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This answer suggests that if a payload were simply bulk propellant, such as LH2 and LOX, and not delicate technology such as an interplanetary probe or communications satellite, then there may be some re-optimization of launch trajectory and perhaps the launch vehicle, to incorporate such things as a higher g-force trajectory.

I know that early in a launch, the thrust (and therefore g-force) is sometimes even reduced so that the total compression forces on the rocket body (rocket thrust pushing up at one end, aerodynamic drag pushing down at the other) is capped at "Max-Q" - see also here - so how much could it actually be increased?

Question: How would a bulk payload like fuel (instead of delicate technology) change boost to LEO optimization?

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    $\begingroup$ In the extreme example of human launches (most fragile payload) in lunar launches engines were shut off to reduce g-load and extra mass had to be added as a launch escape system to save the crew in case of failure $\endgroup$ – Caridorc Aug 30 '16 at 8:09
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    $\begingroup$ If I remember correctly, Max Q has as much to do with conserving Delta-V as it does preventing aerodynamic RUD. It's easier to throttle back and wait til you're in thinner air than it is to try to fight through the huge drag-losses of Max Q. That's what I remember, anyway. $\endgroup$ – UIDAlexD Sep 15 '16 at 13:45
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    $\begingroup$ @uhoh: actually, that's a middle ground / consequence, three factors influencing each other. Exceeding MaxQ would lead to delta-V losses; since MaxQ won't be exceeded there's no point to make the rocket withstand higher dynamic pressures; as result exceeding MaxQ would result in RUD. (also, lightening the construction leads to different aerodynamic profile, which causes drag, and the threshold of delta-V loss, which changes the desirable MaxQ, closing the loop.) $\endgroup$ – SF. Oct 15 '16 at 16:09
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    $\begingroup$ Wouldn't there be a point of max Q no matter how you tweak the launch profile? You could move the point of max Q, but there will always be a point of maximum dynamic pressure during the ascent through the atmosphere. $\endgroup$ – user Oct 15 '16 at 17:43
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    $\begingroup$ There is normally a flight rule on the maximum allowed dynamic pressure, the trajectory and throttling is designed to not exceed this. Should really be called the q limit. But yes, every trajectory will have a point of maximum dynamic pressure "max q" $\endgroup$ – Organic Marble Oct 15 '16 at 18:15
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The bulk payload like fuel may tolerate high g-forces and strong vibrations, but the tanks and the structure of the rocket body must be built for high g-forces also. Not easy if the tanks and the structure should be as light as possible to maximize the amount of bulk payload. A payload of LOX and LH2 would require thermal insulation to limit the loss by boil-out.

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  • $\begingroup$ The context is this question and the presentation linked there, as well as this answer that I've linked to. Presumably it is possible to make tank payloads that can handle greater static and/or dynamic g-forces than deep space probe or communications satellite payloads. In that case, how would that change the "Boost to LEO" trajectory optimization? $\endgroup$ – uhoh Sep 15 '16 at 13:13
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As a rather violent "boost optimization", I'd like to point out to the existing question about unconventional LEO launching, where this answer provides a nice presentation of a gas gun. A quick summary is: gas gun was tested to provide at least 3 km/s initial delta-v. The bullet is planned to be a small rocket. The technology is currently ready for a high-G rocket - with electronics included! Fuel is explicitly said to be an ideal high-G payload. Cost to LEO per pound of fuel is calculated there, but it's so absurdly low I'd dare not repeat it here :)

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  • $\begingroup$ Are you sure this is an answer specifically to this question, or is this just a comment or passing thought? $\endgroup$ – uhoh Nov 14 '16 at 15:03
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    $\begingroup$ An answer. You haven't narrowed the question to the conventional rockets. Using a gas gun is "boost" optimization, just a rather violent one. $\endgroup$ – kubanczyk Nov 14 '16 at 15:24
  • $\begingroup$ OK you've piqued my interest! Can you summarize a bit some of the info that's in that answer and video? Roughly how many G's are we actually talking about here? Lifting liquids is very different than lifting solids - if you push on them they like to expand sideways. Does that answer and/or video talk about the extra mass of the tank necessary to contain liquids? $\endgroup$ – uhoh Nov 14 '16 at 15:33
  • $\begingroup$ Actually I just realized that this is exactly what @Uwe is talking about in this answer. $\endgroup$ – uhoh Nov 14 '16 at 15:36

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