propelled rocket. Once in space, the VASIMR® engines would turn on and apply a gentle but continuous force for a period or weeks, months, or even years, slowly accelerating to very high velocities.
5. How does rocket efficiency relate to speed? [top]
There are many efficiencies that play a role in overall performance of a rocket ship. The propulsive efficiency of a rocket however is an important one that is related to the ship's velocity. This efficiency increases as the speed of the rocket approaches the speed of the expelled propellant (in a center of mass system). When the rocket moves faster or slower than the speed of the expelled propellant, the efficiency begins to decrease, requiring more than the optimal amount of energy to gain additional speed. For example, the Space Shuttle's orbital speed is around 8000 m/s and the exhaust speed of its hydrogen-lox rocket is about 6,000 m/s. So, after reaching orbital speed around the Earth, the propellant (H20) coming from the Shuttle's main engine is already below the optimum needed to further increase the speed of the orbiting Shuttle... The Shuttle can obviously go faster than the propellant from its engine, but it becomes less efficient the faster it tries to go after reaching orbit. This concept is analogous to the role of the transmission in a car. Shifting gears to faster propellants with increasing rocket speed is a good way to increase rocket efficiency. VASIMR® thruster can expel propellant at speeds ranging from below 30,000 m/s to perhaps as high as 300,000 m/s, or saying it another way, 5 to 50 times faster than the Shuttle's main engine.
6. How much money did USA government spend on VASIMR® technology? [top]
A small NASA-funded experimental program on VASIMR® began in the early 1980s at the MIT Plasma Fusion Center and was transferred to NASA's Johnson Space Center (JSC) in the early 1990s. In cooperation with NASA, the project was privatized by Ad Astra Rocket Company in 2005. Since that time, Ad Astra has continued the development of the technology exclusively on private investment. Over the 25 years at NASA preceding privatization, the total direct government funding for the project by the space agency was approximately $6,000,000. Since the company’s formation in 2005 – with the exception of a small 2010 contract to support testing of a high-temperature superconducting magnet, totaling $142,000 – Ad Astra has received no direct government funds for VASIMR® technology development. At present, via a Non-Reimbursable Space Act Agreement, Ad Astra and NASA/JSC collaborate on several aspects of mutual interest relevant to technology and systems integration – with no exchange of funds.
7. What was the highest efficiency demonstrated by the VASIMR® thruster? [top]
At the International Electric Propulsion Conference in September 2011, Ad Astra presented VX-200 testing results that demonstrate a system efficiency (fraction of the jet power to the input DC power) of 60% (thruster efficiency, fraction of the jet power to the RF input power, of 72%) at 210 kW and 4800 seconds specific impulse (Isp) with argon propellant.
8. While the plasma in the VASIMR® engine gets to be close to the temperature of the sun's surface, how is the engine effectively cooled so its hardware does not melt? [top]
Although the plasma is very hot, the magnetic field does a good job in containing the plasma and preventing heat exchange with the materials of the rocket core. Heat will still be introduced into the system (from plasma interaction, UV, joule heating, etc.) that needs to be removed from the system. This heat removal process is part of the thermal engineering solution for the VASIMR® engine which is refined as the system becomes more optimized.'