Naked Science Forum
Non Life Sciences => Technology => Topic started by: acecharly on 30/06/2012 22:23:17
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any thoughts
Cheers Ace
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In the sixties just such a project was started
http://en.wikipedia.org/wiki/Project_HARP
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There's a problem.
The shell accelerates along the barrel of a gun because the hot gases push on it.
Once it's moving at roughly the speed of sound, it's moving as fast as the gas molecules.
So they can't catch up with it to push on it.
You can get round this using hot light gases (like hydrogen) but, even the best efforts
http://en.wikipedia.org/wiki/Light_gas_gun
get to about 7 km/sec
That's less than the escape velocity from the earth (11km/s) so basically, the answer is not at the moment and possibly never.
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I was thinking of "Escape Velocity" (11.2 km/s)
However, you don't have to reach escape velocity to enter Low Earth Orbit (LEO), which requires about 7.8 km/s (I think). Wikipedia (http://en.wikipedia.org/wiki/Low_Earth_orbit#Human_use) lists a Delta-V of 9.4 km/s with wind resistance (for a rocket) adding 1 to 2 km/s more.
So, BC's gun is actually pretty close to what is necessary to achieve LEO. One might be able to improve velocities with a magnetic accelerator.
However, you also have to take into account wind resistance which would be considerable on Earth, especially for an object that would have to reach the full escape velocity near the launch point. And, thus, it would be far more practical to do on the moon, mars, or other bodies with a thin atmosphere.
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The speed of sound problem can be overcome by a two stage acceleration process the first stage accelerating a gun barrel cotaining the missile which upon obtaining the limiting velocity the ignites its propellent further accelerating the missile.
There are of course better ways of getting missiles up to escape velocity but all use two or three stage systems.
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a name that comes up in such discussions is Gerald Bull, as syhprum mentioned with HARP. got to watch out where you are pointing that thing:
http://en.wikipedia.org/wiki/Gerald_Bull (http://en.wikipedia.org/wiki/Gerald_Bull)
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It is possible to get the velocity with a rail gun, where the projectile is powered by electric and magnetic reaction. Major limiting factor for launching to LEO is the required mountain to build the straight vacuum chamber needed to get the power requirement down. You just need muzzle velocity to be greater than 10km/s then it will be in orbit pretty soon. The need for a mountain is mostly for noise abatement, the sonic boom will be pretty big. It will really only be usable to launch mass, any person launching with it will be a puddle on the floor.
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If the problem is the speed of propagation of the explosion, then could you gain velocity by making a multi-point ignition, or perhaps a continuous area ignition?
I.E. essentially making a multi-stage accelerator with a single stage.
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It's not an issue with the speed of propagation. It's the speed of the molecules.
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It is possible to get the velocity with a rail gun, where the projectile is powered by electric and magnetic reaction. Major limiting factor for launching to LEO is the required mountain to build the straight vacuum chamber needed to get the power requirement down. You just need muzzle velocity to be greater than 10km/s then it will be in orbit pretty soon. The need for a mountain is mostly for noise abatement, the sonic boom will be pretty big. It will really only be usable to launch mass, any person launching with it will be a puddle on the floor.
No kidding, the fireball as the projectile leaves the muzzle and hits the air would put a lot of burden on a heat shield. Maybe a rather long, skinny projectile would be work. Ablate the front end and still have some projectile mass left when it leaves the atmosphere. Or maybe you could use multiple projectiles, one to push the air out of the way as it vaporizes and the payload rides in it's wake.
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It is in Donald Duck cartoons, i had that episode in the early 70's.
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I was reading what distimpson said and i had a similar idea, though wouldn't a javelin shaped projectile have the best aerodynamics to beat earth's wind resistance? That would create the issue of not having much space for a payload but then we are simply talking about firing a projectile into space, not the reason why you would want to do so! lol If you had a barrel long enough then it would be fairly simple to get a projectile into space, however you wouldn't exactly be able to move it very easily
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BC explained that there is a limit to the speed that molecules in the propellant are moving (speed of sound). A longer barrel will not help if you are using hot gases to propel. If as Sean suggested you are trying the railgun route - wikipedia has pages on both railguns and massdrivers - but they aren't ready yet. To get stuff into space it is envisaged that over a kilometre of track is required with superconductors and other rare materials.
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I was thinking of a longer barrel using magnetics, if a plane can accelerate to mach 4-5 then with enough space/energy it would be very easy to accelerate a projectile to similar speeds. I had a dream last night that we used a small powerful local electromagnetic field (self contained) that only had enough power to put a high energy field for roughly 10 seconds, then use this coupled with a fusion generator to eject a ball of plasma into this small field then use electromagnetic pulse to fire it >.< Sure there are a thousand reason why we couldn't do that but i see so many energy weapons on sci fi programs and i am 100% certain that physics wise it is possible, same with 'shields' its just our understanding/manipulating of physics that needs to catch up
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BC explained that there is a limit to the speed that molecules in the propellant are moving (speed of sound).
There is a trick that Bull worked out; if you apply the hot gas sideways at carefully timed moments you can go above that. You need a wedge shaped sabot behind the projectile though and apply the force at an angle.
The other way to go is to use multiple stages, like a light gas gun; if the gas is already moving at high speeds being pushed by a piston when you light it, then it's going to go supersonic (higher than the sonic speed of the hot gas).
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BC explained that there is a limit to the speed that molecules in the propellant are moving (speed of sound).
There is a trick that Bull worked out; if you apply the hot gas sideways at carefully timed moments you can go above that. You need a wedge shaped sabot behind the projectile though and apply the force at an angle.
The other way to go is to use multiple stages, like a light gas gun; if the gas is already moving at high speeds being pushed by a piston when you light it, then it's going to go supersonic (higher than the sonic speed of the hot gas).
True, it could work, (a bit like how the soap flies across the bathroom when you try to get hold of it), but you are reaching into areas where rocket science is doing it the easy way.
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As space is regarded by most scientists to be the region of the atmosphere were aerodynamic flight is not possible 100 km is that designated altitude by the Fédération Aéronautique Internationale. In America the designated altitude for some unknown reason is said to be 80km probably to award themselves astronaut wings during the early 60's when they could barley get a rocket of the ground. The HARP project has launched 420-mm projectiles 143 km over Barbados so it is safe to say that gun launched projectiles have already entered space. Alan Shepards first manned flight into space was only 44 km higher.
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Lets do a bit of balistics.
The kinetic energy it was fires with must have been (at least) the gravitational potential energy it had when it got to the top of its flight
E=mgh= 1m*10*143000 =1,430,000 J/Kg
E= 1/2 M V^2
V^2 =1,430,000*2
v=sqr rt 2860,000
V=1700 m/s
So they shot a massive projectile at about Mach 5 (more really- I ignored air resistance which is "significant" for hypersonic projectiles).
How?
Anyway, to get into orbit is just about possible.
Out of the earth's gravitational pull, (11 km/s) isn't.
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Good point about HARP.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F5%2F52%2FProject_Harp.jpg&hash=1917306bad48da91638d4eb8c20e5ba9)
HARP used a non-rocket spacelaunch method based on a very large gun to fire the models to high altitudes and speeds.
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The project was based on a flight range of the Seawell Airport in Barbados, from which shells were fired eastward toward the Atlantic Ocean. Using an old U.S. Navy 16 inch (406 mm) 50 caliber gun (20 m), later extended to 100 caliber (40 m), the team was able to fire a 180 kilogram slug at 3,600 metres per second (12,000 ft/s), reaching an altitude of 180 kilometers (591,000 ft).[citation needed] In 1966, the HARP gun fired a projectile to 112 miles high, a world record that still stands.
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As far as I am aware a gun can achieve it's greatest velocity when fired at an angle of 45°. If this is the case what speed and distance would the projectile [that reached an altitude of 112 miles fired at near 90°] have attained had it been fired at 45°?
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Projectiles go furthest in the horizontal plane for a given muzzle velocity when fired at 45degrees to the horizontal (although I think this angle ignores air resistance) - but the muzzle velocity itself is independent of the angle of inclination.
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Projectiles go furthest in the horizontal plane for a given muzzle velocity when fired at 45degrees to the horizontal (although I think this angle ignores air resistance) - but the muzzle velocity itself is independent of the angle of inclination.
Surely a projectile fired in the horizontal would hit the earth sooner since it,s trajectory means that it begins to fall the moment it leaves the barrel?
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As I thought 90 degrees hits the ground before the 45 degree vector and 45 degrees has the absolute range of any cannon fired projectile .[/i]http://www.physicsclassroom.com/mmedia/vectors/mr.cfm
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Projectiles go furthest in the horizontal plane for a given muzzle velocity when fired at 45degrees to the horizontal (although I think this angle ignores air resistance) - but the muzzle velocity itself is independent of the angle of inclination.
Reread imatfaal's post.
He is in fact suggesting to get horizontal distance, one must fire gun at an angle.
However, in the case of the HARP gun, the goal is to reach > 100 miles in altitude. In this case, a nearly vertical launch is preferable to minimize the time and air to pass through to reach "space". However, it should be noted that while the projectile may reach 100 miles altitude, it is not traveling in a trajectory, or at a velocity that would allow to it remain in orbit.
Keep in mind that downward acceleration at about 9.8 m/s2 is related directly to time, so the more time from launch, the greater the loss of vertical velocity. And, of course, firing it at an angle, the vertical component of the velocity vector begins lower. And, the more time at lower altitudes, the greater the wind resistance further decreasing velocity.
Thus, for maximum vertical altitude, one benefits from a vertical launch (assuming one is not using wings to create lift).
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As far as I am aware a gun can achieve it's greatest velocity when fired at an angle of 45°. If this is the case what speed and distance would the projectile [that reached an altitude of 112 miles fired at near 90°] have attained had it been fired at 45°?
As far as I remember a gun will have it's greatest range , for a given muzzle velocity, when it's at 45 degrees (assuming a few things, notably a flat earth).
If I wanted the greatest muzzle velocity, I would point it down. gravity might not help much, but it would help.
In the real world, the usual model* for range doesn't work terribly well. The biggest problem is that it ignores air resistance.
If you are trying to ram air out of the way at hypersonic speeds, air resistance is hugely important.
45 degrees is no longer the "right " answer and a calculation of the range isn't going to be very good.
As an indication, the (lower bound to the) velocity I calculated for the HARP projectile was 1700 m/s. The real speed was about twice that.
* Like this
http://en.wikipedia.org/wiki/Trajectory#Uniform_gravity.2C_no_drag_or_wind
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One would note that mortars are frequently fired at high angles.
This probably gives 3 things.
- Going over obsticles
- Gaining Distance
- Adjusting range using a fixed propellant load
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As far as I am aware a gun can achieve it's greatest velocity when fired at an angle of 45°. If this is the case what speed and distance would the projectile [that reached an altitude of 112 miles fired at near 90°] have attained had it been fired at 45°?
As far as I remember a gun will have it's greatest range , for a given muzzle velocity, when it's at 45 degrees (assuming a few things, notably a flat earth).
If I wanted the greatest muzzle velocity, I would point it down. gravity might not help much, but it would help.
In the real world, the usual model* for range doesn't work terribly well. The biggest problem is that it ignores air resistance.
If you are trying to ram air out of the way at hypersonic speeds, air resistance is hugely important.
45 degrees is no longer the "right " answer and a calculation of the range isn't going to be very good.
As an indication, the (lower bound to the) velocity I calculated for the HARP projectile was 1700 m/s. The real speed was about twice that.
* Like this
http://en.wikipedia.org/wiki/Trajectory#Uniform_gravity.2C_no_drag_or_wind
You say [if I read you right] that in the real world firing the projectile at 45 degrees s no longer the "right" answer! At what angle in your world would you fire the gun for maximum range?
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How long is a piece of string?
If the velocity is over about 11 km/s the question is meaningless: you never see the projectile again.
Are you shooting with, or against the rotation of the earth (or at an angle to it?)
It really isn't a trivial question with a "best" answer.
http://en.wikipedia.org/wiki/External_ballistics
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Whilst I agree with the piece of string answer - too wide a question to admit to an answer - you could look at an example from history. During the great war the germans bombarded Paris with a huge siege gun from around 120 Km away. The angle of inclination of the Paris Gun was 55 degrees; but whether this was optimal, an engineering/technological requirement, or just the angle calculated to drop shells on Paris is not something I can tell you
http://en.wikipedia.org/wiki/Paris_Gun
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I would imagine they fired the gun at 55* to prevent overshooting the target and not because different laws of physics apply to German artillery!!!
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For 45° not only must the Earth be flat but it must be infinite in extent.
during WWII about 8 shells coming from the south landed on Maidstone about 65 miles from the nearest German occupied territory apparently 21cm missiles from a K12 they had not lost their enthusiasm for experimenting with long range guns.
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I would imagine they fired the gun at 55* to prevent overshooting the target and not because different laws of physics apply to German artillery!!!
Who mentioned different laws of physics? It could have been
1. that the position they fired from was important - thus to hit Paris rather than overshoot you aim a little suboptimally
2. They calculated/discovered this was optimum and the gun was sited a far from France as possible.
3. The physical gun was not able to support itself at 45 degrees but was a 55 degrees
Without further evidence any of these or a combo or a different answer entirely could be true
For 45° not only must the Earth be flat but it must be infinite in extent.
Air Resistance is a far far bigger effect and will always swamp the difference between a parabolic and a elliptical course
during WWII about 8 shells coming from the south landed on Maidstone about 65 miles from the nearest German occupied territory apparently 21cm missiles from a K12 they had not lost their enthusiasm for experimenting with long range guns.
I didnt know that - glad they didnt manage to get any further northwards, my mum stayed in London all through the war! The psychological damage was, apparently, the most important effect of the Paris Gun and they already had that with the V1s and later with the V2s in the second world war.
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There was no was psychological damage in Maidstone as we were just told they were anti aircraft shells that had gone astray but I knew better as at one location you could see where the shell had come thru the branches of a tree on its way but during the war you believed what you were told to believe !!
PS this happened the day before the first V1 came over.
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There was no was psychological damage in Maidstone as we were just told they were anti aircraft shells that had gone astray but I knew better as at one location you could see where the shell had come thru the branches of a tree on its way but during the war you believed what you were told to believe !!
PS this happened the day before the first V1 came over.
as a personal opinion/memory - which affected you more mentally, in terms of anxiety, worry, and shear terror; the sudden V2s or the dreadful droning warning of the doodle-bug V1s. My parents are divided; one preferred the V1s as it gave you a few moments to prepare, the other hated the V1 for those very same moments of dread listening for the noise to stop...