SubOrbital Re-Entry with Mk1 Crew Cabin

[se5a]

Made a pretty decent Sub Orbital Tourist ship using a stack of three pods, four basic fins, two radial chutes and a nose chute:

https://www.dropbox.com/s/frdsg62l9js9vvu/Screenshot%202016-01-01%2017.28.34.png?dl=0

it'll re-enter ok from 100k sub orbital, and is controllable enough to get back to KSC or even the runway if you're good/lucky

 

In My career game, I finally got the MK1 Crew Cabin and attempted to prettify the above and use that instead of the two pods, so far this is been an exercise in frustration:

https://www.dropbox.com/s/wj5nbiqygv0mgou/Screenshot%202016-01-01%2017.37.11.png?dl=0

For some reason it makes the whole stack far less stable, requiring eight extra winglets, and still has a tendency to tip sideways even with sas on. it's a little lighter I guess, but that much?

it won't re-enter well either, it'll nose into the slipstream instead of bellying into it as I'm able to do with the three pods, causing it to loose speed slower and the front fins to go all explody due to heat. I dunno maybe the extra gyro of the pods was enough to keep it controlled?

the whole stack is 4k funds more expensive in the end as well.

 

Anyone else attempted to create a mini shuttle/reentry vehicle using the MK1 crew?

 

 

 

 

[Snark]

The main issue you're running into is that the crew cabin is too light.  Your ship wants to fly with its CoM in front.  You can see what I mean in the VAB if you temporarily remove all the pieces that aren't part of the reentry vehicle, and turn on the CoM marker.  Whichever end it's closer to is going to want to point forwards.

This is a fundamental fact of life; it's how aero works in the post-1.0 world.

The easiest way to deal with it?  Well, one way is to embrace it rather than fighting it.  Give it a reaction wheel for some decent torque, and/or put some control surfaces on the bottom (AV-R8 winglets are great for this) so it's steerable.  Go ahead and reenter nose-first, but keep your nose up, about 30 degrees above prograde.  That will give you body lift that will help slow you down while you're still high in the atmosphere, so you can enter without frying.

In other words, re-enter like a spaceplane.

If you'd rather stick with a heatshield-type reentry, you just need to figure out some way to move the CoM down on the reentering ship.  One simple way:  Lose the upper-stage SRB (they have crappy Isp, which means they're great for the launchpad but bad for upper stages).  Instead, put on a 2-ton LFO tank with a Terrier engine, and don't decouple it from the crew cabin on reentry.  The mass of the engine will help keep you pointed butt-first, and engines are pretty heat-resistant so it should give you some protection.

 

[numerobis]

What Snark says about center of drag versus center of mass is your likely problem. Your goal seems very similar to what I use as my tour boat: capsule for the pilot, cabins for passengers, SRB first stage. The capsule is denser than the crew cabins, so just that ends up with you going head-first, which is lethal (even if you survived the heat, you would be supersonic at the surface, so you'd never be able to open the chutes).

To make the upper stage balance properly so that I can reenter sideways, I put a LF+O engine on the back: those engines are as dense as the capsule. I either put fuel tanks on either side of the cabin, or I put crew cabins on either side of the fuel tanks.

That design got me a 4-passenger to orbit craft with a T-30. I upgraded it to a 2-passenger to Mun, and my recent project is 8-passenger Mun on the same model (it's getting bendy, I'd my current issue).

[se5a]

2 hours ago, Snark said:

The main issue you're running into is that the crew cabin is too light.  Your ship wants to fly with its CoM in front.  You can see what I mean in the VAB if you temporarily remove all the pieces that aren't part of the reentry vehicle, and turn on the CoM marker.  Whichever end it's closer to is going to want to point forwards.

This is a fundamental fact of life; it's how aero works in the post-1.0 world.

The easiest way to deal with it?  Well, one way is to embrace it rather than fighting it.  Give it a reaction wheel for some decent torque, and/or put some control surfaces on the bottom (AV-R8 winglets are great for this) so it's steerable.  Go ahead and reenter nose-first, but keep your nose up, about 30 degrees above prograde.  That will give you body lift that will help slow you down while you're still high in the atmosphere, so you can enter without frying.

In other words, re-enter like a spaceplane.

If you'd rather stick with a heatshield-type reentry, you just need to figure out some way to move the CoM down on the reentering ship.  One simple way:  Lose the upper-stage SRB (they have crappy Isp, which means they're great for the launchpad but bad for upper stages).  Instead, put on a 2-ton LFO tank with a Terrier engine, and don't decouple it from the crew cabin on reentry.  The mass of the engine will help keep you pointed butt-first, and engines are pretty heat-resistant so it should give you some protection.

 

Re-entering like a spaceplane is what I was doing with the original, and attempting to do with the new design, currently in my campaign game I don't yet have the reaction wheel (or the terrier). I'm attempting to enter slightly nose first till I've bleed off enough speed to not overheat (belly first using the wings/fins as breaks), then tilting forward enough that I've got some directional control which allows me to steer into a shallower descent, slowing to chute speed and aim for the ksc. 

I'm a bit leery of adding more/larger surfaces to the top of the whole stack, previous experience has shown this to be a bad thing for the assent. 

I'll have a play around with the second stage as a liquid engine and see how it works out. 

 

Edit: just re-reading what you wrote, I was and did temporarily remove all the parts that were not on the re-entry vehicle and stuck the CoL on the CoM. the two re-entry vehicles should be pretty similar in that respect. 

Edited January 1, 2016 by se5a

[se5a]

1 hour ago, numerobis said:

What Snark says about center of drag versus center of mass is your likely problem. Your goal seems very similar to what I use as my tour boat: capsule for the pilot, cabins for passengers, SRB first stage. The capsule is denser than the crew cabins, so just that ends up with you going head-first, which is lethal (even if you survived the heat, you would be supersonic at the surface, so you'd never be able to open the chutes).

To make the upper stage balance properly so that I can reenter sideways, I put a LF+O engine on the back: those engines are as dense as the capsule. I either put fuel tanks on either side of the cabin, or I put crew cabins on either side of the fuel tanks.

That design got me a 4-passenger to orbit craft with a T-30. I upgraded it to a 2-passenger to Mun, and my recent project is 8-passenger Mun on the same model (it's getting bendy, I'd my current issue).

Hah! This is nearly exactly what I've done:

https://www.dropbox.com/s/a172az6tnvnqo5h/Screenshot%202016-01-01%2019.36.34.png?dl=0

 

That'll get up a little past 100k SubOrbital. 

It's got a tendency to lean over to the left, always the left on assent. not sure what's up with that...

might spin it around in the hanger and see if it still goes left or if it goes forward.

 

the ThreePod design still out performs it in flight though, I guess those pods have a really good amount of lift on their own. 

Edited January 1, 2016 by se5a

[Snark]

9 hours ago, se5a said:

Edit: just re-reading what you wrote, I was and did temporarily remove all the parts that were not on the re-entry vehicle and stuck the CoL on the CoM. the two re-entry vehicles should be pretty similar in that respect. 

Beware, beware!  CoL is only marginally relevant here.

Flipping happens because the CoM wants to be in front of the center of drag.  if you're looking at "where is the CoM relative to the center of lift", then you're looking at the wrong thing.

The CoL indicator only shows lift from surfaces that have a "lift" component, like wings, control surfaces, fins, and Mk2 spaceplane fuselage parts.  It's nearly useless for telling whether you're going to be stable in a particular orientation.  It may be a little relevant, because surfaces that have a lot of lift will also have a lot of drag if they're not parallel to the air flow... but you won't get any information at all from the CoL indicator about all the many components on your ship that don't have an explicit lift property.  Such as the crew cabin.

"So how do I tell where the center of drag will be?", I hear you cry.  The answer is "Sorry, you don't.  There's no indicator for it, for various complex reasons.  So you just have to eyeball."

So, to evaluate how stable your craft is likely to be:

• Turn on the CoM indicator.
• Turn off the CoL indicator, because it will just mislead and confuse you.
• Visually estimate where you think the center of drag wil be.  When in doubt, assume approximately at the geometric center of the ship, biased in the direction of any high-drag parts like fins and such.

You can use the debug aero display while in flight to get a better idea of what the actual drag is like on the various parts of your ship, but unfortunately that's the only way to do it-- actually fly the thing.  No way to know in the VAB or SPH.

Edited January 1, 2016 by Snark

[Pecan]

11 minutes ago, Snark said:

Beware, beware!  CoL is only marginally relevant here...

2016 forums; now with sound effects.

[Snark]

1 hour ago, Pecan said:

2016 forums; now with sound effects.

Beware, beware! His flashing eyes, his floating hair! 

 

[numerobis]

I re-enter pointing normal with SAS on, no air surfaces needed. If my rocket isn't quite balanced perfectly (or I don't have much torque) I instead turn SAS off and spin flat on the plane between radial and normal. Either way the spacecraft is pointing flat into the breeze and slows down even from quite high up.

To land science equipment like this I first spin flat quite fast, then I roll fast to halve the heat (any side of the materials bay is in the plasma half the time, cooling down the other half).