Precision Landing on return

[LN400]

While I have little problem getting around and returning, one issue keeps bugging me and that is, where I land is very much a lottery, especially when returning from other bodies. Since I first started playing KSP some years ago, I only managed to land on KSC itself less than a handful of times.

I tried to standardise the final orbit before reentry, I tried to standardise the geocoodinates for the final burn, same for the initial periapsis after the burn. So far I have not been able to get consistent results on where I land. One problem is that from an orbit at say 71,000 m, 1 meter + or - and 0.1m/s variance can mean several tens of km off target for touchdown/splashdown if not more than a hundred km.

For those of you who do precision landings, any hints and tips on how to get more consistent results? I would think each design would require its own details on how to de-orbit but I would like to hear from you how you go about it.

[bewing]

Each design does have its own details, but the key is aerodynamics. That is the thing that allows you to adjust your drag profile as you descend. Which is what keeps you from overshooting the runway.

(The runway is the best place to land, because you get a 100% refund anywhere on the surface, and it's got a much bigger total area than the launchpad -- and it's long, which removes one dimension from the details you need to control.)

The lift from the aerodynamics allows you to stay in the air an extra long time, which is your insurance policy for undershooting the runway.

And then a little leftover thrust from your engine (once you are down below 1300 m/s) helps with serious undershoots -- or a little bit of jet thrust, if you can justify the extra mass.

And the more aerodynamic your RV is, the more flexibility you have for eyeballing the parameters of your reentry. So I'm often in a 100x96 parking orbit. Approximately on the other side of Kerbin I burn to put my Pe at 55km as close to "over KSC" as I reasonably can. When my RV is down to 1200 m/s I go prograde -- hopefully over the little patch of desert west of KSC. If I'm clearly short, I go prograde early. I want to be at 1000 m/s over the mountains. Planting a set of flags along the path that indicate what your speed should be when that flag comes into the 100km radius can be helpful. That's more reliable than going by landmarks.

[ToukieToucan]

I would recommend 'Trajectories' mod which will make your trajectory more precise taking rotation of the planet and atmospheric drag into consideration. One issue with the mod is that it can only give precise readings for the current stage so you might need to use RCS/other engine on your last stage. Also, it doesn't take parachutes into consideration but mostly you'll deploy your parachutes at a low altitude which would make the difference negligible (that is if you aren't going for SpaceX precision)

[ForScience6686]

I prefer steeper re entries, but that comes at the cost of the needed fuel to slow down enough to bring the pe well below the surface.  Now it all depends on your drag ratio of the craft, but generally I burn at the desert to the west of ksc, and bring the trajectory to just before the island in booster bay directly east of ksc.  Then use aoa to adjust the descent.  Works much better with space planes as there is more drag and lift available depending on how the return is going.  

[foamyesque]

@bewing uses a very similar entry profile as myself. The more lift you get on your RV, the lower you can put that periapsis (for example, I put my spaceplane periapsis down at 45km, and glide longer; for RVs with less lift you need to cover more distance higher up). The trick to precision landing is to adjust your AoA to manage your energy; even body lift and orientation-related drag for capsules helps a great deal. That's how Apollo reentered, for example.

 

Heck, you can turn a Science Jr into a glider if you open the doors :v

Edited August 8, 2016 by foamyesque

[LN400]

What concerns me the most when it comes to using aerodynamics like described above is heat. Last return almost ended in shouting and cursing. The returning ship was

Pod Mk I

2x Equipment bays with assorted science gear

Heat shield with 40 units.

+ parachutes.

Total mass less than 2 tons.

From 45,000 meters and all the way down to around 16,000 where the heat dropped, KER told me the pod was about to explode almost constantly. I went shield first or so I thought but even then did the pod get heated up to just below critical. Exposing solar panels and parachutes has also ended in rapid evaporation of those parts. Seems to me having anything other than the heat shield exposed is a sure way to see fireworks. This is with an initial apo at 71,000 and peri at 40,000-55,000.

[bewing]

Heatshields have very little drag. MK1 pods have very little drag. If you have no drag, you don't slow down. If you don't slow down, you blow the hell up.

Many players use heatshields, but I think they are a very bad idea when landing on Kerbin. Even simply tumbling your RV will slow your craft so much that you don't need a heatshield. Heatshields are worse than worthless -- if you depend on them, they will kill you. Tailfins on the other hand, are very nice for slowing down with.

 

[LN400]

7 hours ago, bewing said:

Heatshields have very little drag. MK1 pods have very little drag. If you have no drag, you don't slow down. If you don't slow down, you blow the hell up.

Many players use heatshields, but I think they are a very bad idea when landing on Kerbin. Even simply tumbling your RV will slow your craft so much that you don't need a heatshield. Heatshields are worse than worthless -- if you depend on them, they will kill you. Tailfins on the other hand, are very nice for slowing down with.

 

I found it to be different. Shedding off a small fuel tank and a 909 (little more than 0.5 t in mass then), to go heat shield first sees the g-meter make a jump, not just the kick from the separator, it stays higher indicating a greater braking force indicating greater drag. Shedding that to go pod only sees an even greater jump on the g-meter. Same as before, it stays higher so it's not only the kick from the staging. I still have had chutes, solar panels, anything that wasn't shielded by the pod, or engine, or heat shield or whatever, blow up long before I reached the low atmosphere. The idea of going anything but bottom/shield first is to me slightly terrifying. 

[magnemoe]

21 hours ago, bewing said:

Each design does have its own details, but the key is aerodynamics. That is the thing that allows you to adjust your drag profile as you descend. Which is what keeps you from overshooting the runway.

(The runway is the best place to land, because you get a 100% refund anywhere on the surface, and it's got a much bigger total area than the launchpad -- and it's long, which removes one dimension from the details you need to control.)

The runway is also flat unlike the launchpad and you are less likely to hit the vab or other structures. 
Add that your main error will be in the east / west direction not in north south 

[ElWanderer]

10 hours ago, LN400 said:

What concerns me the most when it comes to using aerodynamics like described above is heat. Last return almost ended in shouting and cursing. The returning ship was

Pod Mk I

2x Equipment bays with assorted science gear

Heat shield with 40 units.

+ parachutes.

Total mass less than 2 tons.

From 45,000 meters and all the way down to around 16,000 where the heat dropped, KER told me the pod was about to explode almost constantly. I went shield first or so I thought but even then did the pod get heated up to just below critical. Exposing solar panels and parachutes has also ended in rapid evaporation of those parts. Seems to me having anything other than the heat shield exposed is a sure way to see fireworks. This is with an initial apo at 71,000 and peri at 40,000-55,000.

With that combination of apoapsis and periapsis, you're spending a lot of time in the upper atmosphere where you will generate heat but not much drag. I use a 85km by 30km orbit for re-entry with a pod (and a heat shield with about 20% ablator). It seems to be better to get down through the upper atmosphere quickly, but not to set the periapsis so low that you smash into the lower atmosphere at too high a speed.

Do you get the same critical heat built-up if you get rid of one of the service bays? The heat shield occludes something akin to a cone behind it, rather than a cylinder, so if your craft is too tall you may still have hot air hitting your pod.

[LN400]

1 hour ago, ElWanderer said:

[entire post]

Thanks. I have 2 rescue robots + a 7 man 9 ton behemoth inbound from Mun. They have enough fuel to drop the apo to at least 85km together with the aerobraking. I will attempt steeper descents and see how that goes. Also a good opportunity to see what a cluster of drogue chutes will do on that big one.

 

EDIT: I will have to test the 1 bay design in sandbox game later as my budget isn't open for much non-contractual testing at the moment. The tier 3 R&D is horribly expensive but after that, I should have more dosh to spend on those tests.

Edited August 8, 2016 by LN400

[ForScience6686]

9 hours ago, ElWanderer said:

With that combination of apoapsis and periapsis, you're spending a lot of time in the upper atmosphere where you will generate heat but not much drag. I use a 85km by 30km orbit for re-entry with a pod (and a heat shield with about 20% ablator). It seems to be better to get down through the upper atmosphere quickly, but not to set the periapsis so low that you smash into the lower atmosphere at too high a speed.

Do you get the same critical heat built-up if you get rid of one of the service bays? The heat shield occludes something akin to a cone behind it, rather than a cylinder, so if your craft is too tall you may still have hot air hitting your pod.

This is something a lot of people miss.  In the upper atmosphere you generate a lot of heat, but due to the thin air you cannot radiate that heat off the vehicle very well.  That's why getting to the lower atmosphere helps remove heat while also providing more air resistance to slow you down faster.

 

11 hours ago, LN400 said:

I found it to be different. Shedding off a small fuel tank and a 909 (little more than 0.5 t in mass then), to go heat shield first sees the g-meter make a jump, not just the kick from the separator, it stays higher indicating a greater braking force indicating greater drag. Shedding that to go pod only sees an even greater jump on the g-meter. Same as before, it stays higher so it's not only the kick from the staging. I still have had chutes, solar panels, anything that wasn't shielded by the pod, or engine, or heat shield or whatever, blow up long before I reached the low atmosphere. The idea of going anything but bottom/shield first is to me slightly terrifying. 

You get better drag as you shed weight, since your drag profile stays the same while having to slow down less mass.  This is also why reducing ablator to only what you need is a good idea.  Otherwise you have that much more mass reducing your drag ratio.

One thing to try is holding normal while above 45km.  This greatly increases your drag ratio before you start getting to the heavy heating, hopefully reducing your speed sooner.

[bewing]

1 hour ago, ForScience6686 said:

This is something a lot of people miss.  In the upper atmosphere you generate a lot of heat, but due to the thin air you cannot radiate that heat off the vehicle very well.  That's why getting to the lower atmosphere helps remove heat while also providing more air resistance to slow you down faster.

I would actually divide it into three regions. In the stratosphere (above, say, 53km) you get a decent amount of drag, and your craft can radiate fairly effectively. So this is a safe zone for slowing down. The players who advocate for steep reentries don't seem to recognize this. You can stay above 60km for a very long time and bleed off a couple hundred m/s with perfect safety.

In the upper atmosphere (perhaps 36 to 52km) is a bad zone. Your drag has only increased a little, but your craft is storing up heat quickly.

At 35km or below is the make-or-break point. You have a lot of drag, and a lot of skin heating. It's a race between slowing down and blowing up.

 

[Broco]

Since I do 90% of my missions with Spaceplanes I pretty much end most missions on the runway. I always plan so that I have enough fuel left to make it to the runway if I undershoot. Basically what I do is achieve a low orbit (71-75km) sometimes including aerobreaking and set a maneuver node over the desert continent (ca. 130-140° W) and aim for my trajectory to end on a level between the KSC and the Island Airfield. During descent I can adjust a little bit by tilting my craft (increasing or decreasing angle of attack). That mostly does the job.

For other craft I pretty much choose a steep angle from low orbit, retrograde burn over the ocean between the continents (~100° W) because I'm just too bad at predicting the drag on craft with different weight/part count/aerodynamics.

I make sure I always have enough fuel since I have a space station around Minmus that is being supplied by a tanker from Minmus surface and for the very rare case a craft has run out of fuel in Kerbin orbit I have a Tanker-Spaceplane that can bring up big amounts of fuel into low Kerbin orbit. Might seem like cheating for some but since fuel harvesting is in the game I take full advantage of it for maximum efficiency. Hey, being German must be good for something.