# On the launch pad, does a spacecraft physically stand on its engines? **URL:** \ **Category:** Factual Questions\ **Created:** [May 24, 2024, 2:05pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024 "2024-05-24T14:05:21Z")\ **Posts on this page:** 19\ **Page:** 2
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 26, 2024, 12:04am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/21 "2024-05-26T00:04:19Z")
> [@Stranger\_On\_A\_Train](#): > > 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). 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): > **[Robert H. Goddard. The New York Times.](https://www.astronauticsnow.com/history/goddard/index.html)** > > Robert H. Goddard. The New York Times trashes Robert H. Goddard in 1920 and then (sort of) regrets in ... 1969. Mainstream media. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [May 26, 2024, 12:16am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/22 "2024-05-26T00:16:10Z")
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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 26, 2024, 1:14am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/23 "2024-05-26T01:14:54Z")
> [@TriPolar](#): > > 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](https://www.pinterest.com/pin/527554543841656415/). [Or this one](https://www.newscientist.com/article/2190418-elon-musk-reveals-starship-test-rocket-that-looks-like-1950s-sci-fi/). [This one](https://www.thehugoawards.org/wp-content/uploads/2020/07/2020HugoTrophy_A1.jpg) is nicer. ---
**Author:** ![Measure\_for\_Measure](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/measure_for_measure/32/557_2.png) [@Measure\_for\_Measure](https://boards.straightdope.com/u/Measure_for_Measure)\ **Post date:** [May 26, 2024, 5:23am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/24 "2024-05-26T05:23:45Z")
> [@Stranger\_On\_A\_Train](#): > > 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). Author and physicist Matt Strassler calls these physics fibs or **phibs**. Apparently there are many. I appreciate having them explicated. > **[Why The Higgs Field is Nothing Like Molasses, Soup, or a Crowd](https://profmattstrassler.com/2024/04/16/why-the-higgs-field-is-nothing-like-molasses-soup-or-a-crowd/)** > > The many flaws in the most common popular explanation of how the Higgs field works; violations of basic laws of physics and several misleading ideas. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 26, 2024, 12:21pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/25 "2024-05-26T12:21:28Z")
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. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [May 26, 2024, 7:09pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/26 "2024-05-26T19:09:41Z")
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…) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 26, 2024, 7:29pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/27 "2024-05-26T19:29:22Z")
The explosion (or conflagration, or whatever) mostly isn’t in the bell; it’s in the internal part of the engine. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 26, 2024, 7:30pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/28 "2024-05-26T19:30:00Z")
> [@Francis\_Vaughan](#): > > 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. 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. > [@md-2000](#): > > 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…) 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 ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [May 28, 2024, 6:59pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/29 "2024-05-28T18:59:35Z")
> [@Babale](#): > > 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. 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. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 28, 2024, 10:06pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/30 "2024-05-28T22:06:08Z")
How do the SpaceX rockets land back upright on their pads? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 28, 2024, 11:01pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/31 "2024-05-28T23:01:24Z")
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. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 29, 2024, 12:44am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/32 "2024-05-29T00:44:46Z")
> [@Stranger\_On\_A\_Train](#): > > 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. 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. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [May 29, 2024, 12:04pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/33 "2024-05-29T12:04:38Z")
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](https://space.stackexchange.com/questions/38224/why-doesnt-the-falcon-9-first-stage-use-three-legs-to-land), and you can (just barely) see them deploy at about 0:35 in [this landing video.](https://www.youtube.com/watch?v=1sJlFzUQVmY) ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [May 29, 2024, 2:39pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/34 "2024-05-29T14:39:20Z")
> [@Dr.Strangelove](#): > > The Falcon 9 boosters only use a single engine at relatively low power when landing, 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. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [May 29, 2024, 2:44pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/35 "2024-05-29T14:44:27Z")
> [@Dr.Strangelove](#): > > 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). 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? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 29, 2024, 8:10pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/36 "2024-05-29T20:10:35Z")
> [@TriPolar](#): > > 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. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 29, 2024, 8:53pm UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/37 "2024-05-29T20:53:22Z")
> [@md-2000](#): > > Yes, when it gets back to earth, the booster is essentially and empty steel can 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. ---
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [May 30, 2024, 12:03am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/38 "2024-05-30T00:03:37Z")
One of my antique posts: [What’s the rocket standing on? - Factual Questions - Straight Dope Message Board](https://boards.straightdope.com/t/whats-the-rocket-standing-on/64376) ---
**Author:** ![Al128](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/al128/32/16541_2.png) [@Al128](https://boards.straightdope.com/u/Al128)\ **Post date:** [June 4, 2024, 12:37am UTC](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024/39 "2024-06-04T00:37:56Z")
> [@Dr.Strangelove](#): > > 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. 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. [Previous page](https://boards.straightdope.com/t/on-the-launch-pad-does-a-spacecraft-physically-stand-on-its-engines/1002024.md?page=1)