Mechjeb: Can you reduce decent speed in Landing Guidance?

[RedeyePete]

I am trying to land on Eve and need to keep the speed down to about 1000m/s in the upper atmosphere.

Can you set the maximum speed in the Landing Guidance so that it still compensates and lands at the target?

So far I have used Mechjeb to deorbit, then I take ever to keep reentry speed down but that changes the landing location,

and by the time I am out of the upper atmosphere, the craft is too low for Mechjeb to correct.

At the bottom of the Landing Guidance window is says something about Descent Speed Policy, is that adjustable/mean anything?

I might try burning a bit more to see if I can get the landing a little closer and let Mechjeb just do the final bit.

[Xd the great]

10 minutes ago, RedeyePete said:

I am trying to land on Eve and need to keep the speed down to about 1000m/s in the upper atmosphere.

Can you set the maximum speed in the Landing Guidance so that it still compensates and lands at the target?

So far I have used Mechjeb to deorbit, then I take ever to keep reentry speed down but that changes the landing location,

and by the time I am out of the upper atmosphere, the craft is too low for Mechjeb to correct.

At the bottom of the Landing Guidance window is says something about Descent Speed Policy, is that adjustable/mean anything?

I might try burning a bit more to see if I can get the landing a little closer and let Mechjeb just do the final bit.

I dont think you can do that, but you can always add heat shields.

Also, you can try using the trajectories mod that allows you to predict your landing site with more accuracy.

[RedeyePete]

Hi Xd,

The craft is a reusable SSTO so heat shields aren't vary practical. See Pic below.

Its a flying fuel tank. And I haven't yet tried to get back into orbit with it (it has to meet up with the miner/refueler )

But I hope it makes it.

I will try to do manual burning until I am just past the target and then activate mechjeb.

Some more trial and error.

 

[Gargamel]

19 minutes ago, RedeyePete said:

I am trying to land on Eve and need to keep the speed down to about 1000m/s in the upper atmosphere.

Given the thickness of the atmosphere, I think I spent the least amount of time in this speed range.   It was either way faster, or way slower.   1 km/s was a transition point for me, after the heatshields did their braking, and before the chutes would open up.  It was the point where I would stop worrying about burning up and start worrying about slamming into the ground.    Once you you dip low enough in the atmosphere, you'll soon realize you are doing 200 m/s while still really high above the ground, and it will be a while before you land. 

21 minutes ago, RedeyePete said:

Can you set the maximum speed in the Landing Guidance so that it still compensates and lands at the target?

MechJeb is horrible at doing atmospheric re-entries. 

  I get the feeling you are trying to do a completely powered descent.   Why?  The atmo on Eve is so thick it's practically liquid.   Use parachutes.  My 250t lander only needed a handful of chutes to land.  If you are trying to do a powered landing, that means more weight in fuel.  A chute would save you the fuel and possibly engine mass if it slows you down enough.   If you save mass on the landing stage, you can save mass on the transfer stage, and so forth. 

28 minutes ago, RedeyePete said:

and by the time I am out of the upper atmosphere, the craft is too low for Mechjeb to correct.

Holy burn up batman, flatten out your angle some then.   As mentioned, MJ is not good at atmo landings.  It will correct for a missed target on an atmosphericless body, but it fails miserably if it encounters air.   If you need 'pin point' landings, get the trajectories mod, set up a manuever node so you can get a precise landing, burn the node, and profit.   Use heatshields to slow you down to <1km/s, then pop the chutes when safe.   This might entail waiting till you are really close to the ground, as I have found (on my heavier designs), that the full size chutes will safely deploy at height, but will get ripped off as the atmosphere gets thicker.  I had to wait till 1km AGL to pop the big chutes.   But even drogue chutes make your descent slow enough to make this easy to do. 

[Gargamel]

17 minutes ago, RedeyePete said:

The craft is a reusable SSTO so heat shields aren't vary practical. See Pic below.

Its a flying fuel tank. And I haven't yet tried to get back into orbit with it (it has to meet up with the miner/refueler )

You can put an inflatable heatshield on the nose, and re-enter prograde. 

But your mission specs are similar to what I did.  I built a rover base with ISRU in place to meet up with the unmanned lander, refuel it, and then allow the crew to return to orbit. 

Now I used the inflatable heatshields on the top and bottom of the lander and the rover, as I found the stability goes to hell with only a heatshield on the bottom. 

Ignore the burnt off wheels, this was an early test, and I hadn't fully worked out the stability yet. 

 

But for your lander, You could put an inflatable shield on the bottom of the craft, and add some radially mounted engines that serve as orbital maneuvering engines.  when you jettison the heatshield, you can then also jettison the radial engines and use the main engine for landing.   The radial engines should have a better vac ISP than your main engine, which should offset the extra mass of the radial ones. 

 

 

Edited July 29, 2018 by Gargamel

[RedeyePete]

Yes similar to what I'm doing but bigger. At the start of this I saw a video of someone who successfully orbited a similar craft. I liked it and decided to do something similar.

I did try heat shields front and back on the rover, but the wheels were still protruding and would go boom.

I have not had a happy relationship so far with heatshields especially the inflatable ones.

Luckily I used the large docking rings and not decouplers, so I could undock the nuke stage and fly in a massive ass booster with asparagus staging. Got the job done. Accuracy was not as critical. All I had to land on was a plateau and then drive to my preferred spot.

See pic below, get ready to scoff :o)

 

I do have a penchant to supersize things.

That's my thing. Definitely believe: "Add more boosters!"

On the craft above, there were heat sheilds but I jettisoned them.

I topped it up with a refueler at Gilly.

 

Below: Landed. You can see some of the landing aerospikes burning off weight before driving off.

 

Fun fact: If I fire the 2 decouplers on top, all the wheels get destroyed!

So where is the logical physics there?

I just left them on.

 

[Gargamel]

56 minutes ago, RedeyePete said:

I have not had a happy relationship so far with heatshields especially the inflatable ones.

Chutes and heat shields will make life drastically better.   You can lose the fuel required for a powered descent and replace it with usable payload.   You'll burn, what, 3-4k dv on just the descent alone?  Don't fight the laws of physics, utilize them. 

58 minutes ago, RedeyePete said:

I did try heat shields front and back on the rover, but the wheels were still protruding and would go boom.

Our rovers seem to be about the same size, give or take, and that's why I went with the smaller wheels, to keep them nicely tucked inside the heat shield. 

[Xd the great]

Any chance you considered something that looks like a bfr? Use wings to slow down?

[RedeyePete]

What does "bfr" stand for?

Discussing the rover is a bit of a moot point as it has already landed. I got away with a big booster for powered descent because it was a one way, once off use. And somehow I managed to target the plateau in 1 go.

I'm starting to think that was more luck than anything.

As for the SSTO, It needs to be able to go up and down many times, so heat shields wont do and they also add weight when ascending. Similarly wings would also add weight and drag. That's why the SSTO is a no frills bullet.

I have done a powered descent and landing many times over, so it works. Ascent is yet to be proven.

Accuracy is my only problem. I cannot hit the plateau. You can't set a maneuver node for that sort of accuracy.

I have downloaded the Trajectories mod and will try it.

So long as I can set the trajectory to fly over the target, I'm in with a chance.

It would need to compensate for the rotation of the planet.

Manually or automatically I have to also allow for the deorbit burn, powered descent and atmosphere.

And then to do this over and over again. (It will get refueled in orbit)

[Gargamel]

5 hours ago, RedeyePete said:

I have downloaded the Trajectories mod and will try it.

So long as I can set the trajectory to fly over the target, I'm in with a chance.

It would need to compensate for the rotation of the planet.

Manually or automatically I have to also allow for the deorbit burn, powered descent and atmosphere.

And then to do this over and over again. (It will get refueled in orbit)

The mod will show you the current estimated landing spot based on aerodynamics of your craft.

With a little practice, you can set up a manuever node an orbit ahead, the mod does account for planet rotation, and then you can very accurately follow your node for your reentry burn.  

Play with the mod for a bit before committing an active mission to it, there some settings you'll probably want to get familiar with. 

[JAFO]

7 hours ago, RedeyePete said:

What does "bfr" stand for?

Big Fine Rocket

Big Friendly Rocket

Big F...ing Rocket

 

Take your pick.

But in this case, it's most likely a reference to Elon Musk's SpaceX BFR concept (officially called the Big Falcon Rocket). No prizes for guessing how he really came up with the name, though.

Edited July 31, 2018 by JAFO

[RedeyePete]

Well I finally managed to land on the plateau with the help of the Trajectories mod, but it was still very hard.

I had to line things up before entering the atmosphere and then sit back and hope for the best.

And then the plateau is not flat either. The ship being tall a few times I landed ok only to tip over. Argh!

So the ship is supposed to be reusable, but do I want to go through this again and again?

I think going in with heat shields will be the same problem. - Accuracy!

Next task will be to send an Engineer down as 2 of my rover wheels have been destroyed.

I know Eve is punishing on wheels. One of Scott Manley's videos shows him getting out to repair the wheels again and again.

[Gargamel]

You do know Eve is considered the EndGame for KSP right?  It's supposed to be hard. 

6 hours ago, RedeyePete said:

I know Eve is punishing on wheels. One of Scott Manley's videos shows him getting out to repair the wheels again and again.

After the third time my kerbal ran around my rover fixing wheels, I just turned off crash damage so I could get somewhere.  Yes, yes... we'll just simulate the wheels breaking and being fixed every 5 minutes. 

[RedeyePete]

I had a problem with the ship carrying Danlo the Engineer and a small rover for him to get about with.

Every time I clicked on to another ship it would disappear!

Turns out the ship was set to "Debris" even though it showed as set to "Rover"

I changed it to "Ship" and everything was fine after that.

Landing Danlo on the plateau was just as hard.

I even took screen shots of the trajectory, printed them and marked all my attempts.

I could not see any logic to it. Its like trying to hit a dart board with your eyes closed.

Just keep trying over and over and you will get lucky. Next time I'm going in with wings.

Oh and this craft had heat shields and parachutes.

So after fixing the wheels and refueling the SSTO, I encountered a problem.

Every time I undocked the rover, the SSTO would bounce up and down and eventually fall over.

I tried adding a little fuel, undock, redock and add more, but I would get to a point where it would fall over anyway.

So I found that I could launch the SSTO and quickly undock.

Sadly the SSTO did not make it into orbit after all that. Sigh!

Looks like I'll have to send down a multistage to get Danlo off the rock.