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,

 

[btrueblood]

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.

 

[btrueblood]

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

 

[rb1957]

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 ?

 

[rickitek]

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/rockth.html.

2 psi[/color], 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,

 

[tbuelna]

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.

 

[btrueblood]

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.

 

[tbuelna]

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.

 

[btrueblood]

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.

 

[tbuelna]

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.

 

[seastryder]

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.