Neutral Thrust calculation

[Sovnheim]

Hello everyone,

I've been poking around KSP for a few months now and the forum has been a great deal of help, both for serious problems and crazy contraptions ideas - so I hope you guys can help me on this

I'm trying to figure out how I could estimate at which throttle I need to put my engine so that it hovers gently above the ground.

Let me explain :

Usually my Mun landings are a bit chaotic as I tend to apply too much upward thrust to compensate the effect of gravity.

Usually I just screw around until things work out.

Any suggestions ?

Thanks in advance!

Edited May 3, 2014 by Sovnheim
Answered

[airbus a320]

With Engineer Redux you can read the current Thrust to Weight Ratio, if it's 1 (reasonably close to 1) the gravity force and the thrust are balanced assuming that you are thrusting in the vertical direction... well, reasonably balanced! If the engine on your lander/mothership/cruisemissile is too powerful it could be hard to find the exact throttle position and even a small error could lead to a big TWR variation.

Touchdown without "Unespected Spontaneus Disassembly"â„¢ shouldn't be too difficult (< ~10m/s down at the touchdown)

Edited May 2, 2014 by airbus a320

[Red Iron Crown]

Add an RCS system. After getting close to the surface and doing the big burn to scrub off speed, adjust the throttle so that your ship is gaining downward speed slowly and leave it there. When you are about to touchdown, tap the H key to use RCS thrust to slow you down. If you take too long to touchdown, you'll start climbing again due to the fuel expelled reducing mass. If that happens, adjust the throttle down and try again.

You can also use the other RCS translate controls (I, J, K, L) to eliminate any residual sideways motion.

This is not the most efficient descent or design, and when you get better at landing the RCS will not be necessary, but it is a helpful way to overcome the slow response of the throttle while still giving you a way to adjust your vertical speed.

[Streetwind]

Another idea:

While approaching the Mun, rightclick each of your engines (assuming you have more than one on your lander). On each of them, set the thrust limiter to the same number. For example, 60%. This means that your engine will have a lower maximum thrust even at full throttle.

Random Example: you have two LV-909 on your lander. Each outputs up to 50 kN thrust. Discovering that this is too much and leads to you having trouble landing, you now set the thrust limiter on both engines to 60%. As a result, your engines now have a maximum output of 30 kN only, even if your throttle setting is all the way up. You now have much greater fine control, because moving the throttle up or down causes much smaller differences in thrust. You can go even lower than 60% if it works for you.

And then, after you have landed and done your surface mission, you can set both engines back to 100% in order to have a swift and powerful launch

[Kasuha]

There's no throttle setting that will keep you hovering. As your engines are burning fuel, mass of your ship decreases and so does thrust necessary to keep you at constant height.

With a bit of practice, though, it is possible keep the ship hovering more or less at place, or moving at about constant height. It is not easy.

Constant TWR = 1 is not enough because if you get some vertical speed during the time when you did not match the TWR exactly enough, you need to kill that speed by deviating to the other side.

[FREEFALL1984]

Best to do this by eye, finding a constant throttle location is hard and it'll never stay constant for long since as others have mentioned as your fuel decreases you need to throttle back to stay stationary.

[GoatRider]

I fly model helicopters and quadrotors, and I have a lot of hovering practice. I find it very hard to hover a lander in KSP with the keyboard. But I plug in the controller from my RC flight sim, and my RC reflexes kick in and it's easy. So maybe you'll find it's easier if you use some sort of joystick, one with a throttle and twist rudder. You can make fine and large adjustments to the throttle very quickly with the throttle stick.

[Claw]

I do like airbus said and use a mod to show me the TWR. I prefer MechJeb because of the layout, but you can use either MJ or KER.

I use it to basically eyeball the throttle setting based off of the TWR. So if MJ says my craft currently has a TWR of 2.0ish, then about half way on the throttle gives me a TWR of 1 for hovering.

If I've overbuilt the engine and it has a high TWR, then I'll manually dial down the thrust limiter on the right-click tweakable menu. MJ automatically adjusts the TWR reading to match whatever you set for the tweakable.

So in this way you could also use the tweakable/MJ combo to always give you some comfortable TWR range on the throttle that is the same for each craft. If you always like a maximum TWR of 2.0, you can sort of set that in and work at the midrange of the throttle. I would be careful about setting it so that max throttle is near 1.0 TWR because that doesn't leave you any room to recover if you're coming in too fast.

[8MMW]

With Engineer Redux you can read the current Thrust to Weight Ratio, if it's 1 (reasonably close to 1) the gravity force and the thrust are balanced assuming that you are thrusting in the vertical direction... well, reasonably balanced! If the engine on your lander/mothership/cruisemissile is too powerful it could be hard to find the exact throttle position and even a small error could lead to a big TWR variation.

That's a good hin. Here are some other useful things:

- while constructing the lander in VAB select the celestial body (e.g. Mun) you're going to land on in Engineer Redux menu, it will adjust TWR display,

- as Red Iron Crown mentioned - on relatively small bodies (works on Mun, even better on Minmus) RCS is helpful to kill lateral velocity (if it's not too large) while keeping the lander vertical. That way your thrust is only vertical and easier to control. Tilting the lander to kill lateral velocity also decreases vertical thrust, and makes controlling it harder.

[Sirrobert]

I believe Mechjeb has a setting for this exact thing.

The translatron allows you to fill in a desired speed (horizontal or vertical). If you'd enter 0 in vertical speed, it'll make you hover at constant height.

However Mechjeb constantly calculates exactly how much thrust it needs, and can adjust this WAY more accurate than we can with shift and ctrl, so I doubt you could do it in stock.

Just keep the speed somewhere around 5m/s (downward) and you'll be fine

[ArcFurnace]

Often it's easier to get yourself to be slowly gaining speed or slowly losing speed, rather than maintaining a constant speed (which would require perfect balance). What I usually do on final approach is to use more-powerful burns to get down to the desired speed (5-10 m/s), then adjust the throttle until I'm very slowly losing downward speed (i.e. the acceleration from the rocket is just barely more powerful than the acceleration from gravity). Once my speed starts to get very low (say 2 m/s) I nudge the throttle downwards a tiny bit so that I'm now slowly gaining speed. You can keep slowly adjusting your descent speed up and down like this until you touch down; anything below 5 m/s is usually fine, although you might bounce a little. Be ready to use reaction-wheel torque to counter tipping if you're landing on a non-flat surface.

This is for the last few hundred meters of your descent; you can get away with substantially faster descent speeds until then so long as your acceleration is decent, so that you can brake quickly once you get near the desired height.

[cantab]

Watch the vertical speed dial next to the altimeter, and throttle gently up or down to keep that near zero. The speed on the navball can mislead when it's affected by horizontal speed. It can take a few moments to get in the right area to start with, but once done it's easier to keep near.

[Starwaster]

I believe Mechjeb has a setting for this exact thing.

The translatron allows you to fill in a desired speed (horizontal or vertical). If you'd enter 0 in vertical speed, it'll make you hover at constant height.

However Mechjeb constantly calculates exactly how much thrust it needs, and can adjust this WAY more accurate than we can with shift and ctrl, so I doubt you could do it in stock.

Just keep the speed somewhere around 5m/s (downward) and you'll be fine

Doesn't work very well anymore. Hasn't in awhile.

I remember a time I had to land a rescue ship on Duna and overshot by 10 km. MJ2's Translatron saved the day by auto-hovering my ship as you described as I gently tilted and guided it to the landing site.

If that happened today?

My Kerbals would have to walk to the rescue ship.

[draeath]

Just lock your orientation to retrograde and fiddle with the throttle. Be careful though, if your vertical velocity goes positive or close to zero your prograde/retrograde will wander and the lock will make your ship do Bad Things.

Once you're near a hover and you've eliminated lateral motion (this will happen eventually simply by virtue of burning retrograde to slow - you can speed this process by overshooting retrograde but you'll need more thrust to maintain the same vertical acceleration) you can lock to radial and tune with RCS, while your throttle will handle vertical velocity only.

You don't need MJ to do these locks if you've no problems with the pips on the navball and you're dextrous enough with the controls. A joystick or gamepad would be very helpful, there. Also, remember you can toggle fine controls, and you can use the docking RCS controls for things other than docking

Edited May 3, 2014 by draeath

[Sovnheim]

Thanks everyone, amazing advice !

I'll definitely keep a closer eye at my engineer redux window - I kind of never thought of using it on that as well!

I need to try to use the RCS ! I think it's a good way to improve my landings and explore the Mun. (Rovers are slooooooow)

[FREEFALL1984]

if the idea is to translate long distances along the surface, you're best off just using big sub orbital hops, since maintaining a constant altitude is painstaking and can cause huge amounts of grief as well as burning more fuel than a well timed suicide burn.

[John FX]

What I do is check my descent rate as I adjust throttle and note where the descent rate stays about the same (I do this when still a couple of clicks above the surface when I kill horizontal speed) then when I get closer to the ground I would kill my vertical speed and set my throttle near to the place I noted earlier. Due to using some fuel this will slow me down (slowly) instead of hovering. Then adjust manually, keep an eye on the numbers and landing should be a lot calmer.

Also there is this method for `less hectic` landing

[daniu]

I found IVA landings to be almost simpler than external view ones; this way you at least can't get confused with camera position.

The idea is to kill the horizontal speed and then look at the vertical speed indicator.

At 1000m, the scale changes and you can cut down and stay around 100m/s descent speed.

At 100m, the scale changes again and you should go to 10m/s (final landing speed).

Or different numbers depending on your engines and taste I guess.

I don't remember ever messing this up (although I guess you could have a problem with landing on steep inclines).

[Frank_G]

This method seems common among KSP players but it costs A LOT of deltaV you could spent on other maneuvers. A better way would be using a maneuver node to determine the amount of deltaV needed and then go for a more or less suicide burn. If you find this too risky you could also do the landing apollo style.

I recommend this video by Kosmo-Not http://youtu.be/zBa4c-YA3g8

[daniu]

This method seems common among KSP players but it costs A LOT of deltaV you could spent on other maneuvers.

Yeah I got it from a forum post as well.

But I don't think it wastes that much dV; the horizontal kill burn is pretty much (a little less) than the total suicide burn, the second burn is maybe 90dV, and the last one is a small 10s burn (with 100m descent at 10m/s) which therefore wastes 16.3m/s at Mun's gravity.

Not too high a price IMO considering I get a nearly riskless landing.

Although I've lately not been able to do it anymore anyway because I've only been shooting up unmanned Kethane miners.

[SpeedDaemon]

Hovering using the keyboard throttle is for masochists, and is very nearly impossible. You need a good analog throttle first of all.

I find it's easiest to use MechJeb's Smart A.S.S to lock the craft into any heading at 90deg pitch, then roll it so that the pitch and yaw controls make intuitive sense for you. That way you only need to worry about throttle, and can nudge it over in whatever direction you need. You have to constantly adjust the throttle down as you burn off fuel, and also have to increase a bit if you tilt over to move horizontally.

I usually use the same procedure for final descent when I'm landing.

[Joe32320]

A possible way is to use kOS, which allows you to script commands for your spacecraft, to regulate your throttle. You can design it so that kOS tries to keep your vertical speed at 0m/s.

You have to write the code for it to work(but that can be rewarding in of itself), and personally I have a much larger program that has many different throttle modes to take me from orbit to killing my horizontal velocity and the final descent, and I've coded the ability to unlock the throttle should I need to take over manual control when the computer goes awry.

It's obviously not for everyone, but kOS I find is great for allowing the computer to take over certain things like throttle control, allowing me to focus solely on steering the craft for example.

Link to the mod thread: http://forum.kerbalspaceprogram.com/threads/68089-0-23-5-kOS-Scriptable-Autopilot-System-v0-12-2014-5-1