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#51 munks

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Posted 15 March 2017 - 16:57

When the rocket takes off, there is a huge force pushing it up.

 

Out of curiosity, is the force significantly higher than it would be in open air? Meaning, does the ground help contain some of the pressure beneath the rocket? Said yet another way, is the thrust more effective near the ground because it's sort of pushing off of something?



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#52 Charlieman

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Posted 15 March 2017 - 17:51

You know and I know that if you spend money on a rocket, you'll do your best to make it work.



#53 saudoso

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Posted 15 March 2017 - 20:17

Out of curiosity, is the force significantly higher than it would be in open air? Meaning, does the ground help contain some of the pressure beneath the rocket? Said yet another way, is the thrust more effective near the ground because it's sort of pushing off of something?

No, backpressure effect won't improve thrust.



#54 Greg Locock

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Posted 16 March 2017 - 03:10

It does in helicopters, so I wouldn't be quite that confident. I do know the design of the nozzle is a compromise between operation in a vacuum and in air, I'd have thought the proximity to the ground would act like a higher pressure atmosphere. The effect would diminish very rapidly with altitude, WAG say twice the nozzle diameter would effectively be free field.


Edited by Greg Locock, 16 March 2017 - 03:23.


#55 saudoso

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Posted 16 March 2017 - 09:52

The flow out is supersonic, would be hard for anything to creep upstream.

 

 

Due to the specific impulse varying with pressure, a quantity that is easy to compare and calculate with is useful. Because rockets choke at the throat, and because the supersonic exhaust prevents external pressure influences travelling upstream, it turns out that the pressure at the exit is ideally exactly proportional to the propellant flow {\displaystyle {\dot {m}}} {\dot {m}}, provided the mixture ratios and combustion efficiencies are maintained. It is thus quite usual to rearrange the above equation slightly:[5]

 

https://en.wikipedia...i/Rocket_engine
 
Launch pads are designed to move the flow far from the rocket, not to build up pressure:

 

m024.gif



#56 Greg Locock

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Posted 16 March 2017 - 17:53

Which demonstrates that proximity to the ground has effects. 



#57 saudoso

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Posted 16 March 2017 - 20:38

Like cooking the rocket's ass?

 

Lest time I have news of rocket exhaust reaching faring it wasn't pretty.

 

At all.



#58 saudoso

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Posted 16 March 2017 - 20:46

It's kinda like saying that the combustion in a jet engine sends backpressure into the compressor.

 

Well, it doesn't.

 

It's counterintuitive as hell, it was tough to push it through my thick skull back in the day.


Edited by saudoso, 16 March 2017 - 20:46.


#59 Greg Locock

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Posted 17 March 2017 - 02:13

The reason they use some sort of spacer is practical, i suspect. The consequence is that nobody tests rockets with the nozzle against a surface. But I think you'd agree that if the circumference of the nozzle * height off the ground is less, or equal, to the nozzles open area then /something/ would happen.



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#60 Kelpiecross

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Posted 17 March 2017 - 03:51

It's kinda like saying that the combustion in a jet engine sends backpressure into the compressor.
 
Well, it doesn't.
 
It's counterintuitive as hell, it was tough to push it through my thick skull back in the day.


If it does send backpressure into the compressor it's called "compressor surge".

#61 saudoso

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Posted 17 March 2017 - 09:44

It's not caused by the burn, it just pushes it upstream.



#62 saudoso

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Posted 17 March 2017 - 09:46

The reason they use some sort of spacer is practical, i suspect. The consequence is that nobody tests rockets with the nozzle against a surface. But I think you'd agree that if the circumference of the nozzle * height off the ground is less, or equal, to the nozzles open area then /something/ would happen.

Yes it could, but in practical use, as asked, it shouldn't.



#63 mariner

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Posted 19 March 2017 - 09:58

There may be a back pressure from the pad area but i's probably not critical as the rocket is held by the locks until ful thrust condition is verified then its released.

 

I've never noticed one sort of jump then slow as teh clamps let go so it must be a small effect I hink.

 

The nozzle design of rocket motors is very, very complex as they are indeed trying to compromise between low velocity, high atmo. pressure and high velocity, low atmo. pressure. That is another advantage of multi-stage rockets, the upper 2/3 can have motor nozzles designed for space.

 

NASA being NASA they thought long and hard about this problem , particularly for rocket poweed planes and the "aerospike" engine was one soluton tried.

 

https://en.wikipedia...erospike_engine.

 

Until recently the worlds most powerful engine , of any kind, was the Rocketdyne F-1, five of which powered the Saturn launch vehicle. Everything about it was BIG

 

https://en.wikipedia.../Rocketdyne_F-1

 

In those days engineers couldn't do their job stuck looking at a simulation on a PC but had to build the real thing and fix the problems in full scale. A lot of F1's blew up with combustion instabilty across the fuel mixing screesns until they got it right.

 

 

I strongly suspect none of those engineers wil ever forget the work they did on those engines. A US engineer who worked on the Apollo program told me they thought 500,000 people were on the project at its peak, many of them qualiied engineers.


Edited by mariner, 19 March 2017 - 11:40.


#64 Greg Locock

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Posted 20 March 2017 - 02:21

I strongly suspect you underestimate  the hours and days and weeks and months spent with slide rules and textbooks. Designing and developing stuff was not just 'suck it and see' before computers took over. For example cars were designed for crash before we had supercomputers at our beck and call, using a mixture of lab tests on components, experience, and calculations. I would guess it was in the late nineties we could actually model crash without using prototypes, and it was about 2005 before we could confidently go to Verification Build (basically off tool) before crashing a car. Nowadays the airbag calibration is done on the computer.



#65 Kelpiecross

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Posted 20 March 2017 - 04:30

I suspect there is little or no "ground effect" when a big rocker takes off. But it could be imagined that an (imaginary) rocket could be placed in a fairly tight-fitting closed tube where pressure would build up underneath it - making a sort of combination gun/rocket. Strangely - such a thing doesn't seem to have ever been tried.

As an aside - a good trivia trick question is "What is the heaviest thing that has ever flown"? Not the An-225 at well over 600 tons but the Saturn 5 at over 3,000 tons.

Edited by Kelpiecross, 20 March 2017 - 04:47.


#66 bigleagueslider

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Posted 24 March 2017 - 05:35

I suspect there is little or no "ground effect" when a big rocker takes off. But it could be imagined that an (imaginary) rocket could be placed in a fairly tight-fitting closed tube where pressure would build up underneath it - making a sort of combination gun/rocket. Strangely - such a thing doesn't seem to have ever been tried.

As an aside - a good trivia trick question is "What is the heaviest thing that has ever flown"? Not the An-225 at well over 600 tons but the Saturn 5 at over 3,000 tons.

Remember that it takes several seconds for liquid rocket engines to develop full thrust after being started. The chamber pressure in the Rocketdyne F1 LOx/RP1 engine at full thrust was over 1000 psi, and the nozzle expansion ratio was ~16:1. If the exhaust flow at the nozzle exit was constrained by a "tight-fitting closed tube" on the launch pad, the increased pressure/temperature would likely have caused catastrophic structural failure of the nozzles.



#67 munks

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Posted 24 March 2017 - 16:46

I just discovered that my same question was asked elsewhere, with a fairly similar debate: https://www.reddit.c..._generate_more/. If I may summarize the general consensus, it's that of course there would be some effect as you get closer to the ground, but in practice it would be best to avoid extra pressure for reasons similar to bigleagueslider's.

 

One situation that wasn't brought up here, though, is the case of soft landing with retrorockets. This paper suggests significant ground effects, and not always positive!?



#68 Greg Locock

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Posted 24 March 2017 - 18:23

Good find. It's interesting about  the importance of the gas flow being supersonic. 



#69 saudoso

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Posted 24 March 2017 - 23:38

You went past page 599? I'd love to read it...



#70 Kelpiecross

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Posted 25 March 2017 - 03:03


I should point out that I was clearly not suggesting that a Saturn 5 could be put in a tube - possibly a much smaller artillery-type solid-fuelled projectile.

The similar idea but with the shell being fired in the conventional sense and then becoming rocket-assisted is apparently quite common.

#71 Spa65

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Posted 14 April 2017 - 10:27

In 1968 as an ATC cadet at summer camp, we were taken one day to Spadeadam in the moors of northern England. This was the ground test site for (iirc) the Black Arrow rockets.

 

Two things stuck in my mind:

 

1.  How very flimsy were the walls of the rocket - you could easily bend the walls inward with your little finger. There were however strengthening ribs running vertically.

 

2.  The guy running the demonstration had a film showing the shape of the exhaust plume from the rocket as it flew upwards. A nice narrow plume at takeoff in 1 Atmosphere pressure. However at altitude the plume sprayed out a lot more sideways, due to the lack of air pressure pushing inwards - a much more bell shaped plume. The same effect is usually noticeable on most NASA rocket takeoffs, The wide plume meant that a lot of the exhaust had a sideways velocity component, so resolving the forces meant that the thrust in the direction of the rocket was reduced by a cosine factor. This loss of effective thrust could have been reduced to some extent if the nozzle shape could be altered in flight at altitude, but this had been considered to be too expensive a development. Even as schoolkids, some were unimpressed. However I see that many aircraft jet engines (especially the fast fighters) have such variable nozzles (though I've never seen them on rocket engines). I guess these jets have the variable nozzles to overcome this altitude plume effect. In effect the available thrust appears to be optimised to be greater at lower altitudes for fixed nozzle engines.



#72 Kelpiecross

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Posted 17 April 2017 - 05:33

I think fast jets usually have variable nozzles for a couple of reasons - a jet plane is most fuel efficient when it is travelling as close as possible to the speed of its propulsive exhaust. Also - a jet plane cannot travel faster than the speed of its exhaust. This means that a (for example) Mach 2 jet plane which has a cruise speed of about 500 mph or so needs a much slower propulsive exhaust for fuel efficient cruise than when when it is travelling at supersonic speed. The variable nozzle adjusts its size to vary the speed of the propulsive exhaust appropriately.
Also - if the plane has afterburning which has a much greater volume of exhaust its needs to open more.

Edited by Kelpiecross, 17 April 2017 - 05:34.


#73 Greg Locock

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Posted 18 April 2017 - 02:44

"a jet plane cannot travel faster than the speed of its exhaust"

 

Hmm, not so sure about that. If I am sitting on a speedboat travelling at 50 kph, and throw a stone out the back at 40 kph, the speedboat should accelerate slightly.



#74 Kelpiecross

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Posted 18 April 2017 - 05:53


Oddly enough the boat would accelerate slightly - this would be the equivalent of rocket propulsion where you are throwing away some of the mass of the vehicle to go forwards. But with a jet engine it takes in the same mass of air as it accelerates out the back (neglecting the mass of he fuel) and this (apparently) makes jets different to rockets. Same situation with a propeller plane - it doesn't go faster than the speed of the air coming back off the prop.

#75 Greg Locock

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Posted 18 April 2017 - 07:44

Yes, that sounds right. If I catch a stone on the shore that is at 0 kph , and then accelerate it up to 10 kph, then I have transferred KE and momentum to the stone, so the speedboat will slow. Good, that makes sense.



#76 4444

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Posted 20 June 2017 - 06:41

The flow out is supersonic, would be hard for anything to creep upstream.

 

 

https://en.wikipedia...i/Rocket_engine
 
Launch pads are designed to move the flow far from the rocket, not to build up pressure:

 

m024.gif

 

But there is always that boundary layer on the nozzle wall where the speed of the flow is subsonic.