Oddly, the article fails to mention the UK government attempt to railroad into prison the directors of the UK company asked to make the pipe components, by covering up the fact they had approved the export using 'public interest immunity' certificates to hide evidence from the courts.
Just a couple of days ago the local TV was re-running an episode of the "Yes Minister" documentary on that topic.
Actually, the TV show was released before the events you mention, so the details don't match exactly, but it's close enough for government work, so to speak.
A powder gun like the one used in Harp and a gas gun like Sharp both work in the same basic way – expanding gases. The lighter the gases molecular weight, the faster it expands in air. Gunpowder has a molecular weight of 22, slightly less than air at around 28, but ignited gunpowder is incredibly hot, so that’s why it expands so quickly. The hydrogen used in gas guns, however, has a molecular weight of two so expands extraordinarily quickly in air.
I couldn't understand from the article how the hydrogen was being used in a "gas gun". Since pure hydrogen doesn't ignite, wouldn't we care about the molecular weight of the oxidizer as well?
It turns out the "light gas" is important just for it's low molecular molecular weight. It's never ignited, and one could substitute Helium for Hydrogen if one wanted. The gas is a second stage in addition to, rather than instead of the gunpowder. It's like a spring-air gun, but instead of spring you use an explosive charge.
I love the twist of submerging almost the entire barrel in the ocean, pretty awesome to wrap your brain around how that could actually work out. It was also really interesting to hear Hunter's thoughts that even if the physics all works and launch costs could be cut 1/10th, how it would still be unworkable for someone SpaceX to invest in the R&D alongside their reusable rockets.
It would be interesting to see a quantitative head-to-head comparison, but my guess would be that once you have the technology to retrieve first-stage rocket engines after launch space-guns aren't economical- the fuel used lifting the engine itself may be expensive, but the energy losses from hitting sea-level atmosphere at ~8km/h would have to be astronomical.
The cost was roughly $1,727 per kilogram, adjusting for inflation. By comparison, Nasa estimates that it costs $22,000 per kilogram to launch a modern satellite into orbit using conventional rockets.
Does it, though? I mean, I've played KSP, I know the rocket equation. But with a space gun, your vehicle only winds up moving as fast as the propellant pushing it out- so that mass of fuel (more explosive than fuel, I guess) still has to be accelerated to near-orbital speeds in order for the gun to work.
Shows up in Michael Flynn's "Firestar" series, as well. "The Boomer" is what they call it, I think. It was in the side of a mountain in South America, and it was filled with gas.
With all the talk of Musk's Hyperloop, now I'm curious if the barrel was evacuated (if that's worth it).
Even though this wouldn't be appropriate for use for delicate satellites, I wonder if this would be a critical piece of equipment for industrialized space - raw materials like fuel and oxygen and structural materials. Unfortunately full satellites would be a challenge since googling reveals that experiments involving these things included 10,000 gees of acceleration.
The main problem with having an air-filled barrel is that when the projectile is going faster than the speed of sound, it is compressing all the air in front it, all the way to the end of the barrel. Once the air can no longer get out of the way, it is along for the ride (until outside the confined spaces of the barrel). Carrying 100m worth of air will heat up the barrel and projectile (as the air is being compressed), as well as adding a great deal of friction.
I'm not sure how/if it scales, but here's an article about different lengths of 5.56mm barrels, with graphs of pressure and velocity: http://www.sadefensejournal.com/wp/?p=1093
In reference to the second graph on the page, mach 1 is apparently 1125.33 fps
Do we know if the space guns are rifled or smooth bore?
Smooth bore as far as I know. Even tank cannons are mostly smooth-bore these days.
The problem with those graphs is that they are not optimizing propellant quantity for each barrel length. Adding an axis for quantity of propellant shows that you get diminishing returns to increasing barrel length and propellant, but there is a limit to how much you can increase both without increasing the barrel's thickness (as well as the firing mechanism's strength).
You also have to remember that a gun will require > Mach 25 to get to orbit, and air acts differently at that kind of speed (, and once past ~Mach 3).
Thanks for the insight - I guess it doesn't scale!
Is there a reason the space gun would be at a 45 degree angle and not pointed vertically? I know the reason rocket launches happen as close to the equator as possible, but they still go straight upwards...
Rockets only go up for a little while; most of the propellant is used to build up speed in the 'horizontal direction'. There have been some arguments that a strictly vertically-firing rocket would be best for going outside of Earth orbit, but the current strategy for all launches is to go up until outside the thickest part of the atmosphere, then go horizontal, to build up to orbital velocity.
After reading about the end of of Project HARP - it seems like they intended to use the gun to fire a rocket, and after a certain altitude the engine would ignite to get into space.
Yes, and in the years since HARP, there have been a few rocket-assisted projectiles developed (though none which have been intended for space launch).[1]
I should also mention that unless the projectile was fired at or above escape velocity (intended to leave the Earth's gravity well), it would require some onboard propulsion to 'circularize' its orbit (and avoid crashing back into the Earth on its first time around).
Rifling improves accuracy at the cost of speed (and thus range). That's great for traditional guns, where you care more about effective range (i.e. a reasonable chance of hitting your target) than how far away you can send the bullet; but orbit is a big target and space gun projectiles would have their own rockets to guide them, so there'd be no benefit to rifling there.
(As nickff points out, modern tank cannons are generally smoothbore; the current fashion for armor-piercing is solid kinetic darts that need to go very, very fast to punch through their target, and rely on stabilizing fins for accuracy.)
Biggest problem of rifling is that the "lands" wear down quickly. For a pistol it's probably good thing that it wears down before it has time to break in other ways. For tank gun, you can use some other, more accurate way to keep track of shots fired.
Probably second biggest down sides of rifling is that it weakens the barrel. How much depends a lot on how the rifling is done. But anyhow this has to be compensated with thicker/stronger barrel.
But that gets problematic at extreme pressures. You wan't to use pressure vessel steel, because such steels 1. have good fatigue strength and 2. they tend to send around little less shrapnel when they fail. Problem is that such steels are typically low alloy steels with somewhat low yield strengths. So you would like to make thicker barrel, but the problem here is that you quickly hit diminishing returns. There is usually very little point making barrel thicker if the outer diameter is twice the inner diameter. (Except if you need more rigidity to improve accuracy. But that's whole another subject.)
I would be curious about any potential differences between the two once the projectile leaves the barrel. Would all that super-heated air being shoved out ahead of the projectile make a difference in the shock of it leaving the barrel?
There would be an incredibly loud sonic boom when the projectile left the unevacuated barrel, but since the projectile is going faster than the speed of sound, it would not benefit from the pressure wave; in fact, it would probably slow down, and possibly be destabilized when it penetrated the thick layer of (possibly plasma-phase) air. It is also likely that when the pressure wave left the barrel, the entire barrel would shake itself to pieces (unless it had some way of damping the axial waves going back and forth through the barrel, akin to a 'water-hammer' effect).
evacuating barrel will help reach the speed. Evacuation is very easy to do, yet helps tremendously.
The gun can be built somewhere in the Andes, at 4 km altitude. It can be pretty long there. Plus using either ablative head - like new Russian-Indian air missiles - or small forward jets - similar to Russian 200miles/h torpedo - will help to decrease atmospheric speed losses.
"On November 18, 1966 the Yuma gun fired a 400 lb (180 kg) Martlet 2 projectile at 7,000 ft/s (2,100 m/s)[1] sending it briefly into space and setting an altitude record of 180 km (590,000 ft; 110 mi);"
That means that it was going at least 5 Mach after reaching 30km altitude. Doesn't looks like such a big loss of speed in the atmosphere - only about 1 Mach due to atmosphere as the other 0.5 Mach - due to gravitation.
I love the tone of the article. It struggles to assert that although the gun could be used as a weapon, it had more potential as something to launch a satellite.
You would have to be pretty naive to think the Iraqi's, who always wanted to be the dominant Persian Gulf state in the region, would actually use this for its intended purpose and not weaponize it to intimidate its neighbors in the region.
Doesn't seem like any practical way it could be used as a weapon - can't aim and too easy to destroy. Hussein was well aware of these kinds of limitations - witness e.g. his mobile SCUD launchers.
Guns, unlike satellite launchers, need to be aimed at enemies in order to be useful- something hard to do if its built into a hill.
I mean, of course the Iraqi military (like everyone else) thought about it as a weapon first, but if I were Bull I'd have been happy to con them out of spending the same money on more effective weapons.
it is also helps to be a dictator when it comes to big projects - no need to convince/pitch to investors and various levels of government and elected reps, public opinion, obtaining permits, etc... :) You just do it.
With a mostly static target region (London, Regions of Iran), steering capable projectiles and lots of resources to build multiple of these that immobility is not really an issue.
University of Washington's Ram Accelerator (RAMAC) (https://en.wikipedia.org/wiki/Ram_accelerator) is what I think will ultimately win. It's basically a long tube with mixtures of natural gas, designed to work like a ramjet.
https://en.wikipedia.org/wiki/Arms-to-Iraq