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questions about thrust

questions about thrust

(OP)
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,

RE: questions about thrust

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.)

RE: questions about thrust

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 ?

RE: questions about thrust

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: questions about thrust

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 ?

RE: questions about thrust

(OP)

Quote (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.))


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...

RE: questions about thrust

"...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?

RE: questions about thrust

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 ?

RE: questions about thrust

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

RE: questions about thrust

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.

RE: questions about thrust

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).

RE: questions about thrust

(OP)
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...

RE: questions about thrust

(OP)

Quote (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)


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_S...

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?

RE: questions about thrust

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.

RE: questions about thrust

Quote (rickitech)

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.

RE: questions about thrust

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.

RE: questions about thrust

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 .

RE: questions about thrust

I used to be an engineer for the SSME main engine, and worked in the nozzle group. The SSME engine nozzle was optimized for an exhaust pressure of around 2 psi, i.e. about halfway along its typical trajectory. It could have been designed for higher expansion and more net thrust in vacuo, but at a weight penalty. The result is a trade between the vehicle empty weight vs. total impulse that the engine could deliver. When starting, the main engines were operated at about 100% of rated sea level thrust, which normally would result in a nozzle that would have separated flow along the nozzle wall, due to shock waves forming there. A tweak to the design (slight change in angle of the wall near the exit plane) results in a near-wall pressure closer to 4 or 5 psi, while the core flow remains at about 2 psi. As the exit pressure drops, the nozzle runs more "full", with less of the flow passing the exit plane at a non-90 degree angle, resulting in lower cosine losses, and higher effective exit velocity.

The SSME flight nozzles would ring like a bell (but much more violently) when starting into atmospheric pressure, causing fatigue failures of the coolant tubes, so the number of starts on any flight nozzle were limited. A steam ejector called the "SLAM" ring was used to reduce the exit pressure for engine tests at NASA Huntsville (Stennis). The variable back pressure of the Huntsville test stand also allowed some verification of thrust vs. altitude.

Engine thrust is never quoted as net, no more than a car's engine horsepower would be. Airframe losses (drag, gravity) are the aerodynamicist's/vehicle designer's problem, the engine company tells them the thrust available on the engine mounts.

RE: questions about thrust

Oh, and that NASA article that quotes thrust in "kg" just fries me. Thrust is a force, not a mass.

RE: questions about thrust

if you're comfortable with lbf, then why not kgf (like a bag of sugar at the supermarket) ?

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

RE: questions about thrust

(OP)
i have found answers to my queries with this basic thrust equation which includes the effect of outside environment pressure to the exiting gases https://exploration.grc.nasa.gov/education/rocket/....


Quote (The SSME engine nozzle was optimized for an exhaust pressure of around 2 psi, i.e. about halfway along its typical trajectory. It could have been designed for higher expansion and more net thrust in vacuo, but at a weight penalty. The result is a trade between the vehicle empty weight vs. total impulse that the engine could deliver. When starting, the main engines were operated at about 100% of rated sea level thrust, which normally would result in a nozzle that would have separated flow along the nozzle wall, due to shock waves forming there. A tweak to the design (slight change in angle of the wall near the exit plane) results in a near-wall pressure closer to 4 or 5 psi, while the core flow remains at about 2 psi. As the exit pressure drops, the nozzle runs more "full", with less of the flow passing the exit plane at a non-90 degree angle, resulting in lower cosine losses, and higher effective exit velocity. )


is the 2psi you mention refers to the outside environment pressure or atmospheric pressure because 2psi is very small for the exhaust gas, just clarifying...thanks,





RE: questions about thrust

Quote (btrueblood)

The SSME flight nozzles would ring like a bell (but much more violently) when starting into atmospheric pressure...

Every part of the Shuttle "rang like a bell" when the engines ignited. I worked in mechanical systems on the Shuttle, and the worst load environment was often from engine noise for the few seconds before it left the pad.

RE: questions about thrust

2 psi is where the exit plane exhaust pressure matched the external atmospheric pressure.

Tbuelna, it was fun to stand about 1/2 mile from the SSME test stand at Santa Sue, and watch the shock waves bounce off the cliff behind the flame bucket. And to look down at your chest and see it vibrate. You could shout as loud as you could at the person standing beside you, and they couldn't hear anything you were saying. As loud as the SSME's were, the SRB's were much noisier, and probably a bigger component of the vehicle vibration. I watched a "small", 50,000 lb thrust chamber test that ran LOX/RP-1 (kerosene), and the noise from that was much more alarming - it crackles and snaps and snarls at you. The hydrogen/oxygen flame from the 10x higher thrust SSME was just a big bass roar.

RE: questions about thrust

btrueblood-

Did you get involved in the linear aerospike engine program for X-33? I knew the Orbiter TPS SSM at Rockwell/Boeing and he spent some time working on the ramp surface TPS for the X-33 engine. He explained to me the aerospike engine gave better performance overall for the X-33 than a conventional nozzle. Since there was no fixed outer surface constraining the gas flow, the gas expansion continually adjusted to match the current atmospheric pressure at all altitudes.

I recently saw that Firefly Space Systems successfully ground tested their RP-1/LOx aerospike engine.

RE: questions about thrust

Tb,

I wish...and then again I don't. The NASP and X33 programs were staffing up in a big way, right about when I started. Just before leaving ~ 2 years later, they had the project cancelled, and people were coming begging at our door (I was in the Advanced Combustion Devices group by then). Aerospikes are neat. I had a picture (may still, somewhere...) of a test of (I think) an early SSME (or might have been a J-2) combustion chamber with an aerospike nozzle (circular, not linear). It was done as a proof of concept for some Air Force contract or another. Cooling is a bit tricky, but has been proven do-able by now. The 2D nozzle is interesing in that you can do 2-D attitude control/thrust vectoring by throttling one side or the other.

In some ways, I am very heartened by the likes of Rutan and others who bring grass roots and bootstraps (and ex-software barons with oodles of spending cash) to the aerospace world. I got out of the biz as it was always a roller coaster ride, wondering when your meal ticket gee-whiz project would get cancelled by congress critters.

RE: questions about thrust

It is definitely a very interesting time to be working in the US space launch industry, especially with all of the privately funded efforts.

Back in 2014 I was working at a company doing design of the LH2/LOx ducts for the SLS booster first stage, which uses the SSME. The chief engineer on the project was a middle-aged PhD that had spent his career working on engines designed by someone else. On my last day of work there I stopped by his office to say goodbye, and he told me it was also his last day at the company. I asked where he was going to work at and he told me (with quite a bit of enthusiasm!) that he had been given a once in a lifetime opportunity to do a serious clean-sheet-of-paper rocket engine design. It was something he had dreamed about his entire career.

RE: questions about thrust

Thrust or a force is produced only when a mass is accelerated. This can be quantified through F=ma (Newton's second law of motion). It can also be expressed as the rate of change of momentum or F= d(mv)/dt = m(dv/dt). The larger the mass and rate of change of velocity, the greater the thrust. Thrust or Force per unit time can be expressed as F/sec = m/sec (dv/dt) = mass flow rate x acceleration. The mass flow rate depends on the design of the rocket nozzel and the dp across the inlet and outlet of the nozzel which together determines the efficiency of propulsion.

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