4 ms·
> A terrestrial rocket has to push through a plug of air equivalent to a 30-foot column of water, and physics dictates that the smallest vehicle capable of movi
by fnord77 5y ago
> A terrestrial rocket has to push through a plug of air equivalent to a 30-foot column of water, and physics dictates that the smallest vehicle capable of moving all that atmospheric mass without paying a penalty in momentum is about 30 feet long.
This doesn't make sense - they're talking about momentum, but then conflating that with a length rather than a mass
- jjk166 5y agoThe momentum to length thing comes from Newton's approximation for impact depth, which basically states that the distance a high speed projectile can penetrate something is a function of its density and length. Something like an asteroid or needs a certain minimum length to make it through the atmosphere without slowing down below the speed of sound. It has nothing to do with rockets though, as rockets are not high velocity projectiles - they are powered vehicles and most of the time they are in the atmosphere they're not even going that fast. For space launch this condition would only be relevant if you're being shot out of a massive cannon or something. https://en.wikipedia.org/wiki/Impact_depth https://en.wikipedia.org/wiki/Impact_depth