# B Is Jules Verne's space cannon posible with a breathable fluid?

1. Dec 7, 2016

### farolero

"Liquid immersion provides a way to reduce the physical stress of G forces. Forces applied to fluids are distributed as omnidirectional pressures. Because liquids cannot be practically compressed, they do not change density under high acceleration such as performed in aerial maneuvers or space travel. A person immersed in liquid of the same density as tissue has acceleration forces distributed around the body, rather than applied at a single point such as a seat or harness straps. This principle is used in a new type of G-suit called the Libelle G-suit, which allows aircraft pilots to remain conscious and functioning at more than 10 G acceleration by surrounding them with water in a rigid suit.

Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G.[57] Extending acceleration protection beyond 20 G requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists.

Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application"

https://en.wikipedia.org/wiki/Liquid_breathing

i wonder how many g can an astronaut endure in breathable fluids accounting for bone density of 1.5 and how many g would produce a cannon to shoot a spaceship with his astronaut inside

any help to find out how visionary verne could have actually been?

2. Dec 7, 2016

### A.T.

Aside from the differential density limitations on high constant acceleration, extreme acceleration changes (explosion propulsion, impacts) will also produce shock waves in the fluid.

3. Dec 8, 2016

### phinds

Have you done a computation of what the G force would be in a cannon that could reach escape velocity? Do you understand the difference between that and the way rockets work?

4. Dec 8, 2016

### farolero

yes sure but wih breathable fluid i think an astronaut can stand up to 130 G i seem to recall reading

also the rocket bullet wouldnt need scape velocity but orbital velocity

5. Dec 8, 2016

### phinds

Uh ... the point of his space cannon was to reach the moon and surely you do not think that reaching Earth orbit is the same as getting to the Moon.

6. Dec 8, 2016

### Ibix

$v=at$ implies you need to sustain 130g for about 8.5s to achieve escape velocity of 11km/s. Distance travelled is $s=at^2/2$, so you'd need a gun barrel 47km long. Pointing straight up.

Edit: and that's accepting your un-sourced 130g as a valid estimate.

Last edited: Dec 8, 2016
7. Dec 8, 2016

### jbriggs444

Orbital velocity and escape velocity are close enough to identical for back-of-the-envelope work. [Factor of $\sqrt{2}$]

8. Dec 9, 2016

### farolero

i see thanks so the space cannon that jules verne pictured is not plausible

well maybe just a help in the future for lift off

edit:

here it says that a low orbit is at 160 km:

https://en.wikipedia.org/wiki/Low_Earth_orbit

its dificult for me to believe that to reach an altitude of 160 km you need to accelerate at 1300 m/s2 during 47 km

ill see if i can do the calculation of how long has to be the cannon to reach an altitude of 160 km

anyway they were considering doing a spacce lift and a cannon is similar to a lift

of course this is not plausible yet for the lack of breathing fluid of density one

Last edited: Dec 9, 2016
9. Dec 9, 2016

### A.T.

That was for escape velocity not 160km.

10. Dec 9, 2016

### farolero

yes but im talking of using a cannon like shuttle to put in low orbit, similar concept to the space lift

my point is that when breathable fluids of density one are developed this method could be used to simplify things making jules verne point right

edit:

https://en.wikipedia.org/wiki/Space_gun

11. Dec 9, 2016

### jbriggs444

Low earth orbital velocity is approximately 71% of escape velocity (*). The energy required to lift the shuttle to 160 km is not the problem. The energy required to achieve orbital velocity at that altitude is (**).

(*) The higher you are, the lower escape velocity is from where you are. Orbital velocity is $\frac{1}{\sqrt{2}}$ of the escape velocity from the altitude of the circular orbit. That is lower than escape velocity from the Earth's surface. But for a very low orbit at only 160 km up, the reduction in escape velocity is small.

(**) A launch from the the earth's surface using a cannon (i.e. a single impulsive velocity change) with the right energy still cannot achieve a circular orbit. You'll be in an elliptical orbit and will arrive at the intended orbital altitude with a velocity in the wrong direction. It takes a circularizing burn to enter a circular orbit. Fail to do the circularizing burn and your elliptical orbit is guaranteed to intersect with the Earth's surface.

12. Dec 9, 2016

### farolero

yes but the wikipedia points to adding minimal force rockets to the projectile, otherwise by the newton cannon thought experiment it will fall back to earth it needs rockets to correct the trajectory

13. Dec 9, 2016

### jbriggs444

Yes, that's what I said. "Orbit intersects earth's surface" means the same thing as "falls back to earth". "Circularizing burn" implies "adding rockets to the projectile".

If you have a mental picture of a cannon that fires a projectile vertically at high speed so that it reaches 160 km altitude just as it coasts to a stop and then uses "minimal" rockets to make its orbit circular, then the picture is flawed. For such a picture, the correct adjective is "huge".

14. Dec 9, 2016

### farolero

i quote from the wiki:
"As a result, all payloads intended to reach a closed orbit need at least to perform some sort of course correction to create another orbit that does not intersect the planet's surface. A rocket can be used for additional boost as planned in both Project HARP and the Quicklaunch project. The magnitude of such correction may be small; for instance, the StarTram Generation 1 reference design involves a total of 0.6 km/s of rocket burn to raise perigee well above the atmosphere when entering an 8 km/s low Earth orbit.[3]"
it says the magnitude for the rocket correction is small

15. Dec 9, 2016

### jbriggs444

Let us back up. You expressed incredulity that a 47 km cannon could be required to reach 160 km orbital altitude. I responded that the 160 km orbital altitude is irrelevant. It is the required orbital velocity that is significant.

I assume that your incredulity stems from an assumption of a vertical launch. Does the "StarTram Generation 1" reference design contemplate a vertical launch?

16. Dec 9, 2016

### farolero

yes thats another maybe the cannon should actually aim almost horizontally in favour of earth rotation

17. Dec 9, 2016

### jbriggs444

That misses the point. If you arrive at 160 km altitude with a 7 kps vertical velocity, what delta V is required to deflect the craft to a 7 kps horizontal velocity?

How does this compare to the ~0.4 kps delta V gained from the earth's rotational velocity?

Last edited: Dec 9, 2016
18. Dec 9, 2016

### farolero

thats why you shoot the space ship horizontally then in its parabolic trajectory will after half earth rotation have an almost paralel to earth ground vector that just has to be slightly corrected

anyway im already answered that even in the space cannon verne was a visionary on the haarp tests, we still just are too young to have seen it fully evolved and used

edit:

also the fact that the cannon has to aim horizontally make it posible the 47 km lenght we were needing to shoot astronauts to scape velocity with the aid of breathable fluid

i bet verne said nowhere the cannon was aiming up

19. Dec 9, 2016

### jbriggs444

That's an elliptical trajectory. (Specifically, a Hohmann transfer orbit). A parabolic trajectory is an approximation that holds for extremely low launch velocities such as that of a thrown baseball.

[A craft launched at escape velocity also traverses a parabolic trajectory, but that's not what we're talking about here]

20. Dec 9, 2016

### jackwhirl

@farolero It sounds like you are (or were) having trouble with the difference between orbital height vs orbital velocity. Please read this delightfully entertaining and informative article by Randall Munroe: Orbital Speed

21. Dec 9, 2016

### farolero

yeah sorry i meant elliptical instead of parabolic, my mistake

and thanks jackwhirl ill read that with interest