Variable specific impulse magnetoplasma rocket(VASIMR)

[Monsterboy]

Hi,

I was reading about VASIMR ,my knowledge in rocket science is very limited ,i was able to understand how plasma is produced and used for propulsion but i didn't understand how specific impulse is varied and why is it not possible for other (ion)thrusters or even chemical ones to vary the specific impulse?

http://en.wikipedia.org/wiki/VASIMR

 

[Greg Bernhardt]

I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?

 

[Monsterboy]

I read about specific impulse and what it means for a rocket ,i read a number of published papers but all of them were concerned about design,construction and testing of the engine. I could not find any information about how exactly specific impulse is varied and reason behind the inability of other ion thrusters to do this.

The concept of specific impulse itself is a bit confusing for me,it is one of the ways of measuring rocket efficiency, right? it can be measured either in terms of exhaust velocity(m/s) or in terms of time (sec) i.e the amount of time the propellant will last.

If specific impulse is high then by definition the efficiency is high, it also means that the exhaust velocity is high but if the exhaust velocity is high that means a lot of kinetic energy is lost to the exhaust gases then the efficiency should actually decrease right?
I was actually supposed to present a seminar on this(a month ago) ,none of my teachers had any idea about this so i just explained how the engine is built and how it works and then left the stage, leaving the audience to decide how the specific impulse is varied.

 

[D H]

I read about specific impulse and what it means for a rocket ,i read a number of published papers but all of them were concerned about design,construction and testing of the engine. I could not find any information about how exactly specific impulse is varied and reason behind the inability of other ion thrusters to do this.

Most ion thrusters are on/off sorts of beasts. There's only one flow rate, only one exhaust velocity. This simplifies the design considerably, and also keeps the electrical power consumption down to a manageable rate. However, this means the thrust is inevitably very low. They have a very high specific impulse but that low thrust means they can't be used for anything big, or for anything that needs to get from point A to point B quickly. These thrusters are for tiny vehicles that can get by with going from point A to point B at a snail's pace.

VASIMR is an attempt to address the shortcomings of ion thrusters while maintaining that key advantage of high specific impulse. The specific impulse drops markedly when VASIMR is used to provide high thrust.

The concept of specific impulse itself is a bit confusing for me,it is one of the ways of measuring rocket efficiency, right? it can be measured either in terms of exhaust velocity(m/s) or in terms of time (sec) i.e the amount of time the propellant will last.

Think of it as exhaust velocity. That, along with the exhaust mass flow rate tells you both thrust and energy consumption.

It's also best not to think of Isp as indicative of "efficiency", whatever that means. (There are a number of definitions of rocket efficiency; none are particularly useful.)

If the exhaust velocity is constant, there's an optimal exhaust velocity to obtain a given delta v. Too low an exhaust velocity means low thrust. The rocket doesn't go anywhere unless it carries a huge amount of fuel. Too high an exhaust velocity means too much energy is pumped out as exhaust. The rocket once again doesn't go anywhere unless it carries a huge amount of fuel. There's a happy optimum, exhaust velocity equal to about 5/8 the desired delta v, that minimizes the energy needed to achieve that delta v.

As a good example, the first stage of the Saturn V rocket used kerosene rather than hydrogen as the fuel. Why? The answer is that hydrogen with it's higher specific impulse was too high. Even kerosene was a bit high for the job of the Saturn V first stage. Hydrogen would have been overkill.

 

[cjl]

If the exhaust velocity is constant, there's an optimal exhaust velocity to obtain a given delta v. Too low an exhaust velocity means low thrust. The rocket doesn't go anywhere unless it carries a huge amount of fuel. Too high an exhaust velocity means too much energy is pumped out as exhaust. The rocket once again doesn't go anywhere unless it carries a huge amount of fuel. There's a happy optimum, exhaust velocity equal to about 5/8 the desired delta v, that minimizes the energy needed to achieve that delta v.

As a good example, the first stage of the Saturn V rocket used kerosene rather than hydrogen as the fuel. Why? The answer is that hydrogen with it's higher specific impulse was too high. Even kerosene was a bit high for the job of the Saturn V first stage. Hydrogen would have been overkill.

In most cases, rockets are far more constrained by their available reaction mass than anything else, so this isn't really correct. As for the Saturn V, the reason not to use H2 for the first stage was probably that hydrogen's low density makes it very difficult (and bulky) to store the enormous amount needed for the first stage, and for a given thrust level, a hydrocarbon engine will be smaller as well. When combined with the fact that kerosene is much easier to store than liquid hydrogen, it is easy to see why kerosene would be preferred if a rocket design can be made to use it instead of hydrogen. The upper stages had to use hydrogen, since for a high delta V rocket, the overall payload and delta V capability are much more sensitive to upper stage performance than lower stage performance, but if hydrogen did not have the density and storage disadvantages, the rocket would have performed better still with a higher ISP in the first stage as well.

 

[Monsterboy]

The rocket operates at high thrust-low specific impulse mode while lifting heavy payloads near the Earth and operates at low thrust -high specific impulse after going out of Earth's gravitational influence?

I read that VASIMR can reduce the traveling time to Mars to just 37 days ,while operating at high specific impulse(and at low thrust) does the spacecraft constantly accelerate in order to reach higher velocities?

Why can't other ion engines with their high specific impulse reach the same velocity with constant acceleration?

 

[cjl]

The rocket operates at high thrust-low specific impulse mode while lifting heavy payloads near the Earth and operates at low thrust -high specific impulse after going out of Earth's gravitational influence?

I read that VASIMR can reduce the traveling time to Mars to just 37 days ,while operating at high specific impulse(and at low thrust) does the spacecraft constantly accelerate in order to reach higher velocities?

Why can't other ion engines with their high specific impulse reach the same velocity with constant acceleration?

They can. It just takes them months (or years) to do so due to the extremely low thrust.