Operation Plumbbob underground nuclear tests in 1957 launched one ton shaft lid straight up at 5x escape velocity. Was there any serious exploration of doing that on purpose to launch a satellite after Sputnik?

[u/pakled_guy]

[u/Downtown-Act-590]

What a fun question! Short answer is sadly no, but I hope that longer answer will be more inspiring. During Operation Plumbbop, the engineers accidentally created nothing less than a space gun. We will now start with a long general intro on space guns and come back to Operation Plumbbob later.

Space guns have been a fascinating option for a long time. You maybe also read From Earth to the Moon by Jules Verne as a child. I did and it had a lasting impact. We have been investigating the idea for hundreds of years. And such guns have their advantages, they are potentially really cheap and easy to use compared to a rocket.

But they also have three very large problems. The most obvious one is heating. In contrast to a rocket which tends to fly through the lower atmosphere relatively slowly and speeds up later, space gun projectile does the complete opposite. As heating rate of a projectile varies with cube of its velocity, the heating loads get insane (please refer to [1] for derivation and further read on this).

Second issue is not completely intuitive. Space guns can't launch things into stable closed orbits, because such orbit would always intersect the Earth as it has to (neglecting drag) pass through the launch point. This is a problem already Isaac Newton thought about (and unrealistically hoped he can crack it by launching from mountain tops [2]). The only solution is to give some sort of propulsion system to the projectile to give it the necessary change in velocity to modify the orbit.

And this gets us to a third problem, the acceleration loads in the barrel are very, very high even when the barrel is really long. That wouldn't matter that much per se, but you are trying to fit in a sophisticated satellite with some form of a rocket propulsion system on board to make the necessary corrections so you don't hit Earth as previously mentioned. That makes it hard.

Now notice that "not being able to shoot a projectile fast enough" isn't really on the problem list. You can't really do it with conventional powder gun. As [3] explains, when projectile velocity starts nearing 3 km/s, almost all the combustion energy in the burn products is used to propel these burn products down the barrel themselves and little is left for the projectile. Project HARP space guns firing at well above 2 km/s in the 1960s really pushed the limits of this technology, but they would need to fire multiple times faster to at least reach the orbital velocities. Neverthless, since the early 1950s, we have something better, a so-called two-stage light-gas gun.

In a two-stage light-gas gun, we use a propellant-driven piston to transfer the combustion energy to a helium column through compression. This helium column is then used to fire the projectile and has much easier time following it down the barrel due to its low molar mass. Suddenly really high speeds are possible!

According to [3] in 1957, US had a light-gas gun capable of firing around 8 km/s and there was clearly a potential for more. Ultimately, this technology was even tested in the space launch area during the 1980s/90s, because of the project SHARP. Some of the results were promising, but funding dried up in the post-Cold War era.

So the long introduction is over, now back to the project Plumbbob. As you may know, the manhole cover very likely evaporated due to excessive heating. The closed orbit and acceleration issues are still there. There would also be great technical problems connected to firing a nuclear gun into a desired direction and it would be a long-term useless, extremely expensive project. In 1957, the US had launch vehicles capable of this job already, they just needed very slight tuning. Moreover, even in the space gun area, the light-gas guns were a much more promising technology already at the point.

Did somebody sketch it on a napkin in panic after Sputnik? Perhaps. Do we have information about this being seriously investigated? No, we don't.

[1] Introduction to Flight, J. D. Anderson, Chapter 8

[2] Treatise of the System of the World, I. Newton

[3] Light-Gas Gun Technology: A Historical Perspective, H.F. Swift

[u/pakled_guy → u/Downtown-Act-590]

Outstandingly informative. Thank you so much for writing it up!