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How does a rocket deal with those side stresses while it's attached to the airplane? AFAIK rockets generally do not deal well with sideways forces, because a ro

by hawski 8y ago

How does a rocket deal with those side stresses while it's attached to the airplane? AFAIK rockets generally do not deal well with sideways forces, because a rocket is a giant can.

mirimir 8y ago
It's clear that solid-fuel rockets are OK with sideways forces, because they're transported that way. The can may be weak (except re internal pressure, obviously) but the fuel/oxidizer is ~like rubbery plastic. It's also clear that liquid-fuel rockets are OK with sideways forces, before they're loaded with fuel and oxidizer. Because they're so light. But once they've been loaded, they're very fragile re sideways forces. I've watched lots of rocket-failure video. A ~common scenario is loss of attitude control, for whatever reason, followed by crumpling and fireball. So I don't see how this thing could launch liquid-fuel rockets. Unless it could fly straight up, for long enough to transfer fuel and oxidizer. That'd be insane.
- ThenAsNow 8y ago
  A liquid rocket vehicle would have to be designed to be carried and launched from a carrier vehicle like Strato has, but that's eminently doable. You design in the load paths suitable for carriage and also for engine-on ascent loads. There will be more structure attached to the cylindrical sections. But this added structural mass further detracts from the delta V advantages of being released from a "Stage 0" air platform at altitude and speed. And yes, you also design for the bending loads associated with acquiring the ascent flight path angle you need. Those bending loads will be more severe than what a well-guided ground-launched system will be designed to endure. Strato's aircraft undoubtedly pitches nose-up pretty hard before dropping the rocket, but it's definitely nowhere near vertical. Solid motor casings are inherently quite stiff, to deal with combustion pressures. There would be less attachment/carriage hardware mass penalty than with liquids. Bottom line, there is nothing inherent to Stratolaunch that makes it incapable of launching liquids, and even with the additional structural mass penalty, it's entirely possible for Strato to have lower cost per unit of mass to orbit via a liquid rocket stack than with solids (likely, in fact).
  - mirimir 8y ago
    OK, I'm certainly no engineer. But it strikes me that one could get a sense by looking at LPG and LOX truck tanks. For LPG tanks, I found one source:[0] Material: 16MnR Yield strength (Mpa): 345 Permissible stress (Mpa): 170 Comparison: The wall thickness is ... 10-14mm. I found that SpaceX is using 301 stainless.[1] But I haven't found anything about thickness. Anyone know? 0) https://www.anstertrailer.com/lng-lpg-tank-trailer-guide/ https://www.anstertrailer.com/lng-lpg-tank-trailer-guide/ 1) https://www.popularmechanics.com/space/rockets/a25953663/elon-musk-spacex-bfr-stainless-steel/ https://www.popularmechanics.com/space/rockets/a25953663/elo...

by hawski 8y ago

How does a rocket deal with those side stresses while it's attached to the airplane? AFAIK rockets generally do not deal well with sideways forces, because a rocket is a giant can.

mirimir 8y ago

It's clear that solid-fuel rockets are OK with sideways forces, because they're transported that way. The can may be weak (except re internal pressure, obviously) but the fuel/oxidizer is ~like rubbery plastic. It's also clear that liquid-fuel rockets are OK with sideways forces, before they're loaded with fuel and oxidizer. Because they're so light. But once they've been loaded, they're very fragile re sideways forces. I've watched lots of rocket-failure video. A ~common scenario is loss of attitude control, for whatever reason, followed by crumpling and fireball. So I don't see how this thing could launch liquid-fuel rockets. Unless it could fly straight up, for long enough to transfer fuel and oxidizer. That'd be insane.

ThenAsNow 8y ago
A liquid rocket vehicle would have to be designed to be carried and launched from a carrier vehicle like Strato has, but that's eminently doable. You design in the load paths suitable for carriage and also for engine-on ascent loads. There will be more structure attached to the cylindrical sections. But this added structural mass further detracts from the delta V advantages of being released from a "Stage 0" air platform at altitude and speed. And yes, you also design for the bending loads associated with acquiring the ascent flight path angle you need. Those bending loads will be more severe than what a well-guided ground-launched system will be designed to endure. Strato's aircraft undoubtedly pitches nose-up pretty hard before dropping the rocket, but it's definitely nowhere near vertical. Solid motor casings are inherently quite stiff, to deal with combustion pressures. There would be less attachment/carriage hardware mass penalty than with liquids. Bottom line, there is nothing inherent to Stratolaunch that makes it incapable of launching liquids, and even with the additional structural mass penalty, it's entirely possible for Strato to have lower cost per unit of mass to orbit via a liquid rocket stack than with solids (likely, in fact).
- mirimir 8y ago
  OK, I'm certainly no engineer. But it strikes me that one could get a sense by looking at LPG and LOX truck tanks. For LPG tanks, I found one source:[0] Material: 16MnR Yield strength (Mpa): 345 Permissible stress (Mpa): 170 Comparison: The wall thickness is ... 10-14mm. I found that SpaceX is using 301 stainless.[1] But I haven't found anything about thickness. Anyone know? 0) https://www.anstertrailer.com/lng-lpg-tank-trailer-guide/ https://www.anstertrailer.com/lng-lpg-tank-trailer-guide/ 1) https://www.popularmechanics.com/space/rockets/a25953663/elon-musk-spacex-bfr-stainless-steel/ https://www.popularmechanics.com/space/rockets/a25953663/elo...