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In Could any existing gun reach the Karman Line?, Richard Graf's Kickstarter campaign for his Starfire space-gun is referenced & discussed. Setting aside my native skepticism for a project with such scanty technical details, I have another problem: anytime I hear the word "suborbital," the words that follow come out "blah blah blah."

Let's face it, most useful science that can be accomplished through suborbital flights has been done years ago, and, unless you're going to make your space-gun many, many kilometers long, it's not going to be suitable for space tourism. From John Hunter's presentation this past summer at the Starship Congress (starting at 37:40), we hear that you cannot fire a projectile high enough for it to not eventually come down somewhere. So, if space-gun technology is ever to reach LEO, a boost near the top of the projectile's parabola needs to be employed.

Let's assume two things to make it simple: 1) The max G-force sustained by the booster/projectile will be 10,000 Gs (Graf's Starfire claim), 2) To reduce moving parts, let's make it a solid fuel rocket. Can such a booster be designed to strap on the back of our projectile with current technology?

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US Battleships have rocket assist shells that extend range to 50 miles. I believe gun tube accelerations are in the 10,000 G range. –  C. Towne Springer Feb 16 at 18:00

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I'd expect so. You should be able to design and test a solid rocket to take those G's. The tricky part is to design a structure that keeps the grain from trying to detach. It would just take money, time, and lots of testing. The rocket motor should be facing backwards so the propellant is pushed back into the casing where it's bonded. Interestingly, that is the correct direction if the projectile is spin stabilized. By the time it gets to the other side of the planet it will be pointing the right way to circularize the orbit.

There are already electronics that can survive those and higher accelerations, e.g. 30,000 G's. The applications are, of course, for artillery. There is even work on guidance systems that can survive those G's. It's not that hard to design structures to take those G's. It just takes mass, and a lot of testing to find the weak points.

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Also relevant are planetary penetrators, e.g. these ones for the proposed UK Space's MoonLITE mission and the work on them still continues even if the project was canceled. They are hollow and with space for electronics that are protected in IIRC epoxy and tested by firing them from a cannon into a sand heap. And they apparently survive that without a problem. –  Noordung Feb 12 at 18:13
    
@MarkAdler: are you suggesting that the projectile/booster should be loaded into the cannon backwards? Not that that's a problem if you cap the nozzle with a disposable nosecone. –  Jerard Puckett Feb 12 at 22:23
    
Yes, the orbit circularization motor should be mounted with its thrust vector pointing forward. –  Mark Adler Feb 12 at 22:28

In general, all proposals I have ever seen require a kick motor to circularize the orbit, and they all assume solid rockets because of the G loading. A launch gun, can never launch to orbit, without something to provide that kick, into an actual orbit.

It is very hard to imagine a liquid fueled engine surviving that kind of load. Heck it is hard to imagine any structure surviving 10K G's well.

There are some interesting new low thrust designs out there, that are MEMS or 3-D printed with the goal of being able to use non-toxic propellants. Having very few moving parts make it more likely this might be able to survive. (Things scale funny between micro and macro levels. Consider the case of ants ability to lift things, compared to their mass, but would crush them at a larger scale).

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