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Megarocket

Old 16th Jan 2021, 21:58
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Megarocket

NASA are about to test fire 4 RS25 engines together producing 7 megatons of thrust.
My question is, how do you hold the assembly down and prevent damage to the engines straining against the thrust?
I doubt a couple of ratchet trailer tiedowns would cut the mustard.
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Old 16th Jan 2021, 22:46
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Originally Posted by kangaroota View Post
NASA are about to test fire 4 RS25 engines together producing 7 megatons of thrust.
My question is, how do you hold the assembly down and prevent damage to the engines straining against the thrust?
I doubt a couple of ratchet trailer tiedowns would cut the mustard.
7 Megatons? Perhaps you mean 7 million pounds..... I was a kid in Huntsville, Alabama back in the moon days; they static tested Saturn F1 engines all the time and they were pretty loud.
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Old 16th Jan 2021, 22:47
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A short lived rocket blast! Seems that something went out of kilter about 1 minute in and caused a shutdown of all 4 engines. I was watching the live broadcast and it was good while it lasted.

Rans6...................
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Old 16th Jan 2021, 22:51
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The live commentary mentioned an MCF anomaly on the No.4 engine, but they continued and then went into shut down just before the gimbal cycling part of the test.

Last edited by Cpt_Pugwash; 19th Jan 2021 at 22:32. Reason: Pudgy fingers! and dodgy hearing.
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Old 16th Jan 2021, 23:05
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Originally Posted by Winemaker View Post
7 Megatons? Perhaps you mean 7 million pounds..... I was a kid in Huntsville, Alabama back in the moon days; they static tested Saturn F1 engines all the time and they were pretty loud.
If memory serves, they had to carefully watch the weather conditions before they ran the F1 engine tests - if the conditions were correct, the sound would break windows in Huntsville.
Do you know if they are using the same test stand that was used for the F1 engines (upgraded to modern standards)? Or did destroy the old stand and have to start over?
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Old 16th Jan 2021, 23:44
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I think is the NASA coverage of the event.

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Old 17th Jan 2021, 08:10
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Originally Posted by kangaroota View Post
NASA are about to test fire 4 RS25 engines together producing 7 megatons of thrust.
My question is, how do you hold the assembly down and prevent damage to the engines straining against the thrust?
I doubt a couple of ratchet trailer tiedowns would cut the mustard.
Million pounds I'd suggest.

FWIW the Saturn V (first stage 5 X F-1 engines) totalling 7.5 million pounds of thrust was held down for several seconds on the launch pad prior to being released.
That was done by four relatively small, retractable hold down arms at the base of the vehicle so I'd expect the engineers these days could rig up something to hold down something with a bit less thrust..

There is video here of the first test firing of the Saturn V first stage (sorry for the breathless commentary):


There's video here with a much better commentary which gives the hold down arms a mention:


Edit to add: For clarity in the bottom video the hold down arms are hidden under the "small" quadrant shaped covers. The much large "levers" that swing up are the tail service masts that provided electrical connections, electrical power and other consumables etc to the first stage prior to launch.




Last edited by wiggy; 17th Jan 2021 at 10:01.
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Old 17th Jan 2021, 09:14
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Originally Posted by Winemaker View Post
7 Megatons? Perhaps you mean 7 million pounds.....
39,100,000 Newtons of thrust at launch apparently. Iím afraid I donít understand imperial units of mass/force 😬.

Roughly equivalent to thirty eight 777ís at full power?

...edited to clarify that 39.1 MN thrust when joined by two solid fuel SRBís.

Last edited by NineEighteen; 18th Jan 2021 at 22:17.
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Old 17th Jan 2021, 09:57
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Isn't it right that the Saturn V created the loudest sustained noise ever made by humans thus far?

Watched a Shuttle launch from the beach by Patrick AFB, about 25 miles south, that seemed pretty loud! Love to have seen a Saturn go...
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Old 17th Jan 2021, 12:38
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You think thats loud?
Wait until you hear 27 Raptors fire for the launch of the SpaceX Super Heavy Starship booster.


That video is a bit out of date now, this one is more recent.

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Old 17th Jan 2021, 14:05
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I'd imagine not nearly as impressive as the Mighty Saturn V but...pretty awesome nonetheless.
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Old 17th Jan 2021, 16:24
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Originally Posted by wiggy View Post
Million pounds I'd suggest.

FWIW the Saturn V (first stage 5 X F-1 engines) totalling 7.5 million pounds of thrust was held down for several seconds on the launch pad prior to being released.
That was done by four relatively small, retractable hold down arms at the base of the vehicle so I'd expect the engineers these days could rig up something to hold down something with a bit less thrust...
Ah, but don't forget that there is the weight of the rocket itself holding it down; the arms only have to hold against the delta between thrust and weight.
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Old 17th Jan 2021, 17:07
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If only we had dilithium crystals. They would be so much quieter.

But probably much less fun to watch.
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Old 17th Jan 2021, 17:07
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Originally Posted by Winemaker View Post
Ah, but don't forget that there is the weight of the rocket itself holding it down; the arms only have to hold against the delta between thrust and weight.

You're right - in the Saturn V case you'd be looking at the hold down arms dealing with about 1.5 million pounds (not tonnes ) of excess thrust over mass...of course unlike a static test rig with engines etc potentially bolted in place in the Saturn V launcher case all four hold down arms had to be able to release their "grip" simultaneously ...even almost 60 years down the road still a pretty impressive piece of engineering IMHO.

https://www.hq.nasa.gov/office/pao/H...04/ch13-4.html


On 17 May 1965, engineers tested the ability of the first hold-down arm to sustain a vertical thrust of 725,747 kilograms.
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Old 17th Jan 2021, 17:15
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Originally Posted by Winemaker View Post
Ah, but don't forget that there is the weight of the rocket itself holding it down; the arms only have to hold against the delta between thrust and weight.

I believe also, according to something I read a long time ago, that when the Saturn V engines actually ignited the total overall weight of the complete assembly is too heavy for them to lift it. During the first few seconds when the engines were getting up to full power sufficient fuel is burned off in that start up process that the overall weight of the complete craft is reduced such that the engines do then have the power to lift it. Sounds right somehow to this totally non expert.
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Old 17th Jan 2021, 17:18
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Originally Posted by Saintsman View Post
If only we had dilithium crystals. They would be so much quieter.

But probably much less fun to watch.
... and what will Greta make of all this? Can Nasa designers come up with some electric equivalent that can be plugged in and charged up overnight?
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Old 17th Jan 2021, 17:22
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Originally Posted by yellowtriumph View Post
I believe also, according to something I read a long time ago, that when the Saturn V engines actually ignited the total overall weight of the complete assembly is too heavy for them to lift it. During the first few seconds when the engines were getting up to full power sufficient fuel is burned off in that start up process that the overall weight of the complete craft is reduced such that the engines do then have the power to lift it. Sounds right somehow to this totally non expert.
I'm not aware that was ever the case.

At lift off the five first stage engines generated a total of (nominal) of 7.5 million pounds of thrust ( 5 X F-1s at 1.5 million pounds each)

The nominal lift off weight was usually 6 million pounds, so there was about 1.5 million pounds of excess thrust at lift off.

The fuel burn was about 15 tonnes a second, so yes the weight was reducing rapidly..but OTOH they would still have been in serious ***** if an engine had failed on or just after liftoff.......
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Old 17th Jan 2021, 17:50
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Originally Posted by wiggy View Post
I'm not aware that was ever the case.

At lift off the five first stage engines generated a total of (nominal) of 7.5 million pounds of thrust ( 5 X F-1s at 1.5 million pounds each)

The nominal lift off weight was usually 6 million pounds, so there was about 1.5 million pounds of excess thrust at lift off.

The fuel burn was about 15 tonnes a second, so yes the weight was reducing rapidly..but OTOH they would still have been in serious ***** if an engine had failed on or just after liftoff.......
Well as I said I'm a total non-expert in this field, I am happy to be corrected.
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Old 17th Jan 2021, 19:53
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Originally Posted by tdracer View Post
Do you know if they are using the same test stand that was used for the F1 engines (upgraded to modern standards)? Or did destroy the old stand and have to start over?
If this NASA article is to be believed it looks like at the core of the structure it's the original B-2 stand..

https://www.nasa.gov/sites/default/f...t_stand_v1.pdf



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Old 17th Jan 2021, 22:38
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Yellow, that's popular myth, but it was never actually true for any operational rocket.
Think about it - if you needed to burn off fuel to reach the liftoff weight - why not just reduce the starting fuel load?
Thanks wiggy - good to know some sanity remains at NASA. I'm still angry that we're spending tens of billions of dollars to re-create the heavy lift capability we had five decades ago and simply threw away.
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