Some comments on NTR's and ISP

[truebeliever]

(1) One member of the forum made this comment

"Some math on the efficiency: getting from low earth orbit to a best-case Mars trajectory takes about 3.8 km/s delta v. A J-2 powered Mars ship thus needs a mass ratio of 2.43133, though a 2000 sec-Isp NTP Mars ship needs a ratio of just 1.21371, and less mass means less required funding..."

Okay, first lets do some simple engineering...A 1000 sec ISP rocket requires an outlet temperature of approx 3200 K. For a nuclear thermal rocket, ISP is directly proportional to the square root of the temperature, hence, a 2000-sec NTR will require approximately 13,000 K. What material can handle kind of temperatures? The best ISP of the NERVA program was theoretically (ie estimated vacuum) 825-850 sec, which corresponds to a temperature of approximately 2300 K.  Some of the NERVA tests went to higher temperatures, but they were very short duration tests, certainly not on the order of one-two hours needed for a Mars burn. Designing/developing a  reactor fuel that can handle 3200K is a big jump from the NERVA design, and it is not going to be cheap. A ground test  alone will cost at least 2-3 billion, IF it can ever satisty all of the environmental lawsuits, etc.

(2) It is easy and fun to design space missions studies assuming this or that ISP/thrust performance.  Designing the engine to give that performance is much more difficult, which is why every NASA center (MSFC, Glenn, JSC, etc) has a very large contingent of people doing "architecture" trade studies. But God forbid if you should tell them what can and can't be done in the real world.  

[Tap-Sa]

truebeliever - 1/1/2006  11:04 PM
 
a 2000-sec NTR will require approximately 13,000 K. What material can handle kind of temperatures?

Nothing in a solid form so instead of solid core NERVA we'd need a gas core rocket, a purely conceptual tiger.

[Flightstar]

truebeliever - 2/1/2006  3:04 PM

Certainly not on the order of one-two hours needed for a Mars burn.

I was told just today that the MTV NTR burn would be 36 minutes. I am not that sort of engineer, so does 36 minutes give any clues? Remember, I believe Langley has said previously, that it's not *just* or *only* NTR that is being looked at in a tandem system.

[truebeliever]

Hmm,

The gas core concept has been bandied about by LANL for several years now. It has some serious problems, and it will require more development money and testing  than a solid core NTR.  

I may be mistaken, but I thought I  saw a presentation at a Joint Propulsion Conference that a chemical / aerobrake combination would beat both an NTR and NEP system for a Mars manned vehicle (but not for a cargo, NEP was superior due to payload fraction).

I think aerobraking would be easier to test and cheaper to get working than an NTP system.

PS: I am a big fan of nuclear, but if something else is cheaper and works as well, why not use it? We will definitely need nuclear for surface power on Mars and maybe on the moon. Let's start with it. It is much simplier to implement.  

[SimonShuttle]

Would someone be kind enough to explain what ISP is and how heat makes this a higher value?

I'm afriad I'm only used to velocity, and I'm sure I'm only a handful of people on here who don't know ISP etc. Any help would be great for myself and any people who aren't sure too.

It only has to be a basic few lines or a link.

[Tony T. Harris]

vanilla - 2/1/2006  10:33 PM

SimonShuttle - 2/1/2006  4:27 PM

Would someone be kind enough to explain what ISP is and how heat makes this a higher value?

I'm afriad I'm only used to velocity, and I'm sure I'm only a handful of people on here who don't know ISP etc. Any help would be great for myself and any people who aren't sure too.

It only has to be a basic few lines or a link.

Isp stands for specific impulse, and is a nice way to list the exhaust velocity of the rocket.  I may be wrong, and it may not exactly be the same as exhaust velocity, but it's very convenient to understand it that way.  When you multiply specific impulse (in seconds) with gravitational acceleration (in meters/second^2) you get exhaust velocity (in meters/second).  So more Isp is more exhaust velocity, since the gravitational acceleration is just a convenient multiplier (9.81 m/s2).

Given the extra parameter of gravitational acceleration, your explanation is a very good point of reference.

[SimonShuttle]

vanilla - 2/1/2006  4:33 PM

SimonShuttle - 2/1/2006  4:27 PM

Would someone be kind enough to explain what ISP is and how heat makes this a higher value?

I'm afriad I'm only used to velocity, and I'm sure I'm only a handful of people on here who don't know ISP etc. Any help would be great for myself and any people who aren't sure too.

It only has to be a basic few lines or a link.

Isp stands for specific impulse, and is a nice way to list the exhaust velocity of the rocket.  I may be wrong, and it may not exactly be the same as exhaust velocity, but it's very convenient to understand it that way.  When you multiply specific impulse (in seconds) with gravitational acceleration (in meters/second^2) you get exhaust velocity (in meters/second).  So more Isp is more exhaust velocity, since the gravitational acceleration is just a convenient multiplier (9.81 m/s2).

Another interesting way to understand specific impulse, that I heard recently, is that it is the number of seconds that one pound of propellant can produce one pound of thrust.

There's probably better ways to understand this, but hopefully this is a start.

Cheers.

What system or form of propulsion gains the best ISP? I'm thinking Nuclear options would be best given the engine block and core being the whole deal of fuel and propulsion infrasture, as opposed to LOX/LH2 propellant being required in large and heavy amonts for the conventional way of a rocket burn as we're used to (including the likes of methane etc.)

I suppose this is why the ESAS report say chemical propulsion isn't viable?

Thanks for the help, I'm still learning a lot of this since just getting into all of this from seeing Shuttle launches on the telly

[To The Stars]

Yes, chemical propulsion is more prone, from what I know, to potential failure. Not something you want en route to Mars.

[CuddlyRocket]

Here's a Wikipedia article on specific impulse.

The exhaust of a chemical rocket is simply the escape of hot gas expanding out of the combustion chamber. The heat comes from the chemical reaction of the fuel and oxidiser. Obviously this is a fixed amount, depending on the exact fuel and oxidiser chosen.

A nuclear rocket produces the heat from the nuclear reaction, and can therefore heat the gas to much higher temperatures producing higher exhaust velocities.

Ion engines use electric and magnetic fields to accelerate ions (charged atoms) to very high velocities. This produces a very high specific impulse, but the power requirements mean that it is only possible to accelerate a small amount of propellant at a time, thereby only producing a small thrust and hence acceleration. However, the engine can operate for months, and final velocity achieved is acceleration multiplied by time, so such engines produce much higher final velocities for a given mass of propellant (or the same final velocity for less propellant).

[FransonUK]

A simple way for the likes me of to grasp this is "the faster you can get your energy force coming out of the back (engine), the faster you're going to move in the opposite direction (forward)" sorta thing, right? I'm trying to explain this to a work collegue.

I'm sure someone can explain this better

[JamesSpaceFlight]

>The specific impulse advantage of the NTR comes entirely from the fact that the average molecular weight of the exhaust (hydrogen) is 2, whereas the average molecular weight of a LOX/hydrogen engine is around 15. If it had a pure water vapor exhaust, the average molecular weight would be 18 (for water) but there's lot of reasons you don't want to do this. It's called running at "stoich" for stoichiometric, and it's a good way to burn up your engine.<


Absolutely, Vanilla. A very important lesson for any aspiring engineer on here.