# Aerodynamic Stresses on rockets and missiles in lower atmosphere **URL:** \ **Category:** Factual Questions\ **Created:** [May 16, 2014, 5:03pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595 "2014-05-16T17:03:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 16, 2014, 5:03pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/1 "2014-05-16T17:03:32Z")
Am I right in thinking that the stresses on rockets and missile bodies in the thicker lower atmosphere are a great limitation on capability. Is there a limit on thrust, beyond which the vehicle will experience stresses far beyond that it can tolerate or do current rocket engines/motors not reach this level? ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [May 16, 2014, 5:15pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/2 "2014-05-16T17:15:10Z")
Yes, the point of maximum dynamic pressure, “max Q”, is a key design limit case - many structural loads reach their peak there. It’s proportional to the square of velocity times air density, so it actually decreases in the upper atmosphere even with the missile velocity continuing to accelerate. Some missiles with throttleable liquid-fuel engines, like the Titan IIRC, actually reduce thrust while in the max-Q region so they can be built lighter. ---
**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 16, 2014, 5:57pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/3 "2014-05-16T17:57:18Z")
This is one theortical advantage of solid fuel and hybrid rockets, the fuel can be a structual part of the rocket engine during the high stress portion of flight. However, in the case of the Shuttle SRBs which had a short intense burn time they had to form the shape of the fuel to reduce thrust during the high stress period. I’m not sure of the details, this problem with liquid fuel rockets where this a high structural load on the fuel tanks may have caused some of our early failures before the Mercury launches. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 16, 2014, 6:17pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/4 "2014-05-16T18:17:50Z")
I would presume that this is one of the reasons that liquid fuel rockets are used for most space applications, the ability to throttle up and down, which solids do not have. Although, I wonder if launching from an aircraft could help reduce this problem. A Minuteman missile has been [launched successfully from a C-5](http://www.youtube.com/watch?v=It7SQ546xRk). ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [May 16, 2014, 6:46pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/5 "2014-05-16T18:46:03Z")
> [@AK84](#): > > I would presume that this is one of the reasons that liquid fuel rockets are used for most space applications, the ability to throttle up and down, which solids do not have. Actually, solids do have the ability to throttle, but you have to know before making them how you want them to throttle. By shaping the fuel grain you can change how much fuel is burning at different times during launch, thus varying the thrust. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [May 16, 2014, 7:11pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/6 "2014-05-16T19:11:49Z")
> [@AK84](#): > > I would presume that this is one of the reasons that liquid fuel rockets are used for most space applications, the ability to throttle up and down, which solids do not have. And the ability to be shut off and restarted, for orbital or lunar vehicles. > [@](#): > > Although, I wonder if launching from an aircraft could help reduce this problem. Yes, the Orbital Sciences Pegasus rocket is launched from an airplane for that reason. Weight limits make that impractical if you get much larger. > [@](#): > > A Minuteman missile has been [launched successfully from a C-5](http://www.youtube.com/watch?v=It7SQ546xRk). That was mostly a defensive measure - Soviet ICBM’s were presumably targeted directly at land silos, but could not be against aircraft. An ICBM is suborbital, of course - it doesn’t have to get very high or go very fast compared to a satellite launcher. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [May 16, 2014, 7:14pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/7 "2014-05-16T19:14:39Z")
> [@TriPolar](#): > > This is one theortical advantage of solid fuel and hybrid rockets, the fuel can be a structual part of the rocket engine during the high stress portion of flight. However, in the case of the Shuttle SRBs which had a short intense burn time they had to form the shape of the fuel to reduce thrust during the high stress period. I’m not sure of the details, this problem with liquid fuel rockets where this a high structural load on the fuel tanks may have caused some of our early failures before the Mercury launches. TriPolar got it. > [@](#): > > **Question:** What is the maximum dynamic pressure, and why does the shuttle throttle down at max-q? > > **Answer:** Dynamic pressure – also known as q-bar – is the force exerted on the shuttle by the atmosphere as the shuttle is passing through it, and it is measured as a force per unit area (pounds per square foot). The thicker the atmosphere and the faster the shuttle is traveling through the atmosphere, the greater is this force. If you’ve ever stuck your hand out of a car window while the car was moving at higher speeds, then you can picture the speed/force relationship. > > Maximum dynamic pressure, or max-q, is the point at which this force peaks, and occurs during ascent about a minute after liftoff. The combination of the shuttle’s rapid acceleration and relatively low altitude – that is, denser atmosphere – produce this peak pressure. In order to keep the force exerted on the shuttle below what it is designed to handle, the main engines are programmed to throttle back to about two-thirds of their rated power – about 67 percent. In addition, the solid rocket boosters’ propellant is loaded in such a way that the SRB thrust also is reduced during this period. Lowering the thrust lowers the acceleration, so the shuttle speed increases at a slower rate. The slower speed, compared to what it would be if throttling didn’t occur, results in reducing the maximum dynamic pressure the shuttle endures. As the shuttle climbs higher, and the atmosphere thins, the thrust is returned to its normal value. [http://spaceflight.nasa.gov/feedback/expert/answer/mcc/sts-107/01\_21\_09\_22\_16.html](http://spaceflight.nasa.gov/feedback/expert/answer/mcc/sts-107/01_21_09_22_16.html) ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [May 16, 2014, 7:36pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/8 "2014-05-16T19:36:28Z")
Not sure if all rockets are like this but the [Atlas rocket](http://en.wikipedia.org/wiki/SM-65_Atlas) was pressurized. It would collapse under it’s own weight just sitting there. We had a display of one at the AF museum that just collapsed on itself because it lost pressure. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 16, 2014, 8:03pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/9 "2014-05-16T20:03:33Z")
> [@ElvisL1ves](#): > > That was mostly a defensive measure - Soviet ICBM’s were presumably targeted directly at land silos, but could not be against aircraft. An ICBM is suborbital, of course - it doesn’t have to get very high or go very fast compared to a satellite launcher. I am pretty sure the Minuteman reached near orbital speeds in its flights, it certainly was more than Mach 20. ---
**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 16, 2014, 8:17pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/10 "2014-05-16T20:17:18Z")
> [@Magiver](#): > > Not sure if all rockets are like this but the [Atlas rocket](http://en.wikipedia.org/wiki/SM-65_Atlas) was pressurized. It would collapse under it’s own weight just sitting there. We had a display of one at the AF museum that just collapsed on itself because it lost pressure. AFAIK, all liquid fuel rockets are pressurized to some extent; the question is whether the pressure is needed to maintain its structure. The Falcon 9 is semi-pressure stabilized; it is strong enough to be handled unfueled without being pressurized (so ground handling is fairly straightforward), but it needs pressurization to handle the fuel and payload. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [May 16, 2014, 9:12pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/11 "2014-05-16T21:12:17Z")
> [@AndrewL](#): > > Actually, solids do have the ability to throttle, but you have to know before making them how you want them to throttle. By shaping the fuel grain you can change how much fuel is burning at different times during launch, thus varying the thrust. If it was a solid fuel mixed with laser propulsion, then you’d be able to throttle by adjusting the power/width of the laser, I’d assume. ---
**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 16, 2014, 9:40pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/12 "2014-05-16T21:40:50Z")
> [@Sage\_Rat](#): > > If it was a solid fuel mixed with laser propulsion, then you’d be able to throttle by adjusting the power/width of the laser, I’d assume. That’s basically the same thing hybrids do. The fuel is a solid, a liquid oxidizer is sprayed on the fuel. The oxidizer can be throttled. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 18, 2014, 5:06am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/13 "2014-05-18T05:06:19Z")
So, when in flight in terms of altitude does the atmosphere cease to be a major consideration. ---
**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 18, 2014, 5:29am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/14 "2014-05-18T05:29:13Z")
> [@AK84](#): > > So, when in flight in terms of altitude does the atmosphere cease to be a major consideration. It depends on what you’re considering. The atmosphere is certainly still relevant to satellites at 400 km; smaller satellites (like cubesats) and ones with large solar arrays (like the ISS) are most affected. At 1000 km, though, there’s not much left. For the Shuttle, Max Q happened at ~11 km. At 45 km, the dynamic pressure is about 10% of what it is at Max Q (see [here](http://www.aerospaceweb.org/question/aerodynamics/maxq/maxq.jpg)). I’m not sure when it hits 1%, but it’s probably not long after–I’d guess in the ballpark of 60 km. [Here is a chart](http://www.astronautix.com/graphics/r/rsa4f306.gif) from a different rocket, but the general shape of the curve is representative; you can see that near the end the curve smooths out a bit, but it still drops off fairly quickly (close to an exponential). ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [May 19, 2014, 2:03am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/15 "2014-05-19T02:03:04Z")
> [@Dr.Strangelove](#): > > AFAIK, all liquid fuel rockets are pressurized to some extent; the question is whether the pressure is needed to maintain its structure. If anyone would know, I suppose it would be you. 😉 While changing channels yesterday I bumped into that documentary about you. There aren’t many movies where you can tune into the tail end of the closing credits and instantly know what movie it is. ---
**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 19, 2014, 2:30am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/16 "2014-05-19T02:30:05Z")
> [@dropzone](#): > > If anyone would know, I suppose it would be you. 😉 While changing channels yesterday I bumped into that documentary about you. There aren’t many movies where you can tune into the tail end of the closing credits and instantly know what movie it is. At this point, I can’t help but to associate “We’ll Meet Again” with nuclear combat (toe-to-toe with the Russkies). Makes me want to pull out my [slide rule](https://www.fourmilab.ch/bombcalc/)… ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [May 19, 2014, 3:18am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/17 "2014-05-19T03:18:52Z")
**dropdad** thought that was the perfect song for that scene because he used to play it before every bombing mission. Cool link! > [@](#): > > Anyway, when some champion of human liberty in a Che Guevara T-shirt and Mao jacket was haranguing his audience with claims like “A single Hiroshima bomb set off downtown would annihilate this university and all of us in the blink of an eye”, what better way to burnish one’s Strangelovian credentials than to whip out a handy-dandy nuclear bomb computer slide rule, whip—slip—slide, and interrupt, “Naaah…fifteen kilotons at five miles? Surface burst? Why, that’s only a quarter to a third of a pound per square inch overpressure—it’ll probably break some window glass but that’s about it.” Flipping the slide rule over, “The flash isn’t even enough to cause sunburn, and the immediate radiation is next to nothing.” For some unfathomable reason, this never seemed to either carry the argument or suitably impress chicks. I blame the atomic culture of the 50s and 60s for my gallows humor. Every day since October '62 has been gravy because I, a ghoulish 8-yr-old who watched too much news, kissed my ass goodbye then. Really, though, we lived far enough from a target then that we had plans for skipping over the Blue Ridge when it happened, giving us a few extra months, maybe. Now I’d be–lemme see. Air-burst, 2 megaton, over O’Hare (8 miles away)–dead. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [May 19, 2014, 3:32am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/18 "2014-05-19T03:32:50Z")
The Airborne Laser attack envelope to disable a launched ICBM (or smaller range) was optimized for initial lower atmosphere launch for precisely that reason, to disable the missile by heat weakening it enough to fail (also so the nuke or gas would stay in the bad guy’s territory). I believe that program was finally shut down, but it had some incredible optics going on, from what I knew about it. (It was the first national security rejection I ever was given for researching an article and submitting it for review.) ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 20, 2014, 4:33am UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/19 "2014-05-20T04:33:05Z")
Ignoring, the obvious logistic issues, would launching at high altitude help? Say Afrom the peak of one of the 8000 meter peaks? ---
**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 21, 2014, 12:30pm UTC](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595/20 "2014-05-21T12:30:44Z")
Probably yes, but there are a lot of competing issues with locating a launch pad. The tiny gain to be had won’t outweigh the rest of the issues. There are not all that many very high peaks, and those that there are are going to fail some of the other important criteria. The boost to velocity from launching close to the equator is important. Unless you need a polar orbit, when you really want to get as far from the equator as possible. Despite the logistic challenges, Sealaunch makes a business of exploiting the gains to be had here. Were it not for some pretty poor luck they would likely be a thriving and more well known launch business. When you launch you need to fly out over uninhabited planet for quite a distance. Dropping a flaming wreck of a rocket on the populace never goes down well. So you tend to locate launch sites so that the track goes out over the ocean, desert or tundra. Finding a high mountain that is the correct side of a large uninhabited area, close enough to the equator, and under your country’s control isn’t easy. It is hard enough finding a flat area to build a launch complex that meets all these criteria without looking for a mountain as well. Mount McKinley (6km) would probably be an interesting choice for a polar orbit launch. Mauna Kea could be useful otherwise, but is only 4200m, and you would annoy the astronomers. At 19°N the latitude is appealing. The really big peaks are all in Asia, mostly part of the Himalayas. Some are around 29°N which is useful (similar to Cape Canaveral), and you certainly have a good track of uninhabited land to launch over. Logistics of course are insanely impractical. [Next page](https://boards.straightdope.com/t/aerodynamic-stresses-on-rockets-and-missiles-in-lower-atmosphere/688595.md?page=2)