Help getting a Mk3 SSTO to orbit

[Val]

2 hours ago, Jarin said:

I will definitely have to try the small nosecone thing for rapiers, and see what kind of performance boosts I can wring out of it.

You won't get much out of it if you're already breaking Mach down low. It is only a help for crafts with high drag, that have trouble breaking through the Mach barrier.

 

2 hours ago, AeroGav said:

I'd reckoned on 1 rapier and 1 nuke per 30 tons of launch weight, but the ship was overall lighter than expected.

It is certainly possible to make crafts with that kind of Takeoff weight per RAPIER. It's even possible to go well above 40 t. But in my opinion when you get much above 25 t / RAPIER, time to orbit becomes so painfully long, that it's not worth pursuing, except for the Achievement.

 

8 hours ago, NightshineRecorralis said:

Less wing, but which should I remove?

You really don't need much wing at all, if you're only building crafts for delivery to Kerbin orbit. Just enough that you're able to hold altitude and keep accelerating after you pass the end of the runway.

Angle of Incidence is a good way to reduce how big your wings need to be and reduce drag from the fuselage, at the same time.

Angle of Incidence gives wings Angle of Attack, while the fuselage is pointed prograde (zero angle of attack). Real aircraft have Angle of Incidence, too, for the same reason.

And that is even more important in KSP, because KSP wings don't have the cambered wing shape, that real wings have. KSP wings don't provide lift unless they have Angle of Attack, so if you don't attach wings with Angle of Incidence, then the fuselage will also always have Angle of Attack, which increases drag, especially with Mk2 and Mk3 parts.

Here are some example craft.

Z-2 High-Five

The canard has 5° AoI and the main wing 4°. Lifts an Orange Tank to LKO, with plenty dV to spare to go to higher orbits.

• Propulsion: 4 RAPIERs.
• Max Payload (LKO): 36 t.
• TakeOff weight: 110 t.

S-108 Long

This one has even more AoI. 6° on the forward wing and 5° on the rear wing.

• Propulsion: 16 RAPIERs.
• Max Payload (LKO): 100 t.
• TakeOff weight: 290 t.

You're also very welcome to take inspiration from my crafts.

 

Edited November 10, 2016 by Val
Formatting

[AeroGav]

1 hour ago, Val said:

 

You really don't need much wing at all, if you're only building crafts for delivery to Kerbin orbit. Just enough that you're able to hold altitude and keep accelerating after you pass the end of the runway.

 

 

Is there a formula for how much wing?  I did a little test with a 20ton craft,  one with 2 pairs of big S wings (lift rating 20), one with one pair (lift rating 10), and one with 3 big S wing strakes instead of wings (lift rating 6).  The one with the largest wings lifts off very early,  has trouble keeping the nose down, and won't break the sound barrier at sea level.  It does break it easily at 10km and doesn't have any trouble climbing that high at low speeds.  During the speedrun , I was flying angle of attack less than 2 degrees to stop it climbing, which indicates too much wing and parasite drag.   Yes, heat is a problem, but when things get too hot, you climb higher, level off and try to get more speed.  Climbing higher reduces engine power so once again drag limits how fast you can go air breathing. On the other hand, the large winged craft did have one advantage when i finally fired up the nukes - since we'd hit our air breathing top speed and were now getting most thrust from closed cycle power,  i pitched back up to 5 deg AoA which is best L/D ratio,  and the large wings generated a load of lift and we got above 35km really quickly.  The heat bars disappeared almost straightaway.

The craft with one pair of big S wings could break the sound barrier at sea level but it took a while, and it used slightly less fuel if you climbed to 8km first.

The one with 3 strakes only was a little scary.  It struggled to get airborne before the landing gear overstressed, but obviously went sonic right off the deck.  The amount of wing was about right for the speedrun - holding 3.5 to 4 degrees AoA - and I was able to get  maybe 50 m/s faster as a result.  Because this was accomplished at slightly higher altitude (due to needing less thrust) i was at similar temperature to the big wing aircraft at that point.  However, after starting the closed cycle motors, we climbed sluggishly, I was still below 29km when passing 1800 m/s and ended up pitching up to less optimal angles trying to get out of the atmosphere before the cockpit exploded.   Made it to orbit glowing yellow.    We did have 10% more delta V in orbit than the biggest winged craft,  mainly because the ship was lighter for same fuel load.  

Bear in mind I only flew each launch once and started with the biggest winged aircraft and did the smallest last, so my technique may have been improving.  I think the gains from fitting the minimum possible amount of wing are modest, getting the engine mix right and eliminating the other forms of drag does most for you.  You also have to consider what happens if you screw up re-entry and end up coming down away from KSC - will you be able to  put down on hilly terrain without damaging it, will the crew even survive 

Quote

You won't get much out of it if you're already breaking Mach down low. It is only a help for crafts with high drag, that have trouble breaking through the Mach barrier.

I was able to get peak :L/D ratio up from about 2 to nearly 5 on the OP's craft by fitting cones and by reducing the trim drag.   It could break the sound barrier anyway but it can't hurt?

Spoiler

1.  created a T-tail by putting big-s wings horizontally attached to vertical stab.   moves CoL aft and gives us more LF capacity, brings the dry CoM closer to the full one

2. moved the tailplanes to the front as canards

3. replaced some of the strakes with big -s ones for extra LF

4. dihedral on the outboard main wing, on the t-tail and the canards, for a bit of passive roll stability

5. replaced the ailerons with the biggest ones available (big S) and then turned their authority down to 36%.  Why?  Because a big aileron operating at a lower deflection angle, generates less adverse yaw than a small one operating at 100% when trying to pick a wing up

6. cones on the back of the engines for lower drags

7. reaction wheels...   cuts down on some of our cargo space sadly.   also batteries went inside in the name of streamlining

https://kerbalx.com/AeroGav/Kendrel2

 

These were fairly easy changes to make. It wasn't a full redesign, nor did it involve taking all the wings off and re-attaching them at an angle, which would require then straightening out the engine nacelles and landing gear afterward.  

I just moved the shuttle fins to the front, replaced the basic swept wing tailplane with a big S version, replaced the nonfuel strakes with big S ones, swapped the ailerons.  And some dihedral on the outboard wing segments .  Nothing too close to the root in the part tree,

Edited November 10, 2016 by AeroGav

[Val]

2 hours ago, AeroGav said:

Is there a formula for how much wing?  I did a little test with a 20ton craft,  one with 2 pairs of big S wings (lift rating 20), one with one pair (lift rating 10), and one with 3 big S wing strakes instead of wings (lift rating 6).

For LKO SSTOs, my rule of thumb is that wing area should be between 1:10 and 1:5 the mass in ton.

So I recommend a 20 t craft to have wing area between 2 and 4. That is enough to get it airborne and enough to land at around 60 m/s, when coming back empty. Slow enough for easy runway landings, and not too hard to land in terrain.

But even for my Duna landing capable craft I had only 1:3 ratio. It needs updating for 1.2.x, though. Gear is too small, and the nose needs to be more pointy.

Duna landing video

[AeroGav]

Like the Duna landing vid.   Interesting you manage to touch down at such high pitch without tumbling forward.    Is this because the main landing gear legs - the first ones to touch - are close to CG?

As regards putting incidence on the wings, i think it works best with such high wing loading values as you are recommending.

The drawback of incidence is that if you are ever in a situation where the airplane needs less than 5 degrees AoA to fly,  one of two things can happen

• the plane starts generating more than 1g and the flight path arcs upwards into an unwanted climb
• you end up with the fuselage at negative aoa and the wings at a slight positive.   This means you are no longer avoiding fuselage drag and worse, the fuselage is creating negative body lift

With small wings however these situations should be rare.

My method of going through the sound barrier on larger winged aircraft involves climbing up at 240 m/s to approx 8km then either setting prograde or drastically reducing  AoA with trim adjustment.   AoA on both the body and fuselage drop to near zero and we have the lowest possible drag while overcoming the transonic region.  Flight path arcs level at about 10km and falls to 7 or 8km, but by that point we're well supersonic and ready to pull up.  Obviously with incidence you cannot get the AoA of both wing and body to zero,  but then on the small winged craft you recommend, you wouldn't need to shallow dive through the sound barrier, you 'd do it in level flight right after takeoff.

However I  do wonder what happens if you're pushing the limits of TWR, which ultimately needs the least power to get supersonic.

One other thing is the handling issues that incidence creates.  When I've used it, I often find that Prograde is no longer really Prograde.  The autopilot gets confused and flies the plane at -1.5degree AoA, which is worst of all worlds - even though there is sufficient elevator authority to pitch up to 0.

I've found I can dial this out a bit by moving the CoL slightly forward of CG (wing incidence seems to confuse the SPH indicators)  and by setting more incidence on the canards than on the main wing.  My "Badger" design uses incidence and it took a few iterations to tune properly.

OTOH, fitting small wings avoids the major problem in the stock game.   It's really hard to come up with something aesthetically pleasing when you're cobbling together multiple Big S wing panels.   If you can make do with a single pair or at most, a back-to-back rhombus shape, things are much easier !

[Val]

1 hour ago, AeroGav said:

Interesting you manage to touch down at such high pitch without tumbling forward.    Is this because the main landing gear legs - the first ones to touch - are close to CG?

Yes, when they are right below CG they don't produce torque when touching down.

1 hour ago, AeroGav said:

The drawback of incidence is that if you are ever in a situation where the airplane needs less than 5 degrees AoA to fly

It is possible and necessary to make incidence less than 5°. Especially for regular aircraft and low wing loading. My aircraft in KSP often only have 1-2° AoI.

In my experience, 1° Angle of Incidence is always better than none, no matter how big your wings are.

Just use rotate gizmo without snap or Part Angle Display.

 

1 hour ago, AeroGav said:

I've found I can dial this out a bit by moving the CoL slightly forward of CG (wing incidence seems to confuse the SPH indicators)  and by setting more incidence on the canards than on the main wing.

It's not that the CoL indicator that is confused. It is doing exactly what you are telling it to. You are increasing lift on the canard, so therefore the CoL moves towards it.

Actually that is exactly what control surfaces are meant to do. Move CoL.

You move the CoL forward, to pitch up, back to pitch down, and left to roll right and so on. Because that is how you control a craft, both in KSP and in real life.

Though in real life moving CoM instead of CoL is also used on really small craft.

I have a thread with more details on Stability, CoL, CoM and Angle of Incidence uses here: 

 

[AeroGav]

6 minutes ago, Val said:

 

You move the CoL forward, to pitch up, back to pitch down, and left to roll right and so on. Because that is how you control a craft, both in KSP and in real life. Though in real life moving CoM instead of CoL is also used on really small craft.

 

 

 

What i meant was,    that when the wings have incidence, the CoM / CoL display gets slightly confused,  if you position the CoL as far behind CoM as you would normally (without incidence) then the craft is excessively nose heavy and prograde assist no longer gives 0 aoa on the body.   

[Val]

3 minutes ago, AeroGav said:

What i meant was,    that when the wings have incidence, the CoM / CoL display gets slightly confused,  if you position the CoL as far behind CoM as you would normally (without incidence) then the craft is excessively nose heavy

No, it's not confused. It's Center of Lift. It shows where the combined lift forces apply. When that point is behind CoM, then yes. The craft becomes very nose heavy.

Especially if you've already given your canard more AoI than the main wing. Because that equates to having built in elevator up. When you then put CoL behind CoM again, then it needs to deflect even harder.

You have to remeber. CoL is only behind CoM when the craft is pitching down. In level flight CoL is always right on top of CoM.

The reason KSP "encourages" you to design your craft with CoL behind CoM is because, CoL is our only indication of  Aerodynamic Center (AC). And it's not even a good one.

When you design a normal craft with flat wings (no AoI) and CoL behind CoM then, most often, AC ends up behind CoM as well, making it a stable craft.

The price you pay for that stability is that the craft always has to fly with elevator up, to maintain level flight or a certain pitch. (trim or SAS)

When you add Angle of Incidence into the mix, 2 things can happen.

1. If you're using Mk2 or Mk3 parts, then they're now affecting CoL less than the wings, because the wing parts with AoI produce more lift in the simulation that calculates the CoL.
2. You can now move CoL around without affecting stability.
   • The Aerodynamic Center is defined by the shape of the crafts outline, when viewed from above or below. Changing the angle of the wings or canards a few degrees doesn't change the overall shape enough to change stability.
   • Changing the angle of wings and canards, does however affect the CoL a lot. By giving forward wing parts slightly more AoI you can move the CoL up to CoM during design phase and get a craft that can fly pretty much without trim or SAS.

Of course AoI is not without some issues. Lift is not linear, which means lift and drag increases faster on high AoI wings than low. This means a craft with higher AoI on forward wings will experience a CoL that moves forward as speed increases. It doesn't affect stability, but it does mean the nose will try to push up as speed increases. Though, in my experience, this can be kept manageable by not exceeding 2° AoI difference between fore and aft parts.

When I build craft I usually start by with 0° AoI and CoL behind CoM. Only once it's nearing flight testing phase, do I add AoI to the wings.

For SSTOs, I uaually adjust the AoI so the craft can hold steady altitude at around 400 m/s at sea level with the fuselage at 0° pitch on the navball. Though for higher TWR builds (18 t / RAPIER) I might go as high as 500 m/s before it starts climbing at 0° pitch.

 

3 minutes ago, AeroGav said:

and prograde assist no longer gives 0 aoa on the body.

I've not noticed anything like this.