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by Polizeiposaune 24 days ago
The weight of the propellant helps hold the rocket on the pad during the test fire, reducing how much force the hold-downs need to exert to keep the rocket on the pad, and stressing the rocket's structure in the same way it will be stressed at launch.

Test fires with a near-empty rocket would put considerably more force on the pad's hold-downs and the corresponding parts of the rocket's structure.

Blue also had a fuelled 2nd stage on top of the booster for the static fire, which is not out of the ordinary.

SpaceX has a "cap" that is held down with cables that it uses when it needs to test-fire a first stage by itself at its McGregor test site; static fires at launch sites are usually done with the 2nd stage on top.
2 comments
m4rtink 24 days ago
It is also more realistic to do it fully loaded - very different forces are acting on a rocket based on how much propellant is loaded.
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kqr 24 days ago
Right. The forces these things produce are massive. I only know the specifics for the Space Shuttle, but when it is at full liftoff thrust (liquid and solid boosters) there's just no way to keep it leashed to Earth. It's going up whether you want to or not.
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lefra 24 days ago
The space shuttle stack has a net thrust (thrust minus weight) of about 9 MN at lauch [1]. High carbon steel has a yield strength of 700 MPa [2]. So you need a piece of steel with a cross section of 0.013 square meter to hold it down. That's a rod 6.5 cm / 2.5 inches in diameter. Hardly impossible. Your nearest road suspension bridge probably has cables bigger than this.

If you want to argue that it's impossible in practice, I'll point out that SpaceX's Starship first stage has a net thrust of 53 MN [3], and it does static fires (without the weight of the second stage on top) [4].

The space shuttle didn't do static fires because of the solid rocket boosters that would need to be teared down and reconstructed afterwards; not because it's physically impossible to hold it down.

[1] https://en.wikipedia.org/wiki/Space_Shuttle

[2] https://www.unionfab.com/blog/2024/03/yield-strength-of-stee...

[3] https://en.wikipedia.org/wiki/SpaceX_Starship

[4] https://www.dailymotion.com/video/xab20qa

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