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Ideal value RCS TWR


domendemo
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5 hours ago, domendemo said:

Talking abou the engine TWR, we are looking for 1.5-2.0

We are? Some people suggest those values for launchpad TWR (I suggest SRBs :cool:) but that is far from ideal values in different situations.

For RCS thrusters I doubt there is many player that do something different that the standard procedure (stick some and, as long it don't run into part limits, go with this). As @bewing point out, those thruster are for precise maneuver. You probably  don't want velocity adjustment of the order of mm/sas he suggested TWR value would imply but yeah, the idea is what he wanted to pass with that hyperbole.

 

 

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I'll try to put some numbers to it and see if that helps. I normally use RCS for one of two things: docking and fine adjustments to maneuvers. In both situations, it's pretty rare that I want to make an adjustment of more than 1 m/s. Doing that over the course of say 10 seconds would be good, so that gives an acceleration of 0.1 m/s per second. In terms of TWR, divide by 9.8 and you get around 0.01. That 10 seconds is pretty arbitrary, so really anything with a TWR of say 0.001 to 0.1 would probably be tolerable.

 

A single RCS thruster block gives 1 kN of thrust. F=ma --> a = F/m so 1 kN will give 1 m/s per second to a 1 ton spacecraft. Normally I mount my RCS blocks using 4-way symmetry and I assume most others do. That would give 4 kN for movement forward/backward. So 4 RCS blocks would give 0.1 m/s for a 40 ton spaceship, 1 m/s for a 4 ton craft, and 0.01 m/s at 400 tons. At the lower mass end, you might not need RCS at all, or you can use fine control to lower the thrust. At the high mass limit, you might consider using Vernor Engines instead. They use LF/Ox instead of Monoprop, and they only thrust in one direction, but they provide 12 kN each. Anyway, that 4-400 ton range probably covers the vast majority of spacecraft people use for docking or orbital maneuvers, so "just slap on some RCS blocks and go with it" works fine in most cases.

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On 10/10/2020 at 10:21 PM, domendemo said:

Talking abou the engine TWR, we are looking for 1.5-2.0

What about the ideal value for RCS TWR?

There is no ideal value.  Generally, the smaller the better, for two reasons:

  • Because a higher value doesn't help you
  • Because a lower value is more efficient

Why a higher TWR doesn't help you for RCS:

If you're using RCS, it's generally because you're in orbit and trying to match up position for docking.  What a higher TWR would give you is that you can accelerate faster with it.  There's zero need for that in orbit, because you've got all the time in the world.  A low TWR just means you dock more slowly-- which is fine.  Bear in mind that you generally only need a few centimeters per second of nudge from RCS, so even a really massive ship can work just fine with a few RCS thrusters.

Why a lower value is more efficient:

Because RCS thrusters have mass.  That's dead weight that you're hauling up from the surface, which eats into your dV.  If you've got a high TWR with your RCS, that means you're carrying more thrusters than you need, meaning you're wasting mass on unnecessary equipment.

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The reason why i am asking this is because i found that if the value is too low, i need to have patient when doing docking as i need to hold the H or N key very long in order to decrease just m/s. I believe it is because of the mass of my vessel is relatively high.

 

 

 

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11 hours ago, domendemo said:

The reason why i am asking this is because i found that if the value is too low, i need to have patient when doing docking as i need to hold the H or N key very long in order to decrease just m/s. I believe it is because of the mass of my vessel is relatively high.

I hear ya, @domendemo. Docking is an exercise in patience, no doubt. It's common to want to speed up when you have a fairly large distance to cover during the approach phase but, of course, you encounter the "where are the brakes on this thing?!?" sinking feeling as you realize your RCS system is no match for the speed component of inertia !!

I've found that having an accurate readout of distance to closest approach and time to closest approach (like the Kerbal Engineer Redux mod can give you) is the best way to handle those loooooong approaches - rather than increase the velocity of your vessel, and knowing that after your initial RCS burn will place you to within, say 100m of your target... in 4 or 5 minutes or longer... then speed up the simulation (physics warp) while monitoring how much time is left to go (with the "." key). It really works best when you know how much time you have to go as well as how far apart you'll be when you're at your closest approach.

If you give it a whirl, you may find that RCS TWR is usually not an issue at all.

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12 hours ago, domendemo said:

The reason why i am asking this is because i found that if the value is too low, i need to have patient when doing docking as i need to hold the H or N key very long in order to decrease just m/s. I believe it is because of the mass of my vessel is relatively high.

 

 

 

If you ever find your RCS TWR too high you can activate caps locks to enable fine and precise attitude control. 

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