# Reaction Wheel Kinetic Launchers

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Here's my idea:

Build a huge wheel-like structure with a ton of reaction wheels. Connect a spacecraft at the edge of the wheel. Start spinning the wheel as fast as you can.

Undock the spacecraft. Off it goes into the unknown.

How much dV could you get?

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How to you make sure you release at the right time? I would love to try this just for laughs but I can see me making a huge mess.

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Since there's the angular velocity limit in the game you can put an maximum bound of 50*r, with r being the radius of your wheel.

The next limit would be the tensile strength of the wheel (or beam, a beam might be more structurally efficient) and rocket, which would probably come into effect well below 50radians/second.

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Just did a quick test with 10 of the largest reaction wheels and a Mk1 command pod way out on struts.

Was at 80/80 Ap/Pe, released in a roughly prograde direction while at Pe. Ap went up to 160.

Not great, but it's not nothing.

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Looks like you can get about a km/s if you're into extreme engineering.

You could possibly create something akin to what @Stratzenblitz75 used in his Duna-and-back-with-electricity-and-Xenon video, albeit with the ion engines on the beam replaced with reaction wheel.

Trick will be making sure the wheel/beam doesn't explode from shift in balance when the rocket/projectile/whatever decouples. Also getting it into space might be hard if you don't want it to disappear by the time you get back to it. That too.

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Posted (edited)

You can actually get quite a boost from a centripetal launcher... Given that your reaction time is fast enough and your launcher can keep itself from tearing apart.

As mentioned by EpicSpaceTroll, tangential velocity is calculated from ω*r, where ω is the angular velocity and r is the radius. However, there is another factor to consider; centripetal acceleration, which is calculated from  ω2*r.

As you can see for a fixed radius, tangential velocity depends depends on ω, while tangential acceleration depends on ω2. This means that as you double your tangential velocity, your centripetal acceleration will quadruple.

So, what does this mean for KSP? Lets look at an example.

Lets say you want to build a centripetal launcher to put launch an object from the surface of minmus into orbit. Lets say you choose your launcher to have a radius of 10 meters.

Since minmus orbital velocity is ~160 m/s, you would have to achieve an angular velocity of 16 rad/s (150 RPM). This would induce an acceleration of  2560 m/s2 or 261 Gs.

Okay, this is probably not viable. Even ignoring the G force, the high rotation rate would make it very difficult to time the release.

Lets shoot for a more reasonable rotation rate then. Say we want 2 rad/s. In this case, we would need a radius of 80 meters. A launcher of this size would induce 320 m/s2 or 32 Gs of acceleration. Something like this is totally within the bounds of human reaction time, and KSP's auto-strut strength.

However, with a radius this large, the launcher's moment of inertia becomes very large as well. Even if you use a lot of reaction wheels, you'll be waiting a long time for this thing to get up to speed.

Another electric power source you can use are wheels. If you are launching from the minmus flats, you can feasibly use wheel power. The bugged differential steering on the large rover wheels could be the perfect power source.

While a cool idea, for most cases a centripetal launcher is likely not a good option. Rocket engines are far less complicated and are much lighter.

That said, screw practically! Centripetal launchers are a fun engineering challenge to build and are an excellent physics demonstration. I encourage anyone who is interested to give it a try!

Edited by Stratzenblitz75
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When the almighty StrazenBlitz posts...

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16 hours ago, FrostedShoe said:

How to you make sure you release at the right time? I would love to try this just for laughs but I can see me making a huge mess.

kOS script would work fine.

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On 1/3/2018 at 10:39 AM, Stratzenblitz75 said:

Rocket engines are far less complicated

Gotta love that this game makes statements like that take place and make sense.

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Some RL kerbals are on it.

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Posted (edited)

Edited by DoctorDavinci