Author Topic: Rocket and jet engine specific impulse (Read 16028 times)

KomodoLizard · « **on:** 03/04/2011 12:31 am »

Why is the rocket so much lower. I just did a quick calculation on the amount of kerosene a rocket would use per second to generate 250,000 to 300,000 pounds thrust, in other words i left the liquid oxygen out of things. Let's just say a B747 or A 380 would be out of kerosene real quick.

The only thing I could find was the jet had the extra weight of the nitrogen in the air to eject out.

Arthur · « **Reply #1 on:** 03/04/2011 01:33 am »

I am no expert, but I think that only a tiny fraction of the oxygen in the air is combusted in a jet engine, so huge volumes of air are free reaction mass boosting the ISP. It is an apples to oranges comparison.

Proponent · « **Reply #2 on:** 03/04/2011 04:09 am »

As already noted, the jet engine gets it's oxygen from the air. Since burning a kilogram of kerosene requires about 2.3 kg of oxygen, that gives the jet an advantage of a factor of 3.3 in propellant consumption (1 kg of kerosene rather than 3.3 kg of kerosene plus lox). The jet's advantage over the rocket is actually much larger than that, however. I make it about a factor of 40, so very roughly there's another factor of ten to go. Look at a rocket's exhaust: it's very hot, meaning that there is a lot of energy going to waste in the form of heat out the nozzle. Jet engines' exhausts aren't nearly so spectacular (though I did once see a Concorde take off under after-burner at dusk: that was spectacular). I think therefore that most of the jet's advantage comes from its ability to dump a lot of the heat generated into a large mass of air--not necessarily just non-reacting nitrogen--and produce thrust from it.

gospacex · « **Reply #3 on:** 03/04/2011 04:42 am »

Quote from: Proponent on 03/04/2011 04:09 am

Look at a rocket's exhaust: it's very hot

Not really. In some efficient LH/LOX engines, for example, exhaust temp falls below 100C.

IsaacKuo · « **Reply #4 on:** 03/04/2011 04:42 am »

It is indeed because it's pushing against a lot of air that isn't involved in combustion. Modern jets use high bypass turbofans, where the majority of the air entering the fan simply bypasses the turbine engine. It is used as inert reaction mass to push against.

Proponent · « **Reply #5 on:** 03/04/2011 05:56 am »

Yeah, that makes more sense. Forget what I said about heat.

sandrot · « **Reply #6 on:** 03/04/2011 08:34 am »

Quote from: gospacex on 03/04/2011 04:42 am

Quote from: Proponent on 03/04/2011 04:09 am
Look at a rocket's exhaust: it's very hot

Not really. In some efficient LH/LOX engines, for example, exhaust temp falls below 100C.

It gets even better than that!

KomodoLizard · « **Reply #7 on:** 03/04/2011 08:36 am »

Could it have anything do with the very small throat or exit of the rocket's combustion chamber. I seem to remember reading once that thrust was the amount of mass exiting per "time" factor. A garden hose with the nozzle on very fine spray give the water very high velocity but only a small amount of water exits. Turn the nozzle and the amount of water exiting per second gets bigger and thrust increases.

Are there any similarities to the propeller on a late model P 51 Mustang or Spitfire and a 1500 HP Merlin engine as compared to a helicopter with a 1500 HP and the rotor. If we put the helicopter rotor on the Mustang (and ignore ground clearance issues) and gear the Merlin engine down then plane will roar down the runway, it could probably tow a truck against the brakes but it will never get up much speed

But if we put the Mustang propeller on the helicopter it won't budge. So we might need 20 of the and hence 20,000 HP and 20 times the fuel consumption. However, the helicopter will now probably do 300 mph straight up.

The rocket appears to me to be like taking off on a very steep hill in a fully loaded semi and having it to do it in overdrive. Might need a 10,000 HP engine but the truck could do 600 mph

Nomadd · « **Reply #8 on:** 03/04/2011 11:11 am »

Like IsaacKuo said, very little, or less than 10% in the most modern cases, of a jet engines thrust is from turbine exhaust. Almost all of it is from the big bypass fan. You have to treat it more like a prop engine when you figure the thrust.

Vahe231991 · « **Reply #9 on:** 07/23/2022 03:12 am »

According to a diagram of specific impulse by Pratt & Whitney on page 69 of Bill Sweetman's 1993 book Aurora: The Pentagon's Secret Hypersonic Spyplane (attached below), rocket engines have a specific impulse of less than 1,000 lbs thrust per pound of fuel per second, which remains constant across all Mach numbers, while hydrogen-fueled jet engines, ramjets, and scramjets have a higher specific impulse than rocket engines or hydrocarbon-fueled jet engines, ramjets, and scramjets when operating from subsonic speeds to Mach 12. The difference between the specific impulse of rocket engines and that of hydrogen-fueled jet engines is quite surprising given that hydrogen offers high specific impulse thanks to its immense energy content per unit of weight. In an expanded view of the Pratt & Whitney diagram on page 70 of Sweetman's book (see attached file), ducted rockets offer greater specific impulse, generating up to 2,000 lbs thrust per pound of fuel per second compared to a generic rocket engine because it acts as a catalyst for a ramjet or scramjet to ignite at high supersonic speeds.