Eve lander design questions

[MZ_per_X1]

I have gone through many designs to come up with a craft capable of unpowered landing and ascent from sea level.
The ones that can land, don't reach orbit. The ones that can reach orbit can't land. So working backwards, I have a lander that has 8400 m/s delta v at sea level. It flies OK on ascent during testing. However, I can't make it land. I experimented with periapsis between 38-67 km. At around 47 km altitude, speed of about 2800 m/s it sways and flips.
I have a tall and slim design with 6 aerospikes with FL-C1000 tanks and a Vector using FL-TX1800 tank (Making History part) going to 1.25 m FL-T series.
Large inflatable heat shield at the bottom during landing, 4 airbreak feathers at the top end of the dart shape.

Now I know that the centre of mass is probably not close enough to the heat shield and that's why it flips. At the same time I can't lower the COM further. I tried spamming the craft with decouplers. I even thought that instead of an intricate ladder system, I will put an inline cockpit at the bottom under the centre tanks and the vector to help with boarding (and transferring crew).

Making the ship taller and would make the stabilising airbreaks further from the heatshield but that would make the ship too heavy to lift and mess up the TWR. 

I was also thinking of putting something heavy near the basket of the heat shield. Some rcs fuel tanks could be great deadweight. The ship still flips.

I don't necessarily want another inflatable heat shield at the top because it never worked for me. When jettisoning the top heat shield, it either crashes into the ship or gets trapped by the parachutes and then crashes into the ship.

Could anyone help me with the design problem? I might be missing something.

[jimmymcgoochie]

A few suggestions to make it more stable:

• Add a ton- or more likely, several tons- of ore to the bottom; radial ore tanks attached directly to the heat shield or the part(s) it connects to will add a load of ballast to the base of the rocket, making it more stable. You should probably get the ore from Gilly rather than hauling it all the way out from Kerbin!
• Switch the airbrakes for the biggest elevons you've got, airbrakes have really low heat tolerance when deployed so they can burn up pretty quickly but some Big-S elevons are more heat resistant. If you have Breaking Ground, put some big heat shields onto alligator hinges instead as these will provide drag and be really heat resistant, then set them to open up to somewhere between 60 and 75 degrees- this will provide more drag on the side that's tipping and will help your passive stability.
• Add FL-A151S fuel tanks (or some other 1.875m-1.25m adapter) between the 1.875m fuel tanks and the reaction wheels. That sharp step in size will cause more drag at the base of the rocket making it more prone to flipping; yes, it should be shielded by the heat shield, but you'd be surprised how that sort of thing can affect your rocket, and it might help for the ascent too.
• MOAR RCS! Vernor engines near the top to brute-force push the rocket back into line; by the time they lose efficiency you should be slow enough that they're no longer needed.

Hope this helps!

[MZ_per_X1]

1 hour ago, jimmymcgoochie said:

A few suggestions to make it more stable:

• Add a ton- or more likely, several tons- of ore to the bottom; radial ore tanks attached directly to the heat shield or the part(s) it connects to will add a load of ballast to the base of the rocket, making it more stable. You should probably get the ore from Gilly rather than hauling it all the way out from Kerbin!
• Switch the airbrakes for the biggest elevons you've got, airbrakes have really low heat tolerance when deployed so they can burn up pretty quickly but some Big-S elevons are more heat resistant. If you have Breaking Ground, put some big heat shields onto alligator hinges instead as these will provide drag and be really heat resistant, then set them to open up to somewhere between 60 and 75 degrees- this will provide more drag on the side that's tipping and will help your passive stability.
• Add FL-A151S fuel tanks (or some other 1.875m-1.25m adapter) between the 1.875m fuel tanks and the reaction wheels. That sharp step in size will cause more drag at the base of the rocket making it more prone to flipping; yes, it should be shielded by the heat shield, but you'd be surprised how that sort of thing can affect your rocket, and it might help for the ascent too.
• MOAR RCS! Vernor engines near the top to brute-force push the rocket back into line; by the time they lose efficiency you should be slow enough that they're no longer needed.

Hope this helps!

Great tips. Thank you. 

Does it matter if the elevon blades point upwards or towards the bottom of the ship? Or do you mean sideways? 

I like the alligator hinge idea. I just started exploring the DLCs. I will try tomorrow. 

Yeah, I did use the fuel adapter. It might not be visible clearly. It's the orange stripy thing. 

Vernors... Another thing I never worked out but I should really start using. A lot more powerful than an rcs block. 

[Spricigo]

1 hour ago, MZ_per_X1 said:

Does it matter if the elevon blades point upwards or towards the bottom of the ship? Or do you mean sideways? 

I suppose he mean,

That way while launching the thing from Kerbin:

That way when landing on Eve.

 

Since is Eve we are talking about you may attach the parachutes there too and have a decoupler to drop the whole thing before take off. 

[MZ_per_X1]

Well it doesn't look pretty but it does the trick. I still have to fine tune the angles on the hinges and understand more about deploying elevons, their angle and their authority limiter but the first test went better than expected. Those Vernors are truly powerful compared to the RCS thruster block. 

Thanks for your help. 

 

 

Edited September 12, 2020 by MZ_per_X1

[bigcalm]

The problem is that it flips round?  What if you could add something that's really draggy at the top and self correcting?  Something like this --

Yes, I know this is a plane in this pic, but it works perfectly well (actually better) with a symmetrical lander.  As the craft is pushed to one side or the other, the angled upper heat shields effectively deflect it back, ensuring no flipping and putting your lander in danger of being deep fried.  NB - I eject the top heat shields when travelling at around 400ms - they've got sepratrons but it's not particularly necessary for the upper ones.  The bottom heat shield gets ejected only after the parachutes have fully opened and these do need to have sepratrons to stop your craft crashing into it.

 

 

Edited September 17, 2020 by bigcalm

[MZ_per_X1]

1 hour ago, bigcalm said:

The problem is that it flips round?  What if you could add something that's really draggy at the top and self correcting?  Something like this --

Yes, I know this is a plane in this pic, but it works perfectly well (actually better) with a symmetrical lander.  As the craft is pushed to one side or the other, the angled upper heat shields effectively deflect it back, ensuring no flipping and putting your lander in danger of being deep fried.  NB - I eject the top heat shields when travelling at around 400ms - they've got sepratrons but it's not particularly necessary for the upper ones.  The bottom heat shield gets ejected only after the parachutes have fully opened and these do need to have sepratrons to stop your craft crashing into it.

 

 

I found the separatrons for the top heat shield to be weak to pull it away from the craft.

If you slow down enough to below 10 m/s, you can jettison the bottom heat shield and it floats away from the craft. 

[bigcalm]

1 hour ago, MZ_per_X1 said:

I found the separatrons for the top heat shield to be weak to pull it away from the craft.

If you slow down enough to below 10 m/s, you can jettison the bottom heat shield and it floats away from the craft. 

If you have the top heat shields at an angle, this is normally not a problem - I eject at around 25k altitude and around 400ms - still slowing down, but it's not time to open the parachutes yet.  For the bottom one - you may find it's fine for you!  I don't have a lot of weight attached to the heatshield so found my craft kept running into it (even <= 7ms) - some separatrons sorted this out though you're right that they're not very powerful at Eve sea level.

[king of nowhere]

does it really have to use atmospheric braking?

for my eve ascent, i put a convert-o-tron and drills on it, i use rocket braking going down, i refuel and i start again. it only took around 7 tons of extra mass, including extra solar panels and radiators, and i ditch them before liftoff. and the lander is more than 1000 tons (part of a crazy contract requiring me to lift 30 tons of ore and landing it on another planet)

[Spricigo]

 

On 9/17/2020 at 9:24 AM, bigcalm said:

A very Kerbal solution

 

Maybe there is more elegant solutions but THAT would work for sure. 

 

On 9/17/2020 at 6:11 PM, king of nowhere said:

does it really have to use atmospheric braking?

Probably not. But is often cheaper, simpler and lighter than the setup you need for a rocket breaking