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Laythe Ascent Vehicle Problems


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It super unaerodynamic. All the parts sticking out will create drag. Especially the outer tanks that are mounted sideways.

Between 270m/s and 340m/s is the so called transonic region. At these speeds you suffer lots of drag, before punching through the sound barrier at roughly 340m/s when drag will be a little lower again.

So I guess your craft struggles to overcome the sound barrier. 

 

Edited by Chaos_Klaus
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31 minutes ago, bewing said:

KER deltaV is only accurate for airless bodies. You suffer significant dV losses to drag on anything with an atmosphere. So the KER dV value is only a minimum in those cases.

But his KER is set to atmospheric and even then he should have enough delta v. The delta v map actually shows vacuum delta v even for atmospheric ascents, so you also need to look at vacuum values in KER. Drag losses for reasonably aerodynamic rockets are included. So this lander should have way more then enough delta v.

So apart from judging TWR for liftoff, you actually never have to look at atmospheric stats in KER.

 

Edited by Chaos_Klaus
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@bEAstmode

I agree with @Chaos_Klaus that it's probably aeroforces holding you at specifically 300m/s. That said, I'm still surprised your craft doesn't break through the sound barrier (~350m/s) given it has a TWR of nearly 3! The drag of flat faces on tanks (i.e. without nose cones) was radically increased in v1.2, and it still surprises me how draggy those uncovered faces are. That said, you're pushing with not one, but two vectors. By my experience, that should be enough to cram almost anything past the sound barrier. Perhaps doublecheck they didn't have their thrust limiters turned down? Or perhaps they're not staging correctly, or one isn't getting fuel flow? Just ideas. Feel free to post the craft file if you'd like more specific advice!

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6 hours ago, Chaos_Klaus said:

It super unaerodynamic.

This.

Also, Laythe's atmosphere is quite odd, a lot thicker and soupier than it has any right to be, almost all the way to the very top. Laythe's atmosphere is only 50km high, but at 45km you still experience as much drag as on Kerbin@30km, maybe even more.

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Try using that 1.25m to 2.5m adapter-tank, it will also help your stack aerodynamics.  For that matter, nearly 3 TWR on launch is sort of overkill for in-atmosphere work, you might be better off lightening your engine load.

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Unlike all the airless bodies (& even Duna), for Laythe you need something more like an aircraft (even though you are launching vertically) in terms of shape. 

Each stack (which should be the minimum number) should have a nose cone on the front (and on the back if there is no engine there). 

Each change is part size should have a tapered tank or adaptor joining the disparate parts. 

Ladders, chutes, etc that are only used during the descent should be mounted on decouplers and dumped before liftoff. 

Sticky-out bits should be in a service bay.

Do you really need RCS and to dock the craft in orbit? Why not rendezvous and then EVA the crew over (remembering to take the science) and abandon the lander?

 

 

Edited by Foxster
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KER takes account of the lower ISP you get in atmosphere but not drag losses.

For low drag,  all stacks must start and end in something pointy, and all adjoining attachment nodes need to be of same diameter.

I  can see a big problem right away as you have a 1.25m decoupler joining onto a 2.5m tank , which will create a lot of drag.  Should have used a 1.25m to 2.5m adapter tank to handle this transition.   The bottom of this 2.5m tank appears to have left it's 2.5m node empty (flat plate drag penalty is just as bad on the forward or rear facing part of an object) and you radially attached two vectors, which don't occupy the rear node.   Should have used a 2.5m double engine mount or three way engine mount instead and attach the engines non-radially.

Those two FT200 tanks mounted side on is also draggy, and these stacks don't have cones on their top or bottom.

The trick of clipping oscar b into round-8 torodial tanks (like you have done on top of the lander can) saves volume but as far as the aero model is concerned, you have a 0.625m part (oscar b) joined to another 0.625m (the torodial tank) then are going straight to 1.25m where that attaches to your lander can - no adapter used in between.

Finally those RCS quad blocks are much draggier than the linear rcs ports, but reaction wheels are probably sufficient to dock with if your docking port is on the front, which it appears to be.

Edited by AeroGav
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47 minutes ago, AeroGav said:

KER takes account of the lower ISP you get in atmosphere but not drag losses.

For low drag,  all stacks must start and end in something pointy, and all adjoining attachment nodes need to be of same diameter.

I  can see a big problem right away as you have a 1.25m decoupler joining onto a 2.5m tank , which will create a lot of drag.  Should have used a 1.25m to 2.5m adapter tank to handle this transition.   The bottom of this 2.5m tank appears to have left it's 2.5m node empty (flat plate drag penalty is just as bad on the forward or rear facing part of an object) and you radially attached two vectors, which don't occupy the rear node.   Should have used a 2.5m double engine mount or three way engine mount instead and attach the engines non-radially.

Those two FT200 tanks mounted side on is also draggy, and these stacks don't have cones on their top or bottom.

The trick of clipping oscar b into round-8 torodial tanks (like you have done on top of the lander can) saves volume but as far as the aero model is concerned, you have a 0.625m part (oscar b) joined to another 0.625m (the torodial tank) then are going straight to 1.25m where that attaches to your lander can - no adapter used in between.

Finally those RCS quad blocks are much draggier than the linear rcs ports, but reaction wheels are probably sufficient to dock with if your docking port is on the front, which it appears to be.

OK,  I tried a redesign of your vehicle keeping the above principles in mind and got this -

https://www.dropbox.com/s/f0n7zx03nhrn9e5/Laythe Lander.craft?dl=0

kiQZNFX.jpg

Similar mass, the only problem might be that it is longer.    Two aerospikes in the lower stage, one terrier in the upper and asparagus staged oscar b auxilliary fuel tanks.    Can get to orbit on kerbin with loads of delta v spare so it is total overkill for laythe.  Did not need fins because it is wider at the bottom than the top (more drag at bottom) but has most of the fuel mass at the top .

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