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Problem with decoupler ejection force units?


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I just restarted career mode, and in launching my first satellites, I was making them Sputnik-style with no engines on the satellite and just sticking them on top of a decoupler at the top of my rocket. After a couple launches, I was curious about how I could get a circular orbit using a decoupler because it always fires my satellite off into an elliptical orbit once my final stage circularizes at apoapsis.

In the VAB, the TR-18A decouplers (the first decoupler you get) say they have an 'ejection force' of 250. I went hunting around and found that the ejection force is in kilonewtons, or 250,000 newtons. However, my satellite (a Stayputnik and 3 antennas) weighs 0.065 according to the game, or 65 kilograms, so an ejection force of 250,000 newtons would be able to add 3,846.2 m/s of delta-v (250,000 kg*m/s divided by 65 kg = 3,846.2 m/s) or enough to escape the solar system from low Kerbin orbit.

After a bit of experimenting, I figured out that the decoupler adds 38.5 m/s of delta-v, or 2,500 newtons (2,500 kg*m/s divided by 65 kg = 38.462 m/s). This means that the ejection force of the decoupler is in decanewtons, a somewhat awkward unit and one that the game never mentions.

I'd like to ask that Squad double-check the decouplers so that everything lines up. This could mean the ejection force is now written as 2.5 kN with the current units but a corrected number, 250 daN as it is now but with the proper units, or 2500 N with the base unit and a corrected number.

It's just a bit confusing when the units don't line up with what they say they are. This could especially be a problem with the KerbalEdu program, because while I haven't used it, I'd imagine it's much more math-intensive and correct units become much more important in the classroom.

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A Newton is a kg*m/s^2, not kg*m/s. You cannot calculate delta-v from just a force, you also need the forces duration. In your calculation you accidentally assumed it applied it for 1 second.

So I guess you have actually found out that the decoupler applies 250,000 newtons for 0.01 seconds.

250,000m/s^2 * 0.01s / 65kg = 38.46m/s.

They ought to put the force duration on the decoupler though.

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Ok, I'm probably wrong then. If you can go into more detail though, how does the duration matter in this case? The force is going to apply 250,000 newtons, so how does shortening the duration affect how much of that is applied? Decouplers in KSP pretty much provide an instantaneous change in velocity (or 0.01 seconds if you're right), so how does it end up applying only 1/100th of the potential 250,000 newton force contained in the decoupler? I'm probably not phrasing that the best, let me know if I should clarify.

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Because FΔt = mΔv. Force is essentially change in momentum over time, the longer you apply a force the more velocity you get out of it. This is because the acceleration caused by the force occurs over a longer period, and Δv=aΔt.

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