Perhaps this question reveals a naive understanding of engineering, but: When a spacecraft, such as a rocket, stands on its launch pad waiting for takeoff, is the entire weight of the craft resting on the engines at the bottom? Or is the craft somehow attached to the service structures, providing structural support as well as fuel and electric power?
No, definitely not resting on the bells. If they were, when the rockets are ignited, the ground beneath the rocket would be blasted and then the bells would be smashed.
In fact there are channels dug out under the launch pad so that the exhaust from the rockets can escape rather than blasting the pad itself and ricocheting back to hit the rocket.
There are arms holding the rocket up from the sides. These arms hold on until the rockets are firing hard enough to lift the whole spacecraft, at which point they let go.
It is attached to other structures that support the vehicle. Here is a picture of the space shuttle, notice the two large gray supports at the bottom of the craft.
I hate to do this, but:
IIRC they not only support the vehicle, they also actually hold it down for a short period, until the engines are fired up completely and are at max thrust.
https://youtu.be/iBVjV960PRk?t=36 (listen and watch for a few seconds)
The Shuttle wasn’t held by those boxes - they just managed the connections to the Shuttle, including fuel loading connections, plus power and other services. The Shuttle was held down to the pad by the bases of the two solid fuel rockets. There was a skirt around the base of each the SRBs that was bolted (with explosive bolts) to the launch platform. Once the main rocket engines on the Shuttle were running, and the Shuttle had bounced back from the initial twang, when those engines pushed the whole stack over slightly, the SRBs lit and simultaneously the bolts exploded and the shuttle lifted.
The Saturn V was held onto the launch platform by large hold down arms. At lift-off they release. But there were also a set of long bolts holding down the rocket. They basically got extruded through their bolts, and acted to perform a slow release, avoiding the worst of the shock of release. Release only occurred when the systems determined that were are five good engines at full thrust.
Of course, it should be noted that, as soon as the rocket launches, it is physically standing on its engines. And the engines, in turn, are standing on a plume of hot gas.
An excellent and probably underappreciated point. All of the thrust–which usually exceeds gravity by a substantial factor–is transmitted through the engines.
Much of that thrust is transmitted through the upper surface of the combustion chamber. Basically, there is high pressure in all directions, but the upper surface has more surface area, since the bottom surface has a hole in it (so the gas can get out).
The rest of the thrust is transmitted through the flimsy-looking engine bells as the gas expands. It pushes outward (which gives you no net thrust), but also up (which does).
I’m not sure I’d agree with this viewpoint, though. The gas is supersonic, so as soon as it leaves the engine it has no influence on the rocket. The rocket is standing on the momentum of the gas it just ejected, but I don’t think I’d call that standing on the plume.
There are many ways to support a rocket on the pad, including with a rail on the side like an Estes rocket. However, in my experience the most common method is to support it at the bottom rim. The structure there is already robust due to the engines. And the rim is where the loads mostly are already, since that’s where the outer structure leads to. The engines are unobstructed, which is good since anything in the way will be damaged.
The Neutron rocket will “hang” the upper stage of the rocket, which apparently works pretty well in their design. It might work for a larger rocket, but at a certain point the tower you need becomes untenable. Supporting at the bottom means a much simpler pad design.
Probably the nicest method, if you can make it work, is a sea launch. Just float the rocket upright in water and ignite. The rocket is nicely supported on all sides, and the water absorbs much of the energy from the rocket. It’s only been tried a few times (aside from missiles) but they were successful. The Sea Dragon rocket intended to do this at a large scale (it made an appearance in the show For All Mankind).
I just wanted to note that while “twang” sounds like a word just made-up by a random space enthusiast, it is an actual technical term that refers to a visible structural effect and is found in technical analysis. It is obvious in the Space Transportation System (“Shuttle”) because of the large offset mass of the Orbiter Vehicle, but any large rocket will have a predominate lateral modal response due to the ignition impulse that may cause it to deflect several inches.
It is certainly the (mostly) gaseous products that produces the thrust which allows a launch vehicle to defy gravity and fly upward (and eventually forward into orbit, if that is where it is going) but it is a bit misleading to say that it is “standing on a plume of hot gas”. The gas exiting the nozzle provides forward momentum via the principle of conservation of momentum—that is, the impulse imparted the rocket is equal to the mass of the plume multiplied by the exhaust velocity, integrated over time—but it isn’t in any way attached to or supporting the rocket once it exits the nozzle, and the ‘force’ (which is just as ‘fictitious’ as centrifugal force in terms of disappearing with the appropriate selection of reference coordinate frame) is typically measured as applied to the pivot point of the nozzle, which directs it through the center of mass of the vehicle with minor adjustments to control lateral and pitch/yaw body movement. Once the gas has exited past the nozzle exit plane, it has no interaction with the vehicle or even the plume of gas following it any more than a paddle in water would be considered to push on the stream once the rower has completed a stroke.
In fact, you can built a rocket that just has a constriction (the actual “nozzle”) but no exit cone. Such an engine would be almost impossible to vector controllably, and would not gain the typically few percent of additional impulse from the expansion of the gas products in the exit cone, but it would still exchange momentum with the vehicle. Similarly, you could make a (very inefficient) rocket just by being in freefall and throwing baseballs behind you; once the ball is released it is no longer attached, but the momentum continues to propel you onward.
Stranger
Sometimes more than several inches if things aren’t going quite right:
Standing on the plume is the closest you’ll get to explaining the dynamics in layman’s terms.
“Propelled by the reaction forces to the expelled
plume” sounds unnecessarily pedantic.
The problem with that terminology is that while it is somewhat poetic, it provides an idea that seems intuitive but completely wrong. Many people have the notion that a rocket or turbine engine ‘pushes’ against the atmosphere behind it to get thrust in the way your legs push against the ground, and then incorrectly argue that these can’t operate in rarified atmosphere or in the vacuum of space (as one infamous editorial asserted). I think the average layman is capable, with some simple examples, of understanding momentum transfer even if it seems to be inconsistent with the notion of static forces that we normally encounter.
Stranger
Eh, superheated tomato, plasma tomahto.
No, it doesn’t push against the atmosphere. It pushes against the exhaust itself. And yes, the particular bit of exhaust that it’s standing on is constantly changing from moment to moment-- That’s why it’s a plume that it’s standing on, not just a column.
Well that launch went sideways fast. 
They’d salvage the nuts from those explosive bolts and give them to astronauts. Read that in a biography but can’t cite which one because I’ve read so many.
How did you manage to lose the “whole” other sentence in the quote? Were you so desperate to make pedantic keyboard noises?
I think that gets into the nuances of what a “push” is, but from a continuum mechanics standpoint, once the gas has exited through the throat of the nozzle, the momentum component of the rocket thrust equation (the ṁ Ve part) there is no further reaction, and if the plume were dispersed or contained behind the rocket it would make no difference to the force seen by the rocket.
The second expansion term (the (pe-po) Ae) is actually the expanding plume pushing on the nozzle exit cone (and the ambient pressure is literally pushing back retarding expansion). If you removed the exit cone or punched a bunch of holes in it allowing the expanding gas to escape it would reduce the force that the rocket experiences, so that is perhaps is a little closer to saying that the rocket is being pushed by the gas, although again once the gas passes through the nozzle exit plane it has no further influence on the impulse on the rocket regardless of how much it expands.
Stranger
I didn’t think it was salient to the response I was making, and didn’t want to include a statement that just seemed prejudicial or provocative. But I do get annoyed when posts I make are unduly chopped up or context is removed when quoted, so if you feel I did this to your post you have my sincere apologies.
Stranger
Rockets stand on the three long graceful curved fins that extend from it’s side and end in a slightly bulbous foot several feet below the downward end of the fuselage. Like this one. Or this one.