# Aerospace Specific Impulse

1. Mar 27, 2004

### Clausius2

What is the physical meaning of the specific impulse?. All i know is what i've read in rocket informations. Its dimensions are s^(-1), and appear as one of the principal variables in rocket engines.

Thanks.

2. Mar 27, 2004

### Lord Flasheart

I think it means, "The speed and how effeciently the exhaust of a rocket helps you."

The higher the specific impulse, (ISP) the faster you can go. Chemical rockets (ISP = 450) are relatively ineffecient for going to stars, but highly effective for getting off the planet. With the exception of nuclear pulse rockets (ISP = 10,000), acceleration is faster with lower ISP. Thus, an antimatter-matter annihilation rocket will take centuries just to accelerate to near the speed of light, as it's ISP equals around 280,000.

3. Mar 27, 2004

### enigma

Staff Emeritus
You're sort of right, LF.

It is the efficiency of the rocket, but it doesn't define top speed. The combination of Isp and the fuel ratio of the rocket determine your final speed.

Specific Impulse is defined as:

$$\frac{T}{dm/dt * g_0}$$

where
T is the rocket's thrust
dm/dt is the propellant mass flow
and
g0 is the acceleration of gravity at Earth sea level.

The units are 'seconds', not s-1

The term basically sums up how efficient a rocket engine is. For any specific propellant combination, you can make a different engine which produces more thrust, but the mass flow will increase proportionally.

The theoretical Isp, which is what is listed for propellant combinations is a function of the propellants alone. Actual Isp is lower, and also depends on ambient pressure and the geometry of the expansion nozzle. The actual Isp is what is listed if you're looking at specific motors.

Isp * g0 is also the velocity of the exhaust plume relative to the rocket in units defined by g0

4. Mar 27, 2004

### Lord Flasheart

Semi-mea culpa. *grins*

5. Mar 28, 2004

### Clausius2

Good explanations, guys.
Thanks.

6. Mar 28, 2004

### Clausius2

And congratulations for the record of velocity of the X-33 proyect(is this its name?. I've watched it today on the TV. USA is USA, as we say here.

I could question this in the article of the hypersoar program, but since i'm here now: in which web page i could see the internal design of the X-33? I'm referring to Scramjet engine. I haven't had success in looking for one really good.