On a recent launch video, the gantry is at an angle and only loose tethers are attached as the rocket is prepared for launch. What is keeping it upright and in place at this point?

The engines haven't started and it was like this for several seconds at least. I didn't see the earlier part, I assume the cradle lifted it into place and it was removed. Is it just balancing for these final seconds?

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Good answers are being generated, but the fine point is for a large vertical stack, what is it resting on? The main outer shell of the first stage? The engine nozzels are unobstructed, I assume...

up vote 27 down vote accepted

The Transport Erector Launcher (TEL) is used to assemble the rocket in the assembly building, then driven out to the launch pad (The Transport part), then when it is time to launch, hyrdraulic arms lift it up to vertical. (The Erector part).

Then at launch time, it moves away, differing amounts/time lines depending on the pad and version of the TEL. (Vandenberg's is different than LC-39A's and LC-40's (which is a second rev since the first one was toasted in the AMOS explosion)).

At the base, there are hold down clamps. The engines actually start, and run for almost 3 seconds being held down by these clamps, and then finally they release letting the vehicle 'go'.

Hot fire tests, 8-15 seconds long use the hold down clamps as well, to keep the vehicle on the pad while running the engines up to full speed for a few seconds.

Jack in his answer shows the clamps and how they move in a very good animated GIF.

The Octaweb has hold down points, where these clamps attach. The Octaweb is the structure (Before Block 5 this was a large welded piece of metal (Aluminum I think) that the 9 engines are mounted/embedded into. As of Block 5 it is bolted for easier refurbishment and a more heat tolerant metal.

B1032 on TEL

F9 1.1 Octaweb

B1046 rolling out to pad

In the picture below you can see the launch table, with the clamps arranged for a Falcon 9 launch. (More are needed for F-Heavy).

LC-39A TEL view of launch table

In this picture below you can see the underside of the launch table with the plugs in, blocking out where a Falcon Heavy might use the holes in the table.

LC-39A TEL bottom view

In the picture below, you can see the top of the launch table, with the plugs removed for the Falcon Heavy inaugural launch. You can see there are now more launch clamps, since three cores need to be held down.

LC-39A TEL in Heavy mode

  • Awesome. I edited question a little. What exactly is it resting on? The outer casing of the first stage? The nozzles? – MarsJarsGuitars-n-Chars Jun 21 at 19:13
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    Those are some really neat shots of the TEL hardware I hadn't seen before. Great answer. – ceejayoz Jun 22 at 14:55
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    @ceejayoz Thanks I have been collecting these shots for years. – geoffc Jun 22 at 14:59
  • Awesome pictures; that bottom one looks like something out of an Avengers movie. – TylerH Jun 22 at 18:16
  • @TylerH TheTEL is surprisingly large. Youo forget how big rockets get. :) That launch table can handle a Falcon Heavy as well. You can see the plugs that get removed for F-H launches. So I added a photo of the launch table, modified for the F-H. You can see the extra launch clamps as well. – geoffc Jun 22 at 18:22

The Falcon 9 (and Falcon Heavy) use launch clamps at the base of the vehicle to hold it steady pre-launch. In fact, the gantry and umbilicals don't actually provide much support even before they retract - they're there primarily for power and fuel loading.

The launch clamps release soon after the engines reach nominal thrust and can support the vehicle's weight.

Falcon 9 hold-down clamp test at Vandenburg AFB

Edit: This question demonstrates that the launch clamps aren't quite as rigid as they may seem!

Many vehicles have used similar systems - the Saturn V for example used hold-down clamps that pivoted into covers to protect them from the engine exhaust.

Note: The screengrab in the question appears to be from an Iridium NEXT mission, all of which have launched from VAFB, but SpaceX launches from KSC have similar systems

  • Awesome. I edited question a little. What exactly is it resting on? The outer casing of the first stage? The nozzles? – MarsJarsGuitars-n-Chars Jun 21 at 19:13
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    @MarsJarsGuitars-n-Chars No the clamps attach directly to the sides of the base (the octaweb) of the rocket and support its entire weight. The engine nozzles hang below. Edit - geoffc explains this very well in his answer – Jack Jun 21 at 19:37
  • As contrasted to the Space Shuttle; its SRBs were bolted to the pad with frangible nuts which were split by explosive charges at launch release. – Anthony X Jun 23 at 14:10

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