Some launchers, like Falcon 9 recently as in a video clip linked to below, and I think some other launchers since long ago in the space race history, test their engines before launch by firing them while remaining strapped on to the launch pad. I mean firing the actual vehicle days or hours before a real launch, not any horizontal engine test under development. F9 weighs about 500 tons on the launch pad and the engines firing are powerful enough to go to orbit.

How is it possible to hold something like that anchored to the ground without breaking it? Are such launchers especially sturdy and heavy, or is it maybe not as hard as it looks because the forces are similar to those a launcher encounters during a real launch anyway?

What are the main trade offs when deciding if a launcher should be able to do such static engine firing before each launch? How large is the penalty to the payload it can deliver to orbit?


2 Answers 2


Thrust to weight ration of boosters at takeoff needs to be greater than 1, else they do not go up.

However, it is usually not greatly higher than 1. Usually 1.2-1.8 perhaps. This is obvious in boosters that seem slow or fast off the pad.

Thus the boost is gravity plus a bit but not that much. So the stage is able to handle gravity just to stand up. This is really not that much more. It also needs to be able to handle the load on the way up as well. (I.e. The failure in Falcon 9 CRS-7 mission was a strut that could not handle the load).

Thus it comes down to, where on the booster can you hold it, without damage to withstand the thrust. In some ways, gravity and some of the boost cancel out. It is only the difference that is the issue.

Fully fueled, pressurized, the stages are pretty strong.

Consider as well you need a pad strong enough to not be ripped out of the ground by all that thrust either!

It does need to be designed in from the get go, not the sort of thing you add after the booster is complete.

The SSME's on the Shuttle would actually ignite and reach full power while held down, before the SRB's fired. The famous 'twang' of the Shuttle was the SSME's hitting full power, and moving the entire stack before leaving the pad. (Once the SRB's fire, you go, or you die. There is no try).

  • $\begingroup$ So because the thrust to weight ratio is smaller than 2g, the stress on the launcher is in some basic way less when the engines fire than when it rests in gravity on the pad? $\endgroup$
    – LocalFluff
    Commented Jan 14, 2016 at 7:41
  • $\begingroup$ @LocalFluff No, lift-off acceleration adds to gravity. (And gravity is not turned off when launch control hits the button. They'd like to but can't :-) ) $\endgroup$
    – Jens
    Commented Sep 2, 2016 at 13:05

STS did Flight Readiness Firings in the early days where the SSMEs would light up for a few seconds.

The stack was bolted to the Mobile Launcher by eight large bolts at the base of the SRBs. Here's a picture of the nuts both before and after they were blown apart by pyros. (source)

I remember discussion about what would happen if the SRBs fired and the nuts didn't split. There were 2 independent pyro systems to blow the nuts apart. On a flight late in the program one of the systems failed to fire which led to a lot of analysis and Powerpoint. I believe, but cannot find a reference, that the consensus was that the vehicle would still lift off but performance would be degraded.

enter image description here

  • 2
    $\begingroup$ I found this interesting related article talking about some of the problems they had with those exploding hold down bolts. Apparently during STS-126 they saw debris around the launch site from one that didn't get caught properly and spent a good amount of effort re-designing Debris Containment System (DCS) which keeps chunks of the bolts from causing damage during liftoff. Several other reports are linked from that one talking about different iterations and changes to the system over the years. $\endgroup$
    – DuneWalker
    Commented Jan 15, 2016 at 21:53
  • 1
    $\begingroup$ Quite something to see the whole stack rocking back and forth during and after the firing. $\endgroup$
    – Anthony X
    Commented Aug 28, 2016 at 17:29

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