Air Hockey on the Moon (TFN)

Theoretical Future News

Anchorage Daily News March 30, 2030

An artist’s render of what the Lunar Launch System might look like

An Anchorage based start up has a unique idea for expanding the potential of the International Lunar Research Station. Alaska Lunar Launch Systems (ALLS) wants to build a giant air hockey table on the moon. Current development projects on the moon are limited by their ability to get materials off the moon. ALLS is developing designs for a launch system that works like a giant air hockey table.  Jets of high pressure air will push a launch sled and its cargo along a trackway. According to their proposal to the International Space Development Partnership, ALLS launch trackway will be able to launch cargo from the Moon’s surface and into Lunar orbit without the need of engines. This initial boost is intended to reduce how much fuel is needed to get Lunar Resources to where they are needed

While larger research initiatives are looking to develop electromagnetic rail launch systems to decrease the cost of transporting materials off the surface, ALLS believes their approach can be built and scaled far faster.

“When you grow up in rural Alaska you get used to the idea that not all of the parts are going to be at the hardware store, so you get good at making a good enough solution and sometimes that good enough solution is what you needed all along. Our system can be made almost entirely on-site with locally available fabrication facilities available at the ILRS (International Lunar Research Station). I don’t want to knock rail guns too much, they are cool and sci-fi, but they have a lot of fabrication complexity which means more parts gotta get brought in from outside the gravity well.”-CEO Milley Lange

When we asked Mrs. Lange as to how their launch system would find enough oxygen to meet their system’s needs, she was incredibly transparent.

“Oxygen is incredibly plentiful on the Moon, while it isn’t free over 40% of lunar regolith is made up of oxygen. For every ton of ore refined you get some surplus oxygen. Our platform won’t be perfect for all uses, though we do believe that for safely launching high value cargo we are a strong near-term solution for lunar development. At some point economics will change, magnetic launch systems will get cheap enough to import, or ice deposits end up being larger than we currently believe, JAXA’s lunar elevator project gets properly funded, but for the moment those are what ifs. Our technology is being prepared for testing on an upcoming Lunar Expedition later this year or early 2031, the rail gun teams are still at least 4 years away from testing.”

Back to the present for some technical discussions

For a spacecraft to leave the lunar surface and enter orbit the spaceship needs to be going fast enough to achieve what is referred to as escape velocity. Lunar escape velocity is 2380 m/s or about 7 times faster than the speed of sound here on Earth. The question for today will be, what are some of the rough requirements of our Lunar Launch System (LLS) so that we can help get a spaceship/payload into orbit around the moon?

To keep our lives easier (aka Obie aka the author, has no clue how to do some of this stuff) we will assume that Lunar oxygen is relatively cheap and plentiful. We are also going to want to decide what the target velocity of our spacecraft should be. For our first run let’s imagine that our launch system will be the only thing accelerating the spacecraft up to escape velocity. Speaking of acceleration, how much acceleration can our spaceship take when being pushed by the LLS? Whichever acceleration we choose for the trackway will impact what kind of cargo we can get into orbit. A lower acceleration means that we can launch more types of cargo into orbit. When the space shuttle launched crews into orbit the onboard computers limited acceleration to never exceed 3 gravities to protect crew members from injury. Let’s start with 3gs as a maximum acceleration and see what that does to our design and go from there.

Now we need to decide what shape the trackway should use to get our spaceship up to escape velocity. Do we make it circular like a NASCAR track or a straightaway? A circular track has a certain amount of appeal, you can imagine that your track segments might reduce how much trackway you need to get up to escape velocity. Better yet a circular trackway means that your spaceship can be launched into almost any direction, which should be useful for mission planners.  A linear track is a bit more boring but has the advantage of being super simple. We must ask, are the assumptions I made about the two track shapes accurate, is one actually better than the other, or are they roughly equal in performance?

We want to look at the math that defines our two track shapes. Both tracks are trying to get a spacecraft to escape velocity. The straight track’s size is defined by the equations for velocity and distance overtime for a constant acceleration.

To make our lives easier we can replace the variable time with the relationship between escape velocity and the spacecraft’s maximum acceleration.

The circular track size is partially determined by the equation for centripetal acceleration.

The reason we are concerned about the centripetal acceleration for the circular trackway is because our spacecraft will be experiencing the same forward acceleration on both the circular track and linear track and so that math doesn’t need to be done twice.

For both a circular track and a linear track our spacecraft would need to be accelerating for over 96 kilometers, or a bit shy of 60 miles for Americans.

While both tracks need the spacecraft to cover 96.3 km to get to escape velocity, maybe our circular track will let our spacecraft go over the same section of trackway multiple times before launching.

Well darn.  The math shows us that our circular trackway would have a diameter 4 times larger than the length of the linear trackway, which means the actual size of the circular trackway would need more than 12x as much track length to achieve the same escape velocity. This doesn’t mean that the circular concept doesn’t have some merits, but they are limited to scenarios where you can build a very large quantity of trackway pieces.

One thing that should be remembered is that this math applies equally to the air hockey table idea and a more sci-fi sounding mag lev trackway.

If building a 60-mile-long air hockey table sounds daunting, you are not alone. The largest human built project in space is the International Space Station, which can fit inside of a football field. How can reasonably build something much larger than the ISS and do it all affordably?

Well you’ll have to stay tuned for a follow up article, namely because this post is already on long end and I don’t want to lose folks even more than I already have.

TL:DR If you want to build a special track to accelerate a spacecraft to escape velocity on the moon(2.38 km/s), while keeping it human safe (max acceleration =3 g or 29.4 m/s^2), it would need to be almost 60 miles long (96.333.4 km).

Please let me know if you have any questions or comments. I will try to be faster at rolling out follow up details.