Rocket engines, thrust, testing
July 5, 2013 10:31 AM   Subscribe

So, when rocket engineers/scientists test rocket engines, the engines are secured somehow, so that they do not fly off. But those rockets can generate millions of pounds of thrust. How are they secured to some anchor such that they do not fly off into the ether? Here's a video of what I'm referring to.
posted by dfriedman to Science & Nature (9 answers total) 1 user marked this as a favorite
I think the answer is as simple as "with bolts."

In the case you link to, it also helps that the thrust is into the side of a mountain.
posted by ROU_Xenophobe at 10:56 AM on July 5, 2013 [1 favorite]

Think of it like this.
The rocket itself, by design, can structurally withstand it's own thrust - it has to be able to, or it wouldn't be functional. Rocket thrust doesn't decrease as you go faster... it's constant... not like, say, an internal combustion engine... so whether it's moving or not, that thrust is there while it's firing.

Kind of mind boggling that we can build something that can produce and withstand 22 million horsepower though.... wow.
posted by TravellingDen at 11:03 AM on July 5, 2013

This article talks about the ATK facility that was used to test the Ares solid rocket motor. It goes into pretty specific detail about the test rig:

As the motor attained nominal thrust, it pushed against a load distribution and monitoring truss and up against a concrete thrust block designed to withstand more than 4,300,000 pounds (19.1 million newtons) of side force without tipping over.

I'm guessing that squat little reinforced concrete block goes really, really deep.....
posted by JoeZydeco at 11:07 AM on July 5, 2013

Steel cable is stronger than you think. This cable is what they use to hold up the mast on sailboats. You can see that a single piece of 3/4" 1x19 cable has a 63,000 breaking strength.
posted by tylerkaraszewski at 11:07 AM on July 5, 2013

by the magic (really old magic) of civil engineering! (this is engineering humor btw)

It is pretty much the same way you design a foundation for anything. For a tall building engineers figure out how much the building will weigh and how much the stuff and people in the building will weigh, double that total weight and design the foundation to support that weight. In some parts of the country you also design for seismic loading which includes forces acting perpendicular to the weight vector (ie. sideways) and you design for this also, and in cases of loads which are always sideways such as thrust blocks water pipes (look up water hammer), anchors for suspension briges, and cantilever retaining walls. What it ends up looking like is giant blocks of concrete buried underground that spreads out the load to a large area that the soil/dirt/bedrock is able to support without failure.

In the case of the test rigs (and suspension bridges) that block is really, really big. Like bigger than houses big. And i see it all the time, but I am not sure horsepower is a useful measure for rocket engine power...

and dammit joezydeco I was getting there...
posted by bartonlong at 11:09 AM on July 5, 2013 [4 favorites]

Also, I'm not saying that they are actually using cable like this to hold the rocket in place, but just think about the sort of holding power you'd have building something with the equivalent of 100 of these cables. Steel is very strong.
posted by tylerkaraszewski at 11:10 AM on July 5, 2013

If you think of it in terms of weight, it might make more sense. Wikipedia says that rocket in the video can lift 24,500KG-- so to keep it stationary, all you need to do is exceed that, if it was facing skywards.

24,500KG seems a great deal, but if you cut a cube of ice 3m to a side, it'd be over 24,500kg (I think a block of ice is quite easy to picture, plus the math is easier).

So if it was facing skywards, you could balance the ice on the top, and the rocket wouldn't go anywhere.

On it's side, the rocket doesn't have the weight of itself to overcome, so the block of ice on the 'top' would have to be 24500 + the weight of the rocket (900,000kg I think?), but hey-- that's still just a block of ice 10m to an edge, nothing incredibly amazing-- and I don't think you'd be amazed if someone used a lot of bolts and screws and cabling to suspend a 10m cube from a crane.

But instead of the crane, lets cut the ice into a few rectangular slabs, and then lay the rocket on top of it, and strap it down using the same bolts and screws and wires we used with the crane-- the rocket wont move when we fire it up.

Until the ice melts of course.

That's assuming I'm not absolutely incredibly incorrect
posted by Static Vagabond at 11:48 AM on July 5, 2013 [1 favorite]

Yep, just engineering. If the foundation of a building can hold millions of pounds of building in place, it could also hold millions of pounds of thrust in place.
posted by gjc at 1:23 PM on July 5, 2013

Anchorages can be preposterously strong. Have you ever looked at engineering diagrams of the anchorages at either end of the Golden Gate Bridge, which hold onto the ends of the suspension cables? (Sorry for no link; I tried to find one and couldn't.) Each of those anchorages weighs 60,000 tons.

They're dealing with forces a hundred times as great (if not even more) than your rocket engine and they've been doing so for 80 years without shifting at all.

Rocket engines are very impressive but they don't produce as much absolute force as you might think. Civil engineers deal with much worse problems all the time.
posted by Chocolate Pickle at 1:56 PM on July 5, 2013

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