Why can't control this spaceplane at reentry?

[juvilado]

Hi, i have this SSTO that can go anywhere near Kerbin, but when i try to return and land, its almost impossible to control and land safely. As you can see, both CoM and Dry CoM are ahead of CoL, but upon reentry at some time the plane losses stability. Do you find what is wrong with the design? How could i solve it? Thanks

And one more thing: The cockpit is always burning or almost at reentry, even having only heat setup at 70%

Edited September 24, 2016 by juvilado

[Tex_NL]

The CoL is a bit far behind the CoM. Both empty but most especially full. This would make it harder to control but at least it should be aerodynamically stable. However ... those engine nacelles are blunt and will create MASSIVE drag. And since this drag is applied well in front of the CoM it wants to flip.
Try adding some shock cone intakes to the engines. This will reduce drag and increase engine performance.

Edited September 24, 2016 by Tex_NL

[juvilado]

20 minutes ago, Tex_NL said:

The CoL is a bit far behind the CoM. Both empty but most especially full. This would make it harder to control but at least it should be aerodynamically stable. However ... those engine nacelles are blunt and will create MASSIVE drag. And since this drag is applied well in front of the CoM it wants to flip.
Try adding some shock cone intakes to the engines. This will reduce drag and increase engine performance.

Thanks, but, will those shock cone intakes cancel the intake of the pre coolers? or wont they? apart from that, do you recommned using precoolers or nacelles or only fuel tanks?

[Vectura]

All the intakes will add together. This will allow you engines to work at slightly higher altitudes, and will allow it to go faster more easily.

[Tex_NL]

13 minutes ago, juvilado said:

Thanks, but, will those shock cone intakes cancel the intake of the pre coolers? or wont they? apart from that, do you recommned using precoolers or nacelles or only fuel tanks?

AFAIK they will not block the air intake. And even if they did, who cares? The shock cone's intake area is four times bigger.
The precooler is not very special. It just combines a fueltank and an air intake. It does AFAIK not increase engine performance.
The nacelle is heavy, It does have a large intake area but the shock cone performs better at high altitude and speed.

Personally I would recommend the liquid fueltank or just the structural fuselage to save weight in case you don't need the extra fuel.

Edited September 24, 2016 by Tex_NL

[juvilado]

Thanks a lot for your answers. Now what can i do to avoid the cockpit burn?

[steve_v]

4 minutes ago, juvilado said:

Now what can i do to avoid the cockpit burn?

Don't try to re-enter directly from Mun/Minimus return, get into a circular-ish <125KM Kerbin orbit first.
Re-enter at a high (>40°) AoA for maximum drag in upper atmosphere.
Allow plenty of room to decelerate. Abort early and skip off the atmo things get too hot.
Watch your vertical speed like a hawk.

The mk.1 cockpit kinda sucks in this regard. Light, but crispy.

[Tex_NL]

11 minutes ago, juvilado said:

Thanks a lot for your answers. Now what can i do to avoid the cockpit burn?

Instead of the Mk1 cockpit/inline stabilizer combo you could use a Mk2 cockpit/Mk2 Drone combo. The second combo will be a bit heavier and the total amount of torque is a bit smaller but in return it can tolerate more heat and gives full SAS no matter the skill level of the crew.

Edited September 24, 2016 by Tex_NL

[Guest]

Mk1 cockpits survive re-entry just fine. There are just threethings you have to keep under control: your periapsis altitude, your apoapsis altitude, and your attitude. Set periapsis to around 55-60k, and apoapsis ... well, the limit depends on your design and I haven't really explored how high you can go, but ca 250 km is no problem at all. Point your plane vertically and hold that attitude as long as you can or until your surface velocity has dropped to 1000 m/s or so, whichever comes first.

I suspect your problem is that the centre of mass is too far ahead of the centre of lift, and you have relatively little wing area. The CoM/CoL thing means that your plane will want to pitch down while it's going too fast, which means it'll stop aerobraking too effectively and you'll end up in thick air with too much velocity, and burn up there. The small wing means your aerobraking won't be all that effective to start with.

I.e. I would add a good deal more wing, and then place it so that the CoL is only just a little behind the CoM. Also the aerodynamic improvements others have mentioned -- nosecones or intakes on everything, no flat surfaces presented.

You also have a bonkers thrust to weight ratio on that thing, for something that small two Whiplashes and one rocket is ample: it should be able to get you into orbit by just pointing it in the general direction of the sky (say, at 30 degrees or so) and watching it go.

[fourfa]

13 hours ago, Vectura said:

All the intakes will add together. This will allow you engines to work at slightly higher altitudes, and will allow it to go faster more easily.

and @juvilado.  If you have less intake air than you need (which you can see by right-clicking the engines during flight), you will have reduced thrust.  It's possible to have 100% intake air at launch, then starve for air at hypersonic speed when those Whiplash engines (also Rapiers) gain a huge thrust boost.  

But having more intake air than you need at peak does nothing.  It doesn't increase ceiling or top speed.  In fact as all intakes have mass and drag, ideally you want to have just enough intake air but no more.  One shock cone can feed a pair of Whiplash/Rapier engines through all conditions.  I believe you need one precooler per Whiplash/Rapier to cover the hypersonic thrust boost.  Combining both would be way in excess in terms of air.  However - despite its weight, the shock cone makes a very good nose (or tail) cone simply because it has low drag.

Beyond that, lots of good advice in this thread so far.  My advice would be to simplify as much as possible.  Do you need 4 Whiplashes?  For a payload of one pilot?  I never have.  You have those heavy nukes all the way at the back, so you get a big shift in CoM over the flight.  Try two Whiplashes on precoolers on the back, and radial mount the pair of nukes near the center.  RCS Build Aid is showing you how the full CoM (yellow) moves when you burn all the fuel (red).  Try to move the nukes forward and back until the red ball is very close to the yellow ball.  I believe this will put the CoM very close to the center of the ship, so then you should try mounting the wings near the center instead.  You are currently very light on wing - try a total of 5-6 in wing area instead.  Then get rid of everything you don't need - like the radiator sticking out creating drag.  No need for radiators for nukes in the current version of KSP - at least not with a ship like this.

[Val]

On 24/09/2016 at 10:46 AM, juvilado said:

but upon reentry at some time the plane losses stability. Do you find what is wrong with the design?

It looks like it has too little vertical stabilizer. My guess is that, while the craft is stable in pitch axis, because of CoL position, it spins out on yaw axis.

[Guest]

On 9/24/2016 at 1:10 PM, juvilado said:

Thanks a lot for your answers. Now what can i do to avoid the cockpit burn?

(1) Add more wing area for more effective aerobraking
(2) Keep yourself pointed at the sky as long as possible, with your Pe set to around 55k
(3) Put the CoL closer to the CoM so your plane won't nose down until around 30-35 k altitude (add reaction wheels if necessary) 

[swjr-swis]

On 9/24/2016 at 0:10 PM, juvilado said:

Now what can i do to avoid the cockpit burn?

If you're ok with some functional clipping, you can add two of the passive large radiators panels, and offset them into the cockpit. When I do this for the purpose of thermal protection, I place them in mirror on the sides, and clip them in and halfway back. The next parts in the fuselage will hide the exposed half of the radiators.

At the expense of a tiny bit of extra drag, mass, and some EC usage, you can make the Mk1 cockpit survive extreme heat from reentry (or Mach 3+ speeds in the lower atmosphere).

Spoiler

 

Edited September 26, 2016 by swjr-swis
image links

[juvilado]

finally made a fully maneuverable SSTO! Pity it only has 1K Dv left in LKO, i will go on working to get more Dv once in LKO. Thanks for all the tips!

[juvilado]

On ‎26‎/‎09‎/‎2016 at 3:38 PM, swjr-swis said:

If you're ok with some functional clipping, you can add two of the passive large radiators panels, and offset them into the cockpit. When I do this for the purpose of thermal protection, I place them in mirror on the sides, and clip them in and halfway back. The next parts in the fuselage will hide the exposed half of the radiators.

At the expense of a tiny bit of extra drag, mass, and some EC usage, you can make the Mk1 cockpit survive extreme heat from reentry (or Mach 3+ speeds in the lower atmosphere).

  Reveal hidden contents

 

Im not so ok with that (nice and tricky idea though), but just attaching a couple small ones outside the cockpit can save the craft, thanks!

Do you find AIRBRAKES useful in these designs? or not at all?

[swjr-swis]

1 hour ago, juvilado said:

Do you find AIRBRAKES useful in these designs? or not at all?

I do, they can be useful both as pure airbrakes and as control surfaces, but it's often limited to bigger craft due to their size, and I tend to build planes too small to fit them in a visually acceptable way. (hint hint Squad, would be nice to have a small set too...)

[Guest]

I find drogue chutes more useful than airbrakes. I can control airspeed with pitch well enough; my most common problem is bouncing on landing and chutes address that effectively; airbrakes don't bite quickly enough.

[Clipperride]

To add one more tip.  I've found that adding two vernier engines, one on the upper surface behind the cockpit and one on the lower surface near the tail, help to maintain a high angle of attack for longer.  You need to bleed off as much speed as you can while you are still in the thin air above around 20,000m.