how do I land a booster ;ike the falcon 9? i

[moguy16]

i already know about FMRS and how to use it, but when i try to land a booster, it flips over head-first and no matter what i do i can't recover it, also how do you balance the fuel/twr ratio?

can i see some of your designs for inspiration?

[Linkageless]

I don't use FMRS, but have a couple of SSTO boosters I use (so not quite falcon9 style, as they come back from orbit).

When designing, I checked the CoL was high enough and CoM when nearly empty was low enough so it wouldn't flip during reentry.  Part of this was dropping the priority of the lowest tanks so that it burned the low down fuel last (ie the fuel you'll be using for your suicide burn).

To help me out, I have an array of airbrakes, usually only for braking but some I set for pitch or yaw.  For when I'm feeling lazy, I have an array of drogue and main chutes sufficient to land at a reasonable speed, but they land nicely on a few of their vectors, leaving the rest of the engines shut down.

Something worth noting, I'd restricted the gimbal to about 9/100 on vectors and 50/100 on mammoths.  The full range gimbal on those contributed to some terrible instability and oscillations during testing which was solved by this without severely compromising the ability to target KSC.

Here's my large 'Oak' launcher (without payload):

Edited January 9, 2020 by Linkageless
Inclusion of image

[AHHans]

There is a scenario "propulsive landing" or so, in which you are asked to land a booster. (Which has been inspired by SpaceX. The main problem in the scenario being that you have very little fuel.)

[...]

O.K. @Linkageless has already answered in the time I needed to make sure that Jeb doesn't miss Jool. The one thing I still want to add is that don't rely too much on the CoL as displayed in the editor: when you are high and fast (e.g. when re-entering from a (sub-)orbital flight) then the drag can dominate the aerodynamic forces. And if you have draggy parts forward of your CoL, then that can cause unwanted surprises.

[Linkageless]

13 minutes ago, AHHans said:

don't rely too much on the CoL as displayed in the editor: when you are high and fast (e.g. when re-entering from a (sub-)orbital flight) then the drag can dominate the aerodynamic forces. And if you have draggy parts forward of your CoL, then that can cause unwanted surprises.

Too true, hence I started adding airbrakes for stability.  I have them wired to the brakes action group, and only enable the pitch/yaw when I think I need it.  I've not tried pairing it down, but the 'oak' is over-engineered.  Due to it's size and excess of engines at the blunter end I reckon you could do completely without the airbrakes.  There's also the matter of the 4x large reaction wheels offset inside.

I'm now feeling tempted to start stripping it down to produce a more efficient version, but then again, meh, it works!

 

2 hours ago, moguy16 said:

also how do you balance the fuel/twr ratio?

I aim for a TWR at liftoff of 1.4-1.5.  On this, reentry is closer to 5 TWR, hence I only usually need 4 of the 8 vectors and both mammoths shut down.

[moguy16]

1 hour ago, Linkageless said:

Too true, hence I started adding airbrakes for stability

where exactly do you put the air brakes at the top, the bottom or the middle? 

[Rover 6428]

Download Kerbal Engineer and, when landing, open the menu that says "time untill suicide burn" and then full throttle when it hits ~0s. For stability, just flood as much fuel into the bottom, or set fuel priorities so that most of the fuel remains at the bottom, so that it won't be flip happy. You could also incorporate reverse fins for spability.

Edited January 9, 2020 by Rover 6428

[bewing]

1 hour ago, moguy16 said:

where exactly do you put the air brakes at the top, the bottom or the middle? 

To make something aerodynamically stable in a particular direction, you need the CoM at the front, and the drag at the back. So if you want it to fly tail first through atmosphere, you want the drag at the very "top".

[Geonovast]

1 hour ago, Rover 6428 said:

"time untill suicide burn"

Bearing in mind that the suicide burn countdown is incredibly inaccurate in an atmosphere.  I usually hit throttle at -2 to -3 seconds and still come to a stop too early most of the time.

[Rover 6428]

9 hours ago, Geonovast said:

Bearing in mind that the suicide burn countdown is incredibly inaccurate in an atmosphere.  I usually hit throttle at -2 to -3 seconds and still come to a stop too early most of the time.

thats why I said ~0s and not just 0s.

[RizzoTheRat]

I use SSTO boosters and fly them back from orbit so a little different than Falcon but the principal is the same. 

I find with little fuel and the engines at the bottom I've never had an issue with them flipping.  The biggest problem I have is on a standard ballistic trajectory you need to rotate pretty quick as you kill the horizontal velocity. 

To solve this I've based mine on New Glenn rather than Falcon.  I have small wings near the bottom, and fins further up, that are mounted backwards and disabled for launch.  On re-entry I use the now active fins for steering and combined with the wing and body lift I fly a vertical S shape, pitching the engine end up to bleed off horizontal speed while limiting vertical speed, and then pitching down hard so that I get a much more vertical trajectory than I would otherwise.  Airbrakes, mounted at the top, come on when I pitch down and slow it to something like 200m/s, so I need very little dV for the landing.

However not doing a high altitude braking burn like Falcon means most of the deceleration is aerodynamic, so I get a lot of re-entry heat.  After playing around with different engine configurations an engine plate with multiple Vectors is the only one that I've found that reliably copes with the heat.  I'd agree with the earlier comment about limiting the gimbal too.

I use kOS and coupled with Trajectories to calculate atmospheric drag my current script can put it down on the runway every time, but I'm still having issues getting the pitch down right to land accurately in range, ie I target the start of the runway and always overshoot by a bit.  As I have access all the data through kOS I calculate the required thrust to stop in the current distance to the ground, and then put the engines on full when that reaches 95% of the maximum thrust, this means I get down to a couple of m/s several meters off the ground and do a nice slow final descent.

 

Don't have a decent picture of it landed but here it is delivering components of my Mun base to orbit

If you're planning to land boosters, it's well worth looking at Kerbal Reusability Expansion, it has a lot of useful parts like New Shephard and Falcon Landing gear, and the Falcon grid fins.  the bulbus bit at the far end is the New Glenn landing gear.

Edited January 10, 2020 by RizzoTheRat

[Linkageless]

On 1/9/2020 at 8:19 PM, moguy16 said:

where exactly do you put the air brakes at the top, the bottom or the middle? 

Mostly at the top, with the hinges at the lower end.  Hence, they're best left unused for ascent.  I have also put some a bit lower down on that big one, but still above the CoM.

I've not bothered with any mod to help with the burn, just enough TWR and fuel to be able to come to a stop a couple of hundred m early, then slowly descend.

The height at which I start the burn was determined by experimentation, trying later and later until I could reliably come close without crashing and still have a little reserve to do the last little bit.

Edited January 11, 2020 by Linkageless
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