The New York Times famously thought so for almost fifty years, but they eventually came around (I assume this is the infamous editorial you’re referring to):
The whole thing about the gasses exiting the nozzle interacting with the atmosphere is one of the common arguments both flat Earthers and moon landing hoax conspiracy theorists use to support their nuttery.
Mostly it comes down to “rockets can’t work in a vacuum because there is no atmosphere to push against.” Therefore there are no satellites, no interplanetary probes, and no moon landings.
All a conspiracy. Rockets clearly take off and streak into the sky, you can go see that. But what they do after they get high in the sky is obviously all part of the conspiracy of the global hegemony. Probably something to do with chemtrails.
This one is nicer.
Author and physicist Matt Strassler calls these physics fibs or phibs. Apparently there are many. I appreciate having them explicated.
The Higgs molasses is a particular pet peeve of mine, but I don’t think there’s anything incorrect about referring to a rocket as “standing on a plume”, so long as you realize that a plume is an inherently dynamic thing.
Simplest explanation is that the rocket motor bell contains a continuous explossion. (Well technically, a continuous ignition of fuel where it expands tremendously). Part of the “explosion” exhausts downward, part pushes upward against the rocket motor bell. (Much like how a fuel-air mixture burns in an engine cylinder and part pushes against the piston, pushing it downward…)
The explosion (or conflagration, or whatever) mostly isn’t in the bell; it’s in the internal part of the engine.
Yes, and the peculiar thing about that is that if true, it would seem to imply that rockets get less effective (in terms of specific impulse) at higher altitudes where the air pressure and corresponding density are lower. But in fact, rockets become more effective the higher you go (and especially if the nozzle is extended to the optimal length to get the maximum momentum from gas expansion), and are significantly less effective near sea level because of the ambient pressure, even though rockets typically have a more visually apparent and denser plume in near sea level versus altitude. So it is nonsense, but it fits the everyday ‘common sense’ that when we move we have to push against some object with large if not effectively infinite inertia, like the ground, or against some highly viscous substance like water which resists movement. The conservation of momentum is not an obvious principle for someone who has not been through a couple of physics classes and done the math to show why it is true even though it is absolutely fundamental to physical mechanics.
Combustion (or at least, the vast majority of it) occurs upstream of the nozzle in the appropriately named combustion chamber. For most performance analysis, the flow past the nozzle within the exit cone (or “rocket motor bell”, as you refer to it) is assumed to be frozen (net chemical equilibrium) and is evaluated to expand based upon thermodynamics, real gas properties behavior, internal heat transfer for two phase or condensed flows, heat transfer between the exit cone and the flow (especially for regeneratively cooled exit cones), exit cone profile, and the nozzle exit conditions. See Sutton’s Rocket Propulsion Elements, Section 5.3 for details.
There are some conditions where you might look at the chemical kinetics going on in the exit cone but those are generally done to evaluate plume radiative signature and just don’t have any real impact upon impulse performance unless you extended the exit plane out far longer than is physically practicable, and any combustion that is occurring outside of the combustion chamber is a net loss of available potential energy that could have been used to increase temperature or pressure within the chamber.
Stranger
Unless you’re Elon Musk… If you don’t have channels, the rocket may make them.
The first test of his Starship and big booster he assumed the flat concrete pad was sufficient for a launch. The 33 rocket motors apparently blew apart the concrete pad and dug a hole 12 feet deep during takeoff, including flinging dust clouds and chunks of concrete hundreds of yards.
The second launch successfully used a huge water dump to mitigate the blast, so still no diversion channels.
How do the SpaceX rockets land back upright on their pads?
The Falcon 9 boosters only use a single engine at relatively low power when landing, so a concrete or steel pad is sufficient. They also don’t spend a lot of time blasting the pad, so there isn’t that much energy transfer.
The Super Heavy booster has 33 engines and they’re individually much more powerful. SpaceX thought the pad beneath the booster would survive a single launch, and had already been planning the water deluge system. But the force of the booster cracked the pad, allowed the exhaust to superheat the water in the soil beneath it, and the force from the expansion quickly destroyed the rest of it.
With the caveat that hydrogen-oxygen engines are typically run very fuel-rich (i.e., they have extra hydrogen) to improve the exhaust kinetics. Hydrogen has the highest gas velocity at a given temperature and so you can improve the specific impulse by including extra fuel that ends up not reacting with anything in the combustion chamber. It’s also easier to extract the thermal energy when there are fewer modes available (H2O has more vibrational/rotational modes than H2, which ends up as wasted thermal energy).
But that free hydrogen does combust with atmospheric oxygen when available. It’s wasted energy, in a sense, but a hydrolox engine isn’t really limited by energy. They’d run even more fuel rich if they could, but the low density of hydrogen makes the tank size a problem.
And just to tie it back to the original question: the boosters have retractable landing legs, so the rocket doesn’t actually land in its engines. You can see the legs extended here, and you can (just barely) see them deploy at about 0:35 in this landing video.
Yes, when it gets back to earth, the booster is essentially and empty steel can; unlike at lift-off when it is full of fuel and pushing a payload upper stage. So it does not take much power to land upright.
The SuperHeavy, however, they decided to try to catch it with “chopsticks” on landing, rather than build the extra complexity of feet into the rocket. After all, it already has steerable rocket motors and the need to almost hover to land, so catching it should work… Conveniently, the same mechanism can be used to load the starship onto the booster at the pad.
Based on something I saw written about the reverse underwater sprinkler problem I’ll ask is there a substantial amount of thrust transmitted to the throat of the engine nozzle?
No–the throat is the narrowest point where the combustion chamber connects to the nozzle, and the walls are necessarily vertical there. So there is great lateral pressure (and intense heating) but no way for the gas to push up somehow.
The momentum flow at the throat is necessary for the rocket to work and you could add that to the momentum transfer from the nozzle to calculate the total (although it’s easier to just look at the exit plane of the nozzle). But I wouldn’t call that transmitted–you need a molecule bouncing off a non-vertical surface to transmit force. That happens at the top surface of the combustion chamber and the inside of the nozzle.
Well, aluminum-lithium for the Falcon 9 (Starship/Super Heavy are steel).
Just to add some mental visualization: a Falcon 9 booster has just about the same empty/full mass ratio as an aluminum soda can. And the propellant it reserves for landing is the equivalent of about half an ounce of soda.
One of my antique posts:
What’s the rocket standing on? - Factual Questions - Straight Dope Message Board
yep, the fact that steam expands some 1600 times vs. water if superheated (going from liquid to gas phase), clearly throws a wrench into what effectively is a makeshift underground steam driven piston.