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EarthWolf

Member

From: Missouri, U.S.A.

Registered: 2004-07-20

Posts: 59

Re: Impulse and Specific Impulse

Hello,

When reading the specifications on boosters, I've found the terms Impulse and Specific Impulse. What do these terms mean? Are they terms that define how many seconds of acceleration that the vehicle produces?

Cordially,

EarthWolf

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" Man will not always stay on the Earth. "

Konstantin Tsiolkovsky

srmeaney

Member

From: 18 tiwi gdns rd, TIWI NT 0810

Registered: 2005-03-18

Posts: 976

Re: Impulse and Specific Impulse

Isp (Impulse Specific) is a rating used to define a relationship between force and a given fuel/oxidiser mix.

Isp=sqroot((2/k-1)*((R*Tc)/(Mw*g))*[1-((Pe/Pc)^((k-1)/k))])

examples:
Ozone + Hydrogen=373 Isp
Oxygen + Hydrogen=345 Isp

There is a nice table of fuels for rockets and their specific impuse values on page 310 of Handbook of the engineering Sciences, Vol 2,  D.VAN NOSTRAND COMPANY INC. 1967

Impulse (i can only assume we are talking about Impulse total)
It (or Impulse Total) = F * dt

The book includes a nice section on nuclear reactor engineering and the SNAP 10a Nuclear Propulsion System. It also includes early thoughts on Solar Sails.

GCNRevenger

Member

From: Earth

Registered: 2003-10-14

Posts: 6,056

Re: Impulse and Specific Impulse

In laymens' terms, specific impulse reffers to the efficency of a particular fuel/engine combination (at least ones that use exhaust gas for push). The higher the specific impulse, the more push per-pound of propellant you get. The higher this value is, the more momentum change you get for a given gram of propellant.

Take note, that specific impulse has nothing to do with how much thrust an engine makes, ion engines have a very high S.I. but extremely low thrust.

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[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]

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Mad Grad Student

Member

From: Phoenix, Arizona, North Americ

Registered: 2003-11-09

Posts: 498

Website

Re: Impulse and Specific Impulse

Essentially total impulse is the total amount of momentum, and by extention velocity, change capability a vehicle has with a full load of propellant. It's measured in Newton X Seconds (Ns), which in turn is force times time. For example, if a given vehicle has a total impulse of 10 Ns, that would mean it could sustain a newton of force for ten seconds, or vice versa, or any other combination of the two that when multiplied together equal ten, provided of course the engine can throttle to that value. Naturally, a rocket with a smaller total impulse (such as SpaceShipOne) cannot change its momentum (and thus usually velocity) as much as one with a high total impulse (such as the Saturn V first stage).

On the other hand, specific impulse is, as GCN noted, a measure of efficiency. Specific impulse measures the amount of momentum change you can achieve not out all fuel on the vehicle, but for a given amount of propellant. By contrast isp is measured in Newton X Seconds/Newtons, usually just abbreviated to Seconds. In other words, specific impulse measures the number of newtons (or pounds if you prefer English units) of thrust you can get from a kilogram (or pound-mass) of propellant.

In rocketry specific impulse and the rocket equation are probably the two most critical principles involved. The rocket equation: MR= e^delta-V/isp, where MR= mass ratio, delta-V is total velocity change, and isp is the specific impulse of the rocket's engines, determines how much of a rocket's starting mass will have to be fuel in order to reach a given velocity change. In order to go from a standing start at a temperate lattitude to low-Earth orbit, a minimum velocity change of 8 km/s is required, but engineer's usually have to deal with around 9 km/s of delta-V after factoring in losses due to atmospheric drag, steering, and the time a rocket spends going straight up after launch burning fuel but not adding to its tangental (orbital) velocity. This is an immutable law of the universe that must be addressed in order for any rocket to work in its intended role.

The rocket equation is so demanding of orbital launch vehicles, whether or not single-stage-to-orbit (SSTO) LVs are possible is a subject of hot debate. Going to orbit is easy in multiple stages because each stage only has to deal with a smaller portion of the total delta-V, but a single stage has to deal with the whole thing up front. As the rocket equation dictates, the amount of propellant a rocket needs increases exponentially as delta-V increases linearly, which is why Robert Zubrin says "engineers will kill to get delta-V down and isp up." Modern rocket engines are very close to the theoretical specific impulse limits of their fuels, which for liquid oxygen and hydrogen is about 4.5 km/s (around 470 s, guesstimating without a calculator). Until there's a breakthrough in something such as airbreathing engines or a revolutionary new propulsion system that variable won't get much better.

So, plugging these variables into the rocket equation one sees that an SSTO launch vehicle burning oxygen and hydrogen can have no more than about 15% of its starting mass be tanks, engines, electronics, payload, everything but the fuel. That's no problem for an expendable LV, but for a reusable one, perhaps carrying a crew capsule, ouch. It's a tight squeeze. GCN and I have intensely debated whether or not it's possible to develop such a reusable SSTO launch vehicle for a price of under a billion dollars or so. Expensive or cheap, such a rocket will be extremely difficult to make.

This is why specific impulse is sooooooooo important for rocket scientists.

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