SSTO Advice

[TheNoobHunter24]

Looking for advice on building an SSTO, preferably one that can make it to at least the Mun. Specifically I suppose, best profile of ascent for this type of craft, how much delta-v does it need for achieving orbit (I mean after air intake runs out and I have to rely on the rocket), and whats a good TWR as well. Any other tidbits are quite welcome

 

https://imgur.com/a/6SGwS   This one is first attempt. I got it to a 67k periapsis but ran out of oxidizer for the rocket. There's two whiplash jets and then 1 aerospike.

Edited September 7, 2017 by TheNoobHunter24

[MrChumley]

Building an SSTO that can make it to orbit with any amount of d/v left over is a tall order to fill.  I recommend adding a docking port to your space plane so that you can dock and refuel after achieving orbit.

Another tip: Sometimes less is more.  If you ran out of oxidiser and still had lq fuel left on your last test, remove the extra liq fuel instead of adding more oxidiser.

My SSTO flight profile goes something like: Take off and achieve 250 m/s then pull up and climb at 45deg or less making sure that I am still gaining speed the whole time.  Start SLOWLY leveling off at about 10k and make it to 11deg climb angle at about 14k.  Keep it stable through the Whiplash/Rapier power band to boost your speed to at least 1200m/s.  After your jets cut out nose up to 20 or 30 degrees and fire the rockets until AP is in space.. Then hold prograde while drifting into space to minimise drag.. Then circularise at AP.

Good luck with your space plane building endeavours!

[bewing]

You have at least 5 too many air intakes, and air intakes create drag and have mass. Remove extra parts, add fuel, reduce drag.

For flying, you will have a lot of acceleration at 15 to 22 km altitude. So go slow and let your wings do their job up to 12km or so, then go to full power.

Keep your climb shallow, just set SAS to stability hold, and don't maneuver. Just climb. If your nose rises more than 10 degrees, use the F key to let it gently fall back down. Most players lose all their hard-won deltaV by pitching up and down. To get to orbit, you want to increase your horizontal velocity. Vertical velocity accomplishes nothing.

Once you get above 35km lock your SAS to prograde for the rest of the trip.

 

Edited September 7, 2017 by bewing

[AeroGav]

Closed cycle Delta V ?   

Orbital velocity is 2200.    Assume you can reach 1400 airbreathing.      This means you need 800dV from rocket power, but gravity and drag will cost about a third of that,  plus you want some "wiggle room" (minimum 25%).      Once you're in low orbit,  you'll need another 900 or so just to reach the moon's orbit.

Honestly I never do delta V calculations in the SPH however, aerodynamics introduce too many variables.

What I concentrate on is making sure the airplane has the right number of engines of each type for its mass, and the right amount of wing.    Then I build for low drag and decent handling.  By "decent handling" I don't mean aerobatic, it just has to be stable and it has to allow the pilot to easily make small corrections without over controlling.   The CG shouldn't move as the fuel burns off,  which makes holding an accurate pitch angle throughout the ascent easier.  If you can pitch the nose up 10 degrees, that's all you need for takeoff and landing.

 Weight isn't particularly a priority,  it's all about the lift/drag ratio.    

Engines - My rule of thumb is one Panther and one RAPIER per 50 tons.    One nuke per 25 tons or less.    This assumes you get a lift:drag ratio of about 3:1 when supersonic.    The better your lift:drag ratio, the less thrust:weight ratio you need.    Getting TWR much above 0.4:1 with nukes is very costly of your dry mass because they are so heavy.   Getting good TWR with Rapiers in closed cycle mode is easy, but their ISP is horrible compared to nukes.

How do I find my Lift:Drag ratio?       Press Alt + F12 to bring up the cheats menu, go to Physics, select "Aero" category and tick the box for "display aero data GUI".  Note, you can also tick the box for "display aero data in action menus" to get drag data displayed when right clicking a part, good for troubleshooting.

About 3/4 of the way down , you can see it displaying the Lift / Drag Ratio here, 4.237 to 1.

Wing area.

For every 2 to 2.5 tons of mass, you want one unit of lift rating.  So, a pair of big S wings and two pairs of Big S wing strakes (total lift 14) is a good amount for a 30 ton ship, for example.    Wings do add mass,  but they create less drag than fuselage tanks of same capacity.     Also, the vast majority of the drag on your ship comes from the fuselage, more wing means you fly higher and at a lower angle of attack for a given airspeed, which lowers the drag made by the fuselage.   Too much wing will cause the airplane to balloon above 22km before you've hit your top airbreathing speed, causing the RAPIERs to loose power.

Fuel fraction 

Compared to rockets, fuel fractions on spaceplanes, especially ones with nuke engines , can seem really low.   40-45% is about the maximum you can get.  I prioritise drag reduction over weight reduction every time.

 

Flight profile?

Text version -

1.  Climb subsonic, keeping speed below 250m/s and the nose not more than 5 degrees above prograde.

2.  Somewhere between 5 and 10km, you'll find you can't get enough lift to keep climbing without either busting 250m/s or hauling the nose more than 5 degrees above prograde.   This means its time to break the sound barrier - ease up on the nose-up inputs, let the plane fall into a shallow dive if it wants, you can even turn the nukes on briefly for an extra boost.

3.  Above 420m/s cancel the nukes and let the plane resume climbing, but keep control inputs gentle.

4. Try to level off between 18-22km for the speed run.    Fly level here until you're getting near 1400m/s or the acceleration is dying away.

5. Activate nukes/rockets and accelerate to orbital velocity.  Don't yank back on  the stick - keep the nose no more than 5 degrees above prograde.   If your plane has wings that are angled up with respect to the fuselage, you can even fly on prograde lock.Video example

Spoiler

 

Practical example -  I 've written some quite detailed instructions for this craft.    It's a fairly low tech ssto (basic wings with no fuel, Panther engine, only the nukes are high tech) that can go to the Mun and back.        Because of the panther engine,   air breathing top speed is much lower than you'd get on RAPIERs , but the same principles apply.

https://kerbalx.com/AeroGav/Stretch-Ray   (it's the craft from the screenshot above)

higher tech minmus shuttle

https://kerbalx.com/AeroGav/Coimbra-PA-X-LR

[AeroGav]

3 hours ago, TheNoobHunter24 said:

 

https://imgur.com/a/6SGwS   This one is first attempt. I got it to a 67k periapsis but ran out of oxidizer for the rocket. There's two whiplash jets and then 1 aerospike.

My advice

1.   Those subsonic intakes will melt.    Use an adjustable ramp intake  or a shock cone on the front, since they can handle heat. One per jet engine is plenty.

2.  Chemical motors probably won't get you to the moon.  One jet engine on the back (whiplash is ok i guess) with two nukes on the sides, but slide them right forward so your ship is properly balanced  even with empty tanks.   You don't need two Whiplashes to push a tiny thing like this to mach 3 !

3.    Remember to right click on control surfaces and tweak the "limit authority" slider.   Small deflection angles give you most of the effect but with less drag.  Important if you're flying with keyboard.   Also makes fine adjustments easier.   Larger control surface deflecting at a small angle makes less drag than small one at 100%.

4.  In flight,  don't let the nose rise more than 5 degrees above prograde or drag goes up crazy amounts.

5.   Big S wings and strakes are better because they hold liquid fuel, though I understand the modular wings are easier to create custom shapes with.

[AeroGav]

@TheNoobHunter24

I tried to recreate your craft a bit -

https://www.dropbox.com/s/6r745tedeau4bnj/hunter.craft?dl=0
This is what i mean by tweaking control authority.   The rudders should only control yaw.      Note that I used NCS adapters to attach the nukes to the fuselage.  Look at how far forward I had to push them !    Note also, the wings are actually attached to the main fuselage and slid outward and forward with the offset tool.  If you attach them to the nacelles, a bug in the physics engine causes your plane to constantly roll left.   Also, you can see I'm a messy worker.

Spoiler

 

Ailerons - I've fitted slightly larger than necessary but turned the deflection angle down so they make less drag and yaw torque when used.  

Spoiler

 

 

elevators should only control pitch..

Spoiler

 

 

Going to space -

I used the prograde hold to stop the thing climbing above 17km until it had a decent amount of speed (1100ms) .  Then i start the nukes and pitch up gently, but notice how the nose is still only 5 degrees above prograde for optimal lift/drag.    As I'm flying with keyboard,  I use pitch trim to adjust the nose by such small amounts.    Basically press ALT + S or ALT + W to gradually raise the nose up and down.

Over 3000dV left -  the air's really thin at this point so using SAS is the easiest option at this point.

 

[agrasyuk]

On ‎9‎/‎7‎/‎2017 at 1:37 AM, AeroGav said:

 

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those are 4 full ore tanks, sheesh, That's some awesome flying Aerogav. good comment on controlling via trim (alt W-S) , I need to use it more. You obviously have a working design but those patchwork wings! also, do I see no incidence on your wings at all? all the flames though and heating makes me very nervous watching this . what is your idea of "maximum mach at given altitude" table?

OP, some good advice above, good luck on your build. Space-planes are so more exciting then rockets. Can't post my design at the moment (and it doesn't really beat AGav's efficiency monster, with 600Dv will not go anywhere before refuel at the depot) , but for some extra fuel saving perhaps consider using staged drop tanks. it's done IRL, since only tanks are dropped I think it still can be considered SSTO. if you keep tanks modest (for your plane perhaps  pair of size 0) you should not have problem lifting them to at least 5000m , basically its like you get to start already in air. 

All that said, what will be more so exciting then the launch is the reentry  keep us posted on how that is going .

 

[AeroGav]

18 minutes ago, agrasyuk said:

those are 4 full ore tanks, sheesh, That's some awesome flying Aerogav. good comment on controlling via trim (alt W-S) , I need to use it more. You obviously have a working design but those patchwork wings! also, do I see no incidence on your wings at all? all the flames though and heating makes me very nervous watching this . what is your idea of "maximum mach at given altitude" table?

Yes  those patchwork wings are aesthetically challenged, but if you think about it ,  5 strakes have the same dry mass and lift as 1 big S wing, but would hold 500 instead of 300 units of liquid fuel,   and since you're working with smaller building blocks you've got more control of the overall shape, which helps to get your CoL and fuel mass where you want them to be.

The ore lifter ("Partridge")  has 5 degrees of incidence angle on the wings, as does the panther/nerv Mun SSTO.        On these vessels you just lock prograde hold for most of the flight.   The Panther/NERV ship ("Stretch Ray") also has some trim flaps bound to an action group, so you can cause the nose to sit a couple of degrees higher or lower while keeping SAS on Prograde hold by toggling action groups.         You never touch pitch or yaw controls, only make gentle roll corrections to keep the wings level and on a heading of 90deg due East.

I only use pitch trim for vessels without built in incidence which are flown with SAS OFF below 35km.

Adding incidence to the wings is an "advanced technique" which can screw up the plane's handling and the game's stock CoL/CoM indicators aren't much use helping you predict the effects of this.  .  I have editor extensions redux, CorrectCoL and RCS build aid installed but the OP may not, he's also new to SSTOs and in any case his ship has a low drag mk1 fuselage,  so   it's not really necessary.    For my version of his ship , which i linked on Dropbox (I called it the "Hunter") I avoided using any "advanced" construction methods,  but it still has plenty performance.

re: Heat

I configure Kerbal Engineer to show me Critical Thermal % on the HUD, most flights, lets you know how close to boom you are.  The thing is, unlike a space capsule re-entering from an interplanetary trajectory, temperature goes up slowly.  If it starts getting over 90% and you're worried, you can always throttle back or pitch up.    However,  that obviously costs delta V so i'd rather redesign at that point.

The  Partridge was showing heat bars in that video because the Type B nose cones I used to glue the NERVs onto the front wing can only take 2000K.   Heat bars appear at 60% max temp.

If your craft has enough wing it will gain altitude fast enough as speed increases to not blow up, even if you stay on Prograde at full throttle.    If it has too much wing it will tend to bob out of the atmosphere before the airbreathers can hit their top speed.  That's part of the reason why i experimented with "trim flaps" to hold the nose down a bit.

Generally, if you keep all your fuel in wings and strakes ,  don't put the cockpit right at the front, and use a nose cone with 2400K or better heat tolerance, you can't overheat.     The Partridge's fuel is all in its wings, apart from the mk3 fuselage to 2.5m adapter at the front which has LF (but no oxidizer).Speed table?     Mach 5 is ok around 22km in an inline mk1 cockpit.      That's about as fast as you can go airbreathing, RAPIER thrust falls off a cliff after that, if you're able to get much faster you probably are carrying more jet engine weight than you need to.        By mach 6 I'd hope to be above 30km.       Above 35km it's not really possible to overheat,  I've left the throttle wide open before till we were on an escape trajectory from Kerbin with no issues.

Spoiler

 

 

[AeroGav]

6 hours ago, agrasyuk said:

but for some extra fuel saving perhaps consider using staged drop tanks. it's done IRL, since only tanks are dropped I think it still can be considered SSTO. if you keep tanks modest (for your plane perhaps  pair of size 0) you should not have problem lifting them to at least 5000m , basically its like you get to start already in air. 

The only thing with drop tanks is that the fuel is 9x heavier than the tank itself,  so you don't gain that much by chucking it away.   Unlike a rocket, which weighs very little empty, an SSTO has heavy jet engines which make little savings like this less noticeable.   Doubly so if you're carrying nukes at 3 ton each.    Also those size 0 jet fuel tanks have a crummy capacity to drag ratio.   Oscar Bs are 1/3 the drag and 1/3 the length  and hold only 20% less liquid, unfortunately a nuke ship doesn't have a use for the oxidizer they hold.

What would make a difference is putting your jet engines on the back of the drop tanks, and chucking those.

Whilst RAPIERs are expensive (6000 funds) ,  Whiplash are only 2000 or so.   They weigh 1.8 tons each so if you dump them on flameout the rocket stage has an easier time.     The problem of course is to build an airplane whose stability is not affected by the departure of major components like this.

Easiest way I guess is to hang them on nacelles beneath the wings, since that puts them more or less in line with the centre of mass.     Of course that means two Whiplash which is far too much for a little plane like the OP's, but it's good in a bigger design.    

If you're just having a single engine then that means it's going on the back of the fuselage behind the rocket, which either makes your plane tail heavy (unstable) when the jet is attached, or behave like a lawn dart (nose heavy) when it's gone.     You can work around that by attaching a piece of wing to the jet engine that gets shed with it,  so that the presence of the jet stage moves the CoL aft by the same amount it moves CoM, but it's not ideal.

The underwing engine pods mean you don't have to worry about that.   Just remember to offset the wing upwards so that the engine hanging beneath it is at the same height as the plane's centre of mass. that way it doesn't pitch up like crazy when you throttle up. 

The engine nacelle will of course contain a mk1 liquid fuel tank, remember so set that to a higher priority than the fuselage tanks so it's drained first. Remember to enable crossfeed on the decoupler, but I in fact recommend using a small pylon instead of a radial decoupler since they are much cheaper.    On the front you can just go with the basic circular intake like the OPs ship, it's  the lightest, cheapest and lowest drag option.  It won't survive re-entry heat, but doesn't have to.