MK2 SSTO plane difficulties

[Warzouz]

I'm working on a passenger SSTO to go to my space station. The plane achieve LKO all the time, but I'm not satisfied with it.

Issues :

- Not enough dV after LKO (only 100/200)

- Unstable around 12km. The plane don't always go straight so I often end on a non equatorial orbit which cost 100/400dV to fix.

- I need to dive to pass 340m/s. I usually go to 12km then end at 500m/s around 8/9km. Then I have a hard time to pull it up.

- Plane don't lift of, I wait for the end of the runway. I had canards but it ship la CoL to much to front

- CoM/CoL are very near. I've a hard time to keep CoL behind CoM. The plane is very long, arrow-like.

- I add a bay to put the RCS fuel tank and some solar panels (to avoid more drag), but that's not really nice and add to the CoM/CoL problem.

I think I need 3 engines... but where to put the air intakes...

Some clever insight ?

EDIT:

Finally I did that and went better

- 3 engines

- Probe core and more fuel

- Bugger wings

- Bigger landing gears

- Removed storage bay

Edited May 29, 2015 by Warzouz

[Ferdoni]

Use RAM's instead of Shock Cones and add a lot of radial intakes. Plus your elevons are very small compared to your SSTO. I would advice canards on the middle of the plane (to change the CoL not too far to the front)

[Warzouz]

With staying at 2 engines ?

As for the elevons, the plane is nicely maneuverable (under 10km)

On reentry, the CoM seems to be behind the CoL, hard to tell. Last try I ended in flat spin.

- - - Updated - - -

Further more : are precooler useful ?

[Ferdoni]

With staying at 2 engines ?

As for the elevons, the plane is nicely maneuverable (under 10km)

On reentry, the CoM seems to be behind the CoL, hard to tell. Last try I ended in flat spin.

I don't know what your max altitude now is so I can't judge if you need more engines or not. But I think it's enough.

As for the elevons, yes I can read, but I meant it for the instability above 12km what you stated above. In wich elevons are far to small because the atmosphere is too thin so you need a bigger manoeuvring surface.

And reentry shouldn't be a problem, you have to raise your nose to glide into the thicker atmosphere. (and check the CoW and CoL ratio's in the SPH with empty tanks)

Further more : are precooler useful ?

I don't know, as far as I know it's just another air intake

[steve_v]

On reentry, the CoM seems to be behind the CoL, hard to tell. Last try I ended in flat spin.

Your fuel is mostly in front of the COL, check that COM is still in front of COL with empty tanks.

Further more : are precooler useful ?

Sort of, they don't actively cool anything but they do have a high thermal mass for their size, so they should draw heat from parts attached to them.

[georgTF]

Off the top of my head:

- i'd swap the cockpit for the inline variant. It's lighter, allows a MK2 rocket fuel adapter to be fitted in front and allows the use of a shielded dock instead of the Mk2

- try using smaller winglets in an X configuration (4 of them) to improve heading stability. [instead of 2 tails]

- while you don't want control surfaces (elevons, winglets) anywhere near the mid/front of the plane, you may get away with using some strakes or small wings. Just wings, no control surfaces. They are displayed in a neutral state in the SPH but they can violently alter your CoL during actual flight.

- don't be too keen to eliminate drag by using fewer wings. They are not that draggy and you do need lift. Try using swept wings because they can move the CoL further back. Maybe try using 2 pairs, one attached higher and the other lower.

- try a cockpit + 1 passenger module first, then if you're succesful, try adding room for 4 more kerbals.

- you're not using the rearmost node. Consider 2 TurboJets + 1 Aerospike as it provides serious thrust and good Isp with all three engines firing. The extra speed might give you a higher initial Ap.

- keep the cargo bay and put it where it least interferes with CoM moving about as fuel is burned. You might want to try placing it next to the cockpit and stuff it full of heatsink-like parts because re-entry heat will be an issue for long, thin planes.

- Ramjet intakes have a slightly higher heat tolerance, though i haven't used the shock ones so idk. Ramjets are good though. And you won't get any extra mileage out of airhogging so don't bother.

The main issue is to check where you CoM is when the plane is fully loaded and empty then aim for a build that keeps the damn thing almost in place or moving slightly forward as fuel is spent. In my experience an empty plane copes easily with a difference between CoM and CoL pictured above, though take off would be difficult. Once you're happy with the CoM, Find a good spot for wings then worry about elevons. An SSTO during ascent should fly more like a rocket than a plane, hence keeping control surfaces further back is usefull.

To be frank, i don't know how usefull this advice is, i will say i have had dozens of failures with the Mk2 spaceplanes and am seriously hating almost all of them. Plus it cost me 10 dead kerbals in career mode. You are not alone in suffering. I currently have only 2 that work. And one of them is the one that killed so many kerbals...

[oh and LOL is me, since i haven't unlocked the Aerospike in career mode i have no safe Mk2s afterall. Pics added.]

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Note that othermost elevons are set to ignore Rolls because they tend to overwhelm the craft. Also both craft were built to reach Mun/Minmus after refuelling in LKO. The one i can't use can probably do it.

Edited May 29, 2015 by georgTF

[Shyrka]

I think I need 3 engines... but where to put the air intakes...

You could double up the intakes on the front of your engine nacelles by radial attaching a pair of tail connectors and putting the intakes on the front. Like this:

[Starhawk]

Try to place half your fuel in front of your CoM and half your fuel behind CoM. Use the tweakable feature in the SPH to empty the tanks and see where your CoM ends up.

As others have noted, you want it to move as little as possible.

You don't need extra RCS fuel, since the inline docking port you used comes with monopropellant. So the cargo bay is probably not worth it's weight.

You have enough engines, in my experience. You may want more wing, although all those Mk2 parts do give some lift, so maybe not. I would say you want about 6.5 total lift coefficient.

To lift off from the runway, either angle the wings slightly upwards at the front, or angle the whole plane by setting the rear landing gear much higher up on the fuselage than the front gear. Either approach will give you a built-in angle of attack on the runway.

The precoolers do have significant thermal mass, but they also add drag and you don't need the extra air intake ability they offer, so they may not be worth it.

It may be worth a try to place a single rudder on the central core rather than two rudders on the radially attached tanks. You may want to try a static wing part rather than an active control surface. It may be worth placing a reaction wheel near the rear to help with stabilization.

Hope this helps.

Happy landings!

[Warzouz]

OK, I've swapped to 3 engines add some more fuel. and slightly bigger wings. I don't have to dive to much now.

Switched to inline cockpit and frontal docking ring, event I don't like the shape.

I get to orbit with 500m/s left. Comfortable. I'm testing landing now

EDIT : Hmmm, I changed the landing gear and extend wings. Now I can't pass through sound barrier anymore...

[Ferdoni]

Can you post some pics of the final design? I'm very curious

[RuBisCO]

This works with 500 m/s dv left.

[Ferdoni]

This works with 500 m/s dv left.

How many LF+O do you use?

[Falkenherz]

As regards liftoff problems, this can also be a problem of your elevons being too far behind the CoM; bad leverage, regardless of where CoL is in relation to CoM.

Your plane definitely has not enough engine power when you cannot pass to supersonic. I found that one RAM intake per engine is enough, no additional air intaces required (as it used to be in 0.9).

Instability, i.e. plane veering left and right, happens to me often, too. Usually this is when thrust is not strong enough, so more engines should partially solve this, too. The other matter would be the way your control surfaces are arranged and work. Sometimes you can remedy problems by blocking yaw, pitch or roll for certain surfaces. I always have slight imperfections remaining, though. It might also be SAS which sometimes over- or undercompensates during course corrections.

[eddiew]

Re OP: I do feel that that's a three-engine ship, really. Moving the crew tanks forward will help keep your CoM in place when the fuel drains too

This works with 500 m/s dv left.

This may sound weird, but if you had rapier-aerospike-turbo-aerospike-rapier, it may well fly just as well and have more delta-v in orbit. The turbo would punch you past mach 1, the rapiers would get you further on-air, and the reduced weight by removing an engine would save fuel ^^

(Disclaimer; may not work at all, but it'd be interesting to find out?)

[RandomRyan]

needs more R.A.P.I.E.R engines, instead of the shock cone intakes, use ramp air intakes

needs more lift,add the Big-S spaceplane elevon thingy.(works for me)

the Com and Col (for me) needs to be little bit inline

Although im new here, im 13, i played this game since January,i have built SSTO's for days now!

(7 exploded) rest in kerbal peace.

[Warzouz]

(click to enlarge)

I opted for a probe core tested if from runway to runway : 900m/s left at 75km. The nose cone heat a lot on reentry, it nearly exploded. Before reentry I send all fuel up front.

So :

- 37T

- 3 engines

- Lift factor should be around 2.9 (core) + 2*3.46 = 9.82 (sic)

Rapier loose power around 22km, I engage close at 23km. and follow prograde to reduce drag.

[moronwrocket]

- Plane don't lift of, I wait for the end of the runway. I had canards but it ship la CoL to much to front

- CoM/CoL are very near. I've a hard time to keep CoL behind CoM. The plane is very long, arrow-like.

- I add a bay to put the RCS fuel tank and some solar panels (to avoid more drag), but that's not really nice and add to the CoM/CoL problem.

IANASSTOD - I Am Not An SSTO Designer, but I have created a couple.

The first issue is a "feature" not a "problem". You want your plane to have the minimum drag possible, hence you aren't going to have a hell of a lot of lift. Waiting until the end of the runway to pull to 45* isn't a problem, usually.

CoM/CoL - The issue is that you are using Mk2 spaceplane parts. Adding more Mk2 parts forward of the CoM isn't helping your CoL problem because they also contribute lift. What you need are more non Mk2 parts ahead of your CoM, or dragging your wings further back. You could try swapping out fuel tanks in the Mk2 center for additional fuel tanks in your outriggers - remember to strut them back to the center or else you'll have problems during liftoff with bouncing. Also, try putting all your crew cabins right behind your cockpit, and your fuel tanks towards the rear. That way as they empty the CoM should pull ahead of your CoL more, making your plane more stable for reentry. For liftoff, your plane just needs to be "stable enough" - it's not doing acrobatics, it's just going to pitch up.

The inline docking port comes with 75 monoprop, more than enough for 2 docks in my experience.

[GoSlash27]

Warzous,

Wet wings are your friend

A couple notes:

More intakes than necessary is just dead weight and drag.

The flight profile is at least as important as the spaceplane design

Maximum efficiency isn't achieved when you can't add any more, it's achieved when you can't take any more away.

Best,

-Slashy

Edited May 29, 2015 by GoSlash27

[Jaeleth]

If you can't take it off runway you have too little lift, one can tell that looking at the small wings, it appear underpowered too.

1. Replace air intakes by the top ones ( cant remember the designation right now) those big ones that look like modern fighter jet intakes.

2. Increase wing area

3. Add one more engine, at least, a whiplash and more fuel, place tanks in front so that you can play with canards which is a fair option for extra manueverability.

4. Add intake coolers ( cant remember proper name also : P )

[georgTF]

Switched to inline cockpit and frontal docking ring, event I don't like the shape.

Just wanted to chime in and say that i don't like that look either, but i've had rotten luck with the pointy cockpit.

Oh and it's usefull because it allows fuel tanks to be placed forwards.

[edit]

While i'm rabbiting, i'll add that i do hate the inline docks and prefer the shielded one when the node isn't needed for an air intake. The shielded dock is also very heat resistant. But overheating isn't really a huge problem except during re-entry. It's more a sign of things working well. I think an ascent profile that very nearly melts the plane is the best one. I really do want maximum thrust out of airbreathing engines and that means moving quickly through air. Also heat. ^^

Edited May 29, 2015 by georgTF

[DancesWithSquirrels]

If your CoM is behind your CoL on your return, pump fuel from back tanks to the front before re-entering.

[georgTF]

If your CoM is behind your CoL on your return, pump fuel from back tanks to the front before re-entering.

Tricky as you can't see the CoM in flight. Best to have empty or near empty tanks on re-entry and perform an unpowered landing. If the craft is light enough, it should behave well below 2km alt. Obviously, check where the CoM is located with empty tanks in the SPH.

[DancesWithSquirrels]

"Tricky as you can't see the CoM in flight..."

You can to a degree eye-ball it. First, note your CoL and Com in the hangar prior to flight. Remember those points. In normal (not orbital) view the center of the view defaults to through the CoM. Rotate so you're looking at the ship from the side. As you pump fuel back and forth the view will slowly pan to keep the CoM centered - keep pumping until the point you want is centered in the screen, or at the very least until it's ahead of the CoL.

For more accuracy you can zoom in until only part (the middle) of your ship is in view, close enough to read the graphics - you should be able to more closely estimate the view center/CoM from there.

If you've been panning around by hand and aren't looking at the CoM, toggle between orbital and normal views to reset it.

Edit: If after pumping all available fuel to the front you still have a CoM behind the CoL, well, you're probably up for a rough reentry. Sorry...

Edited May 29, 2015 by DancesWithSquirrels

[Warzouz]

OK, I sticked with the final design (I added it in the first post).

- Lift off at 100m/s

- Increase speed to 200m/s then up to 11km with constant speed (during this part the plane swing a little)

- Follow prograde (it'll dive a little to 10km) until speed is around 500m/s

- Pitch up without loosing speed (tricky part) and then increasing speed to 800 m/s

- Make the real ascent to 22km/24km

- Switch to close cycle as engine start to fail

- Pitch at 5° until 35km, then prograde until apo is 76km.

- Orbit insertion should be around 300m/s

I end with 800/900m/s left and on a quite clean equatorial orbit.

- Before reentry, transfer fuel to front tanks.

I still have to figure reentry parameters to arrive very near KSC. I succeeded twice but I was very short the first time on overshoot a lot the second time.

I think I should place the place vertically but nose DOWN. So when air flow would force the nose prograde, it'll dive faster instead of gliding and overshooting KSC.

NB on close cycle only, the plane has 1875m/s of dV

[cybersol]

Looks good and sounds eerily similar to my ascent profile for efficient spaceplanes. Cheers!