h a l f b a k e r yLike gliding backwards through porridge.
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I'd like to see his math before I look at it to closely. I
make it 40g on a 45ft cannon just to reach escape
velocity, and that completely ignores the air
resistance a mach 32 projectile will experience. |
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I'm not saying it's impossible, but I am saying there
are reasons that NASA still uses rockets. |
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He's just waiting for you to call him and offer your mathematics counsel, [MechE]. |
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Is my name [AeroE]? I'm not an aerospace guy, I just
have my doubts about a backyard space cannon. If
he can get it working, more power to him, but like
most interesting proposals, show me the math
before you can expect my support. |
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The problem is that even if the gun has enough
energy to reach orbit, the initial speed with air
resistance will cause the projectile to burn up. Put
it on a balloon and send it up into the thinner air and
you'd have a chance, but not from any normal
persons back yard.
Also that is just altitude, and misses that hard right
turn at the top. This gun would need to point hard
east and be right on the coast so that he doesn't
destroy all his easterly neighbors with sonic booms. |
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The problem is that even if the gun has enough
energy to reach orbit, the initial speed with air
resistance will cause the projectile to burn up. Put
it on a balloon and send it up into the thinner air and
you'd have a chance, but not from any normal
persons back yard.
Also that is just altitude, and misses that hard right
turn at the top. This gun would need to point hard
east and be right on the coast so that he doesn't
destroy all his easterly neighbors with sonic booms. |
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//but like most interesting proposals, show me the math before you can expect my support// |
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...aaand this is why I'm having to start high school all over again.
<mutters something about not enough aspirin on the planet> |
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Funny, the kickstarter page says: "The first phase of this project will be to conduct a series of test launches to certify the Starfire gun while firing in the vertical position for the first time." But it doesn't actually say that this has been test fired in the horizontal position. You'd think if he had he would have at least mentioned what the muzzle velocity was. |
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[MechE] are you sure about your calcs. I think 40g for 15m will get you 108m/s, not escape velocity or even orbital velocity. |
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A = 40g = 40*9.8m/s^s
v = At
x = At^2/2
t = sqrt(2x/A) = sqrt(2*15m/(40*9.8m/s^2)) = 0.2766 s
v = 40*9.8m/s^2 * 0.2766s = 108m/s |
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If the acceleration is less than 10,000g by "a substantial safety factor" and the tube is 45 ft long, the muzzle velocity will be "substantially" less than 1700 m/s. According to Wikipedia that's what a tank mounted gun can currently achieve. That's maybe impressive for a home built gun with that wimpy looking tube, but seems rather short of breaking new ground for space launch. Of course the idea is that this is more scalable by extending the tube, but he says that, "a barrel twice or even three times as long would be practical." Unfortunately, a barrel 3 times as long would only get him close to 3000m/s. Apparently HARP, which he is comparing himself to, got up to 3600m/s. It seems like if he hasn't even done that basic feasibilty yet, he's not serious about actual space launch. He's just trying to get a little cash out of his glorified potato cannon by launching T-shirts. |
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It seems he has no grand plans of actually getting to orbit (that realism is encouraging), but I'm not sure how big the market is for suborbital cannon launches. Maybe we can halfback some way to launch a net to catch space debris... |
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Didn't we discuss this awhile back ? something about using a continuous charge of explosives in the barrel, starting with deflagrants at the breech and working up to octol near the muzzle... I don't think it was my post though. |
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Yeah, I dropped a few of orders of magnitude there
(I think I forget to square Vf in my original calc, what
can I say, work's keeping my brain pretty fried
lately). Which makes it more like 400,000 gravities,
and a much more absurd number. |
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When I saw what the project was, then saw Canada, I
expected to see Troy Hurtubise's name somewhere. |
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(Vf)^2 = (Vi)^2 + 2*A*D
Is probably the easiest kinematic equation for these simplistic calcs. Vf = final velocity, Vi = initial velocity, A = acceleration and D = distance. |
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D = Vi*T + 1/2*A*(T^2) is also useful, where T = time. |
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As a worked example, say a barrel length of 20m, and a final velocity of 11000 m/s - so we're in the right order of magnitude for a space cannon. |
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(11000)^2 = 0 + 2*A*20 (S.I units are sooo handy) |
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A ~ 3 000 000 m/s^2, or about 300, 000 g's. |
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Which is really a lot, 20m is a very long barrel for a backyard cannon, and we're still ignoring air resistance. |
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If you could make backyard space cannons, Hitler would have bombarded London from the relative safety of Berlin, without going to the trouble of bothering with pesky pulsejet V1's and all of those hideously expensive V2's. |
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Lastly, I'm quite positive that you simply cannot launch an orbital projectile from the surface of the earth, irrespective of initial velocity. All possible trajectories either return to earth or escape - they never result in an orbit. Happy to be corrected here but I'm quite certain this is the case. Maybe you could aerodynamically convery vertical velocity into horizontal velocity during passage through the atmosphere to correct this, otherwise your projectile needs to have a rocket on board to impart horizontal velocity after launch in order to go into orbit. |
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I imagine with some seriously fancy math you could make a circular orbit with fixed fins. |
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Aerodynamics won't do it. The issue is not
actually that an orbit isn't possible, it's that the
orbit will pass through it's launch point, which will
take it down with air drag in a few orbits.
Likewise, any attempt to circularize with
aerodynamics will leave the payload with an orbit
that still goes through the atmosphere, since
that's the last point where the fins had any
influence on the path. |
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How strong the circularization burn would actually
have to be depends heavily on the attempted
orbit, and the launch trajectory. If you fire
straight up, you've minimized your atmospheric
interference, but you're going to have to burn a
lot of fuel to turn a very tight ellipse into a circle.
Your'e going to do much better firing essentially
flat, which makes the final burn essentially the
completion burn of a Hohmann transfer orbit, but
you're going to have to go through a lot of
atmosphere to get there (not to mention needing
to make certain there's nothing downrange tall
enough to get in the way). |
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The last time I looked, you could get up to orbital altitude with about
one mile per second (ignoring air losses). Enough speed to stay in
orbit would be about five miles per second. (Which is in addition and
at right angles and cannot ignore air friction and which cannot be
done from the ground.) |
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Trying to cannon something into orbit is a good way to get your
missile in the back of your head. Ignoring air and the planet's
rotation, the very best you can do is fire the cannon horizontally and
then duck (you'll have at least an hour to get the cannon down off the
platform). |
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If you point the cannon up at all, the projectile is still going to try to
orbit right back through the cannon again, but the planet will be in
the way. At least it will hit somewhere else. |
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The concept of launching an object from the ground fast enough for it to have orbital velocity once it exits the atmosphere (even ignoring the need to adjust the direction to get a stable orbit) always seemed completely ridiculous to me. People always talk about it with the qualifier "ignoring air resistance", but my impression is that air resistance is the largest challenge by orders of magnitude, not some small detail to work out later. The only valid reason for ignoring it is that we currently can't even approximate it, and if we ignore it, we can still show that shuch a feat is impossible. The problem is that then other people continue ignoring air resistance and waste their time trying to overcome those minor issues. |
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Re-entering the earth's atmosphere from orbital velocity is itself not a trivial problem. At near-orbital velocity (after rentry burn), the heating from the very thin upper atmosphere is a non-trivial engineering problem, but it also provides a great benefit since it gradually reduces the velocity to terminal velocity, and then the terminal velocity gradually decreases as the craft gets to lower altitudes, providing a low cost way to dump all that extra speed. |
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The idea that any man-made object could withstand the heat from moving at orbital velocity through the atmosphere even at the altitude of a tall mountain seems very unlikely to me, and then you've got to multiply that speed by some astonomical factor so that you're still at orbital velocity after being slowed by the trip through the atmosphere. |
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I haven't heard of humans actually succeeded in making an object travel at orbital velocity (say 7000m/s) through the atmosphere. Looking at the wiki page on rail guns, The US Navy is hoping to have achieved 5800m/s by 2020 to 2025. Therefore I assume that we might not even have an accurate understand of aerodynamics through thick atmosphere at that speed to even be able to calculate how fast the projectile would need to be traveling to have orbital velocity after punching through the atmosphere. |
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Maybe the best way to study this sort of velocity at low altitud is by watching the occasional meteor or meteorite that makes it into the lower atmosphere. Those have a tendency to explode. And the velocity they are traveling once they get into the lower atmosphere (assuming they magically reversed direction) might not even be enough because these meteors have already been slowed down significantly as they passed through the upper atmosphere. |
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Now I could be wrong about the assumptions above, so I'd be interested to read about experiments that have been performed that might shed light on what velocities actually would be needed to reach space at orbital velocity. |
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I wonder if any maths were preheated in the book King David's Spaceship. |
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Did that ship start off with a bang ? |
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Hmmm....about the air resistance:
Use a laser to ionize the air and then make a huge
electrical discharge along it. The air superheats and
rapidly expands. Maybe there is an additional pulse of
low air pressure due to the inertia of explosively
expanding air. Then fire the object up through the
evacuated region. |
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To get into proper orbit, just bounce it off a convenient
Russian Satellite. "In off the red" type of thing. |
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Maybe instead of a laser (I suppose if local authorities allow you a cannon in the back yard, what extra harm a giant laser) you could use lightning! Fire the projectile along the path of a wire guided bolt of lighting! Bonus: maybe use the charge from the lightning to power the railgun. |
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I am wondering about frictional heating on entry or re-entry which is the more often encountered real life circumstance. I wonder why shuttle tiles are not Teflon coated? They are heat resistant but not especially low friction as fas as I can tell. |
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Real world test - do teflon coated bullets fly farther? That may be googlable. |
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No luck. Some of you bakers have guns. Here is the test. |
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1. Accurate rifle with paper target at distance.
2. Sight on the same point each time.
3. Uncoated vs coated bullets. If coated is faster due to less frictional drag, coated bullets will have less time to drop before hitting target and so as compared to uncoated, bullet holes for coated bullets will group higher on the target. |
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From my understanding of heating during reentry, it's the compression of the air in front, not the friction, that causes most of the heating. So teflon might not help much. At much more reasonable velocities (like a bullet), I assume that friction might be a larger component. |
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// Use a laser to ionize the air and then make a huge electrical discharge along it // Umm, where is this charge coming from? I guess if we run a wire up our space elevator and stick it out over the projectile launch zone... |
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But seriously that type of concept would be the one to explore, but I don't see how a tunnel through the air would stay open long enough if created with a single event. I thikn it would need a continous explosion propogating thorught the air just the right distance ahead of the projectile. While that could reduce the air resistance if it could be aranged, I have no idea if that would reduce the air resistance by 1/2, 1/10th, or 1/100th. |
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// To get into proper orbit, just bounce it off a convenient Russian Satellite. // |
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That's not a completely unreasonable concept. But did you pick Russia out of a sense of friendly rivalry, or do you think their typical Molniya orbit might be more useful for such a maneuver? I assume we're actually aiming for a dead satellite that is mutually agreed upon. Of course bouncing off it is probably not such a good idea, but if the mini-spacecraft could deploy a net on a long cable, maybe we could do some slightly less violent momentum transfer. If the timing is just perfect, let's say we can arrange to have the projectile pass within say 100m of our target satellite. Before it passes, the net is deployed with the cable running perpendicular to the path of the satellite relative to the projectile. When the satellite gets caught in the net, it will pull the cable sideways, so the acceleration of the projectile will be somewhat gradual, and it will swing in circles around the satellite. But ideally we would cut the cable after it has swung about 180 degrees. At this point it will actually be traveling faster than the the satellite it hitched a ride from. |
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Since we don't want to be traveling faster than orbital velocity, we should coordinate this with a planned deorbiting. Using a satellite that is still live will also help with tracking to get the aim right. We launch the projectile just as the satellite is finishing firing its rockets to deorbit. When they meet, the dead satellite isn't traveling at orbital velocity any more but we arrange the relative velocity of our projectile such that after the slingshot maneuver, it is traveling at exactly orbital velocity for a nice stable orbit. I guess for the first attempt, we need to be sure that whether or not we hit or miss, the deorbiting satellite will still crash in an acceptable part of the ocean. Assuming we actually get this to work reliably (yeah right), we might be able to include less fuel for deorbiting because we know the satellite will be further slowed by slingshotting a projectile satellite. Or if we have one projectile launched satellite in orbit that we want to replace with a new model, with the two objects havinghte same mass, after the 180 degree rotation, I think both objects will have essentially switched courses, so the new one will be in orbit, and the old one will continue the ballistic trajectory back to earth. |
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Unfortunately we can't just shoot our projectile straight up and use this method to do a right turn when we get to orbit because we don't have cables that could handle that much acceleration. I don't have the numbers but I researched that some when I was contemplating an idea for the N-prize. It seems to me that if we could aim well enough (and overcome the atmosphere problems), this could be used to correct the path of a projectile that already had a significant sideways velocity, but as [MechE] pointed out, we end up going through a lot more atmosphere to get a significant sideways velocity. |
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It's a fun concept, but I think a space elevator is likely to happen before anyone launches an object into orbit using a space gun. |
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// Use a laser to ionize the air and then make a huge
electrical discharge along it // Umm, where is this
charge coming from?
// |
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From the ground into free air! I've seen lightning do a
similar trick from air to air. I think it would work better
if we could get it to reach to a more conductive region
of the atmosphere. |
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But how do you get the electricity to discharge through miles of ionized air rather than taking a shortcut to ground which is comparatively close? Lightning occurs when one region of air builds up a charge and discharges to ground or another region of air with a different charge. If you could build up some charge at the top of the path, then we might be getting somewhere. |
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Mmmm.... a wire won't work because it won't be
straight. |
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A discharge from ground to air needs to work in the
opposite direction to that of ordinary lightning, but
there is a big danger of a local arc. |
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So one electrode could be at the rail gun laser point,
and another at a similar laser somwhere else. Then
fire both lasers at the same time, allowing the upper
atmosphere to conduct between the two. |
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You use the cannon to fire shells with an
extremely high statically charged core inside a
very high dielectric outer casing, and use the high
powered ionizing laser to burn off the casing at
the right height. Once that happens, you get a
discharge between the shell and the static source
near the cannon. Repeat as necessary to extend
the distance the shells can travel, until such time
as the full path is ionized, allowing the final arc to
travel the entire distance, at which time you fire
your actual payload. |
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(And then run for cover before all the people who
had molten metal droplets and shell fragments
rain down on them show up with the lawyers) |
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Fire the first round from the railgun with a trailing
wire. Charge the wire to summon the lightning.
When it comes, fire the second round to ride the
lightning through the atmosphere. Easy peasy. |
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//If you could make backyard space cannons, Hitler
would have bombarded London from the relative
safety of Berlin, without going to the trouble of
bothering with pesky pulsejet V1's and all of those
hideously expensive V2's.// (Link). |
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Oh they tried, [spider] - they tried really hard. And yet with all of that work, plus Gerard Bull's traitorous HARP medalling as well, no one has yet come close to a space cannon of any sort. |
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"On 4 July 1944, the Misdroy gun was test-fired
with
8 rounds (one of the 1.8 m (5.9 ft) long shells
travelled 93 km (58 mi)). The gun burst during the
testing,[26] putting an end to the tests." |
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Which means it went well into the stratosphere ...
Interestingly, superguns intended as weapons are
fired at angles steeper than 45 degrees for
maximum range, which implies that the projectiles
pass through most of the earth's atmosphere (in
terms of quantity, not distance, obviously). |
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<HARP wiki link> "On November 18, 1966 the Yuma gun fired a 180 kg Martlet 2 projectile at 3,600 m/s (12,000 ft/s) sending it into space briefly and setting an altitude record of 180 km (590,000 ft; 110 mi); that world record still stands as of 2013." |
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So suborbital projectile launch is definitely possible, though likely quite difficult from one's backyard. As part of an orbital system I don't anticipate much success. |
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