I see the thrust of rocket engines, and things like vector engines, being tested a lot at the moment with all the Space X stuff happening. How come these rockets that are testing their thrust don’t actually lift off? They don’t appear to be held down so well as to not let the thrust overpower it.
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You’re probably gonna have to show an example, cause all the thrust tests I’ve seen have big old pylons or otherwise strapping the thing to the ground.
The may seem a little small, like only being mounted around the base, and a fraction the height of the rocket, but they are definitely there.
Also compared to the rocket itself which is hollow and full of non structural rocket bits like fuel, those pylons are solid steel and concrete. They don’t have to match up with the overall width of the rocket.
They’re bolted down with the same mounting points that would be used to attach the engine to the rocket.
Sometimes they’re test fired while attached to the rocket, in which case there are clamps or bolts holding the rocket to the pad.
When they test rocket thrust, they often mount them sideways against a barrier, and the engine is bolted to the ground. For vertical tests, they bolt them and usually have something above the rocket as well.
Using math, they *generally* know how much thrust force the rocket will most likely have. So they know what is needed to counteract that in order to keep the rocket from zooming away. There’s never gonna be an incident where the rocket is so hugely powerful that they had no idea it was that strong.
For short tests, the rockets are nearly full of fuel and the thrust of the engines is mostly balanced out by the weight of the stage. A rocket would typically have a thrust/weight ratio of about 1.2.
For long full duration tests, the standard hold-downs aren’t strong enough. [SpaceX uses cables that attach to the top of the stage.](https://www.nasaspaceflight.com/wp-content/uploads/2018/02/Block5.jpg)
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