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[–]bobbobbybob 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (3 children)

That's a bit weird. Second stage rockets have a different profile to keep the pressure / flame shape balanced. Which is why there is so much work on variable rocket cones.

[–]Tom_Bombadil[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (2 children)

You're correct about the improved relative efficiency of the second stage rocket. Unfortunately, that doesn't resolve the problem.

This problem will still be encountered as soon the rocket reaches a height where the atmosphere is functionality a vacuum.

All known current generation rockets depend on atmospheric backpressure for efficiency. To the best of my knowledge, anyway. If you can prove this to be incorrect then please let me know.

This is analogous to the way a gun fires a bullet. The gun barrel is similar to the atmospheric pressure, where a shorter barrel is similar to a lower atmospheric back pressure.

As the barrel length is shortened the max velocity of the bullet exiting the gun is reduced. Also, the nozzel flash increases significant. When you get to a barrel length of zero, the bullet velocity is trivial, and the nozzel flash expands in all directions.

In a vacuum a rocket is functionality useless.

Here a great example of how questioning about this issue is typically evaded...

https://www.livescience.com/34475-how-do-space-rockets-work-without-air.html

The explanation was oversimplified and intentionally misleading.

You can imagine how effective such a rocket would be when "landing in the moon".

The result would be landing in a crater; that was newly formed by the impact of the astronauts and the landing craft.

I don't like it either. I do my best to accept hard facts, and to then realign my world view.

That's a bit weird.

Agreed.

[–]bobbobbybob 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (1 child)

Hard vacuum is going to be an issue, but I thought we relied on conservation of momentum one we've left orbit. Your gun analogy is good, but doesn't hold true all the way down to zero pressure. The velocity at which we expel compressed gasses allows steering.

Your spaceship has a very high velocity already. It maintains that velocity through surfing the gravity well. So the steering tubes are merely used to trim that surfing line.

The moon landing is another issue completely. I was a big NASA fan, and bought the anniversary album. And then measured the shadows. Divergent shadows make me very sad.

I guess I'm not arguing that rockets don't work in a hard vacuum, since you are correct. I've now read that link, and to me, that sums it up perfectly.

For things like the ISS, there's a smarter method, https://www.airspacemag.com/multimedia/how-things-work-space-station-steering-22912477/

And I'm intrigued that they use that, since I'm pretty sure its one of the random ideas I put out onto newsgroups in the early 90s. Along with sonic weapons like the LRAD.

More on CMG here:

https://www.academia.edu/5497143/ISS_CMG_Momentum_Desaturation_Design

[–]Tom_Bombadil[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Your spaceship has a very high velocity already. It maintains that velocity through surfing the gravity well. So the steering tubes are merely used to trim that surfing line.

It would need to be going near escape velocity to escape to the moon. Escape velocity is ~40,000 mph. It was my understanding that the rockets were traveling at 17,000 mph for near Earth orbit.

The additional velocity required to get to the Moon to reach the edge of the "gravity well" would be tremendous.

Also, this acceleration would need to occur within the Earth's atmosphere for the rockets to function; which is unlikely...

Significant travel beyond low Earth orbit appears to be impossible using currently known technologies.

I think we're stuck...