questions about thrust 2

[rickitek]

hello everyone,

is the thrust produce by rocket engine(thrust produce by exhaust gas)dependent on the outside environment. or is the amount of thrust produce in a vacuum space also the same in pressurized space,

thanks,

 

[RPstress]

It is dependent on the speed and mass of the exhaust material, and since this is reduced in any atmosphere it is at a maximum in vacuum. It is also true of waterjet propulsion. The jet is expelled above the water to maximize its velocity and so the thrust. (It is still useful if expelled below the water, see

https://en.wikipedia.org/wiki/VA-111_Shkval,

but is less so.)

 

[rb1957]

rocket thrust depends (in part) on the delta pressure between the exhaust and the surrounding pressure. Maximum delta p when in vacuum (space), but the nozzle design is optimised for a specific delta p.

another day in paradise, or is paradise one day closer ?

 

[KENAT]

Simplistically rockets operate on momentum, principle.

Every action has an equal and opposite reaction.

So throwing a bunch of hot gas out at very high velocity has a given momentum. The rocket itself sees equal momentum in the opposite direction.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[rb1957]

another way to look at it is like an "added mass" problem.

it's harder to run in water (than in air) 'cause it takes more work to move the water out of the way. similarly, a rocket exhaust in space doesn't have to push an atmosphere out of the way; and the added aerodynamic drag is another "loss" ... there's no drag in space, so the rocket thrust (which is higher in space) accelerates the rocket more (without aerodynamic drag).

another day in paradise, or is paradise one day closer ?

 

[rickitek]

https://en.wikipedia.org/wiki/VA-111_Shkval,[/URL] but is less so.)]

so youre saying that the jet velocity and thrust is at maximum when the jet is near the water surface, my thought on this is that when the jet of water strikes the surface of the water it creates an upward force towards the vehicle and this adds to the thrust. just my analysis, correct me if im wrong...

 

[VEBill]

"...when the jet of water strikes the surface of the water it creates an upward force towards the vehicle..."

No.

When the jet of water is accelerated out the back of the vehicle, that in itself causes thrust in the opposite direction.

The column of moving water doesn't carry force like a support pillar.

I suppose if the jet of water somehow bounced off the surface, then it might subsequently ricochet off the vehicle adding an imperceptible additional thrust for a few miliseconds, but that's nothing to do with what's really going on.

Why are JetSki questions here?

 

[rb1957]

I think you misunderstood the water-jet reference ... I think it's referring to a propulsor using water are the fliud (I'd use a personal watercraft as an example).

in nay case, if the exhaust hits a nearby water surface, pushing this water out of the way won't increase the rocket thrust ... it'll create a lot of spray ! it might reduce the thrust as I can see some back pressure building up (increasing the pressure near the exhaust, reducing the delta_p of the exhaust ...).

another day in paradise, or is paradise one day closer ?

 

[tbuelna]

Isn't the exhaust gas produced by a LH2/LOx rocket engine primarily water vapor?

 

[RPstress]

I apologize for mentioning waterjets! I thought it might be a help, as it's a real everyday application where thrust is maximized by minimizing resistance to rapid ejection of the fluid.

 

[btrueblood]

tb, steam plus a bit of excess H2 (the unburned hydrogen gives higher specific impulse, owing to its lower mass and subsequent higher exhaust velocity).

 

[rickitek]

so the outside environment pressure indeed affects the exhaust gas and the thrust that it creates. my next question now is how the outside environment pressure affects the exhaust gas? is it because of friction or resistance between the outside layer of the exhaust gas and the outside environment? or the higher the outside pressure the harder the exhaust gas needs to work to push itself to get out.

note: in answering this question please neglect for awhile the effects of drag to the body of the vehicle and other factors, just focus on the exhaust gas ITSELF and the effect of the outside environment to the flow or performance of the exhaust gas ITSELF, because my goal here is to find the effect of the outside environment pressure to the exhaust gas ALONE.

thank you so much...

 

[IRstuff]

To the first order, it's the pressure differential, since that's what goes into the continuity equations. But, it's a fairly tiny effect.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //

http://www.engineering.com/AskForum/aff/32.aspx

faq731-376 forum1529

 

[rickitek]

Each Space Shuttle Main Engine operates at a liquid oxygen/liquid hydrogen mixture ratio of 6 to 1 to produce a sea level thrust of 179,097 kilograms (375,000 pounds) and a vacuum thrust of 213,188 (470,000 pounds).

http://www.nasa.gov/returntoflight/system/system_SSME_prt.htm

based on the above info from NASA it seems that the difference of thrust at sea level and outer space is really big. is this mainly due to the effect of drag or resistance against the body of the vehicle and not the resistance between the exhaust gas and the outside environment pressure?

 

[tbuelna]

The nozzle geometry is very important. Take a look at the aerospike engine concept, which provides improved performance over a large range of ambient pressure.

 

[IRstuff]

The question was specifically about the velocity of the exhaust gas, which is different than the thrust, which involves drag and gravity, since thrust is usually net, not raw.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //

http://www.engineering.com/AskForum/aff/32.aspx

faq731-376 forum1529

 

[tbuelna]

is the thrust produce by rocket engine(thrust produce by exhaust gas)dependent on the outside environment. or is the amount of thrust produce in a vacuum space also the same in pressurized space,

The OP asked about the effect of the ambient environment on thrust produced by a rocket engine. However, I would agree that the velocity of the exhaust gas exiting the nozzle is an important consideration for the engine performance at various ambient conditions, such as sea level versus the upper atmosphere.

 

[GregLocock]

I suspect the nozzle shape would need to be different for optimum performance in air and in vacuo.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[VEBill]

A highly exaggerated thought experiment...

If one took a ship equipped with a typical rocket motor down near the surface of Jupiter for a flyby, one might find that the atmospheric pressure near the surface of Jupiter was higher than one's rocket chamber, let alone tank, pressure. So when one opened the valves, atmosphere would be sucked up into the chamber and into the fuel and oxidizer tanks. This would cause a short pulse of thrust in the direction opposite of what one would want. It would of course end very badly.

This is not intended to be practical, but it may help clarify the extreme cases.

 

[RPstress]

Nozzles need to be longer and wider (at the exit) the lower the ambient pressure. Nozzles which compensate for different pressures include ones with a big nozzle extension at altitude. For greatest efficiency the exit pressure should be close to the ambient (i.e. very low at high altitude). The longer the nozzle the greater the pressure drop inside it (which starts out at the pressure of the combustion chamber and then drops with distance in a 'normal' De Laval-type nozzle).

https://en.wikipedia.org/wiki/Expanding_nozzle

.