SSTO Help

[Aethon]

Stock aero v.1.0.5

Ok.  I'm not a plane guy.  I've only built one (plane) SSTO in v 0.90.  

I built this over the past 2 days and many iterations.  It will make orbit, but with almost no leftover fuel.  

I climb out at 30 degrees, breaking mach 1 around 5000m.  At 10,000m I reduce pitch and start building speed, trying to be in a slow climb around 17,000 m where the RAPIERS are in their sweet spot, putting out about 200kn of thrust.  At around 22,000m and 1450m/sec the thrust starts to drop off dramatically and heating begins to become an issue, so I switch modes and raise the nose.  

Should I be milking more speed on airbreathers?  If I keep going on air at 23,000m and slowly pitch up, my speed starts dropping as thrust falls off.  The craft masses just under 20 tons.  I've clipped 2 FL-T100 LFO tanks into the front and back, but they don't seem to make much difference.  I'm guessing I'm up against the tyranny of the rocket equation. 

Do any of the experts see any glaring problems with the craft, or have any good advice for me?

 

Edited December 2, 2015 by Aethon

[Mister Dilsby]

1450 m/s at 22km should be enough speed to make it the rest of the way in closed cycle mode, assuming you're carrying enough LFO. But I think you might get higher and extend your apoapsis a bit more on airbreathing with additional intake air. From what I see you just have the two nacelles--I would start by getting rid of those and replace the two advanced nosecones with shock cones. 

[JTpopcorn]

You also might be able to cut down on drag by reducing your main wing size.  Maybe a tiny bit more speed from airbreathers but you wont get that much more out of it.  If space TWR is good enough ditch the duel rapiers for a single rapier with a wiplash inline (cliped partly into the rapier) Then you cant spin out do to engine out, and you can leave the whiplash on after the rapier switches modes. If you use the short mk2 to mk1 adapter, you will save .2 mass from the engine swap + gain low atmo performance.  Worse case just make the plane longer by adding more fuel.

-JT

[AeroGav]

Rapiers produce max thrust at 1250 m/sec so since the heating problems in 1.05 I no longer see the point in going faster.  Once I hit that speed i use excess thrust to climb, it should be possible to get a slick, 20 tonne craft to at least 25km airbreathing this way on just a single rapier.  I assume you're not using any aerodynamics mods?

 

Looking at that craft

1. You're using engine pre-coolers for intakes - good.  Since 1.05 this is the lowest drag way to feed an engine.

2.  You have RCS blocks - I'd be careful since there are far fewer physicsless parts these days, or they add significantly to the drag of the parent item.  Consider couple of advanced reaction wheels in a cargo bay instead.

3.  It looks like you're using a 2 into 1 adapter.  I'd heard these were real draggy, and best left off.

4. The two mark 1 fuselage pods either side of the main fuselage are extra frontal area, and contribute a total of four drag nodes (2 from the front of each, 2 from the rear). If you don't need them to mount engines, consider putting whatever is there, inline in a longer main fuselage.

5. Tail cones are the lowest drag part to begin or end a fuselage.   You can also clip them to the back of your engines, and offset them so they're not occluded.  A bit gamey but hell, the game messes with us enough times, gotta take your chances when they come.

6.  That tail fin is tiny ! Well, if you can fly it with that, more power to you !

 

I disagree with what was said about making wings smaller,  but it is a matter of preference.  Personally I'd make them even larger.  There's a nice chart someone did in the tutorials section,  lift drag ratio vs angle of attack, basically you get best L/D at 2degrees AoA subsonic and 5 degrees at 2000 m/s.

Edit - 

 

 

The critical phase of your flight is probably the transition to closed cycle mode, when you're at 24km+ and doing 1250 m/s.  The air is very thin at that point so even though you're going mach 3.7 you need quite a lot of wing area to keep your AoA below 4 degrees, otherwise you're incurring unecessary lift induced drag.   Extra wing area makes re entry and landing easier too.  Wings don't cause parasite drag with stock aerodynamics so the only penalty is the mass of the wing itself, which is tiny compared to your engines and fuel.  Also the Big S delta wings and strakes offer the same lift/mass ratio as the other parts, and the same high temperature tolerance, but they also have considerable liquid fuel tankage to boot.

https://drive.google.com/file/d/0BworS5V0CvPcNEU5UDktN3NJY0U/view?usp=sharing

Edit - here's something i built just before 1.05.  It's an attempt to min max the stock aero model.  It does walk a fine line with overheating in 1.05.  It's also the ugliest thing i ever built !  It's got a RAPIER with a NERV clipped into it and a tail cone clipped onto those for minimum drag.   It can get to orbit on NERV and airbreathing RAPIER alone, no oxidizer, in fact it makes it with 30% fuel remaining.     Small amount of wing incidence means you can fly much of the ascent with Prograde set on the SAS.  There's an action group to trigger the NERV, IIRC.

 

Edited December 2, 2015 by AeroGav

[GoSlash27]

I concur with everything AeroGav has said here.

I would also point out that you don't actually need 2 RAPIERS for such a small payload and therefore could build a much more efficient spaceplane with the parts. A single RAPIER and precooler would allow you to eliminate those 2 sponsons on the sides, dramatically reducing your drag.

 Focus on eliminating unnecessary drag instead of adding thrust. If it can get to 360 m/sec, it will make it the rest of the way.

*edit* Also, I don't know what you have clipped into the airframe back near your engines, but be aware that clipping doesn't hide the drag. Whatever that is, it's just as draggy as it would be out in the open.

Best,

-Slashy

 

Edited December 2, 2015 by GoSlash27

[AeroGav]

One  last thing, your ascent profile is very different to how i fly, but then your craft has more engine and less wing, so it may pay to treat it differently. Also the experimentation i did seemed to show that how you fly the first 20km has very little affect on overall fuel use, most of which is down to how much you get out of airbreathing above 20km and how the closed cycle bit is done.       

 

I tend to fly a constant angle of attack rather than shooting for a specific pitch angle, and stay subsonic a lot longer.   Generally I fly up from the runway using pitch trim at 2degrees AOA, making manual inputs to surpress oscillations, until i'm getting to 240m/s at around 10-16km, which is the sound barrier.  Drag doubles at the speed of sound (330 m/s) , so i try to delay going through it, then punch through fast in a shallow dive when it can be put off no longer.   When i'm delaying crossing the SB i add more back stick to keep my speed below 240, until i'm getting to 7 degrees or more AoA, at which point I've gone as high as i can subsonic without lift induced drag killing me, so i release the stick and let it dive through to 430, at which point we're through the drag peak.   Because of heat issues, i also try to not exceed 1000m/s until 20km  or right before it.

[Val]

Most of what you do seems right. The ascent profile sounds good.

You'll probably overheat if you go any faster, airbreathing, with that cockpit, so no need to try. The only nose parts that can survive going faster is Shielded Docking Port and Heat Shields.

I have a few suggestions.

• The clipped LFO tanks are VERY bad for drag. Costs you fuel and takes longer to get up to speed. Remove them.
• Replace the Intercoolers with FL-T200 or 400.
  (You'll still have a comfortably overpowered spaceplane with 15-20 t per RAPIER. The Chibi Skylon in my signature can takeoff, weighing up to 65 t, and get to orbit with just 2 RAPIERs)
• Replace the nose cones with Adjustable Ramp Intake or Shock Cone Intake.
  (Technically you only need one Shock Cone Intake for 2 RAPIERs, but I guess it'd be a hassle to redesign.)
• Give your main wing some Angle of Incidence. That will reduce drag further and make it less likely tailstrike during takeoffs and landings.
  (Use the rotate gizmo in non-snap mode and rotate the wing so the leading edge goes up a little and the trailing edge down. For the wing in your picture you need to lift the leading edge about one wing thickness or two. Hope that makes sense.)

Edited December 3, 2015 by Val
Grammar and spelling

[GoSlash27]

Val,

 I would've agreed with your intake advice in 1.04... but what he's doing is technically correct in 1.05. Engines don't need nearly as much intake area as they used to.

 The precooler is now the magic bullet intake, but only if it's incorporated in a way that doesn't add drag. This design adds drag by employing unnecessary sponsons.

Best,

-Slashy

[Val]

@GoSlash27 I don't disagree, but on such a small plane Intercoolers just take up too much space, in my opinion. My suggestions were intended to have minimal impact on the design, while still improving it's performance.

Edit: Also I just noticed I got ninjaed by 3 posts. Were only 2 replies when I typed my reply.

Edited December 3, 2015 by Val

[GoSlash27]

Val,

 I'd definitely agree with that. The design as it sits would work better with your proposed fixes.

Best,

-Slashy

[Aethon]

Hey great advice all.  Thanks!  

I'll faff about with it again tomorrow, add more fuel and angle the wings.  I thought the precooler was the way to go these days.  I also tried it earlier with just one RAPIER and it just wouldn't go.  Could be operator error though.  I appreciate all the help.

[GoSlash27]

10 hours ago, Aethon said:

Hey great advice all.  Thanks!  

I'll faff about with it again tomorrow, add more fuel and angle the wings.  I thought the precooler was the way to go these days.  I also tried it earlier with just one RAPIER and it just wouldn't go.  Could be operator error though.  I appreciate all the help.

Aethon, The precooler is the way to go, but it has to be used in a way that doesn't add drag. For example, if you already have a fuselage, it costs no drag to stuff a precooler in the stack. But if you add a parallel fuselage just to house a precooler, you have added a lot of new frontal area, which increases drag.

I'm sure the reason your plane wouldn't go on one engine is because it's not streamlined enough. Getting good speed in air breathing mode is all about reducing drag, and this is a pretty draggy design.

Best,

-Slashy

[AeroGav]

22 hours ago, Aethon said:

Hey great advice all.  Thanks!  

I'll faff about with it again tomorrow, add more fuel and angle the wings.  I thought the precooler was the way to go these days.  I also tried it earlier with just one RAPIER and it just wouldn't go.  Could be operator error though.  I appreciate all the help.

I think the reason for angling the wings 2 or three degrees is that your engines can be level, putting all of their thrust into forward lift, while the lifting surfaces are angling up and still making lift.     At optimal AoA you're getting a lift drag ratio of 20:1 so 1 KN of thrust is getting turned into 20+ of lift.  That's a very worthwhile tradeoff.   Without positive incidence,  in order to generate lift, you have to angle the whole ship upwards - that means some of the thrust which could be making forward momentum which would then  be exchanged for lift in a 20:1 ratio, is instead being turned directly into lift on only a 1 for 1 basis, which is a bum deal.     Your spaceplane is mostly type II fuselage , which does generate lift like, though I don't know if it gets the same L/D ratio as proper wings.

One last reason, which I alluded to before, is that you can fly some of the ascent with Prograde hold set on autopilot and it'll still climb.   The "Vark" craft I posted above takes over 20 minutes to reach orbit, this helps to save wrist cramp.

 

Incidentally, I was doing a "low tech spaceplane" for a "no rockets" career mode challenge yesterday.  Made a craft with 10 Junos and 1 swivel that barely made orbit (it jettisoned the Junos when they flame out, so was costlier to operate than a rocket, it also punched off its fixed gear on takeoff and could only land in water.....), then improved it when I unlocked the Terrier.        Finally I wanted to see how far I could take the  thing if I pushed re-usability out the window.  The fuselage consisted of 3 FT400 fuel tanks...  I put a stack separator in behind the forward FT400, and added it's own Terrier.  After the first two rocket tanks ran dry, I'd stage and leave the airplane behind at 33km.   However , it didn't even make orbit.   Did the airframe still help that much even at that altitude?   Or did the extra mass of the heatshield, parachute, second Terrier negate the benefits of dumping two empty FT400s,  wings, tail fin and canards?

 

[GoSlash27]

AeroGav,

"I think the reason for angling the wings 2 or three degrees is that your engines can be level, putting all of their thrust into forward lift, while the lifting surfaces are angling up and still making lift. "

That does play a part, but it's not the main reason. The main reason is that angling the entire fuselage against the airflow greatly increases drag and it's the drag that your engines are fighting to get you to the transition point. Keeping your nose pointed directly prograde reduces the thrust and time required to get hypersonic, which greatly reduces the mass and fuel required, which leaves more payload capacity or DV depending on which you're after.

 Building in wing incidence allows you to generate lift as cleanly as possible.

Best,

-Slashy

 

[AeroGav]

RE: 2 into 1 Adapter

I must admit,  I was just regurgitating advice given previously on this forum, but hadn't tested for myself, besides which the device may have been buffed in last patch , especially given the appearance of the new and somewhat more aero 3 : 1 mark 3 adapter.

I built 2 craft, both had a mark 2 cockpit and short mark 2 rocket fuel fuselage behind it.

One had a mark 2 to mark 1 fuselage adapter, giving one centreline engine attach node,  and three radially mounted mark 1 fuselages as engine pods, for a total of 4 engine attach nodes.

The other used one of those much-maligned 2 into 1 splitters,   and only required two-way instead of 3 way symmetry on the engine pods to achieve the same 4 engine config.

The pods were outfitted with tail cones at the front end, and the "Swivel" chosen as the engine in all configs.   Getting a stable config was a challenge, I had to put mark 1 structural fuselage immediately before each engine to move the CG forward a bit, and also attach a pair of fins to the centreline engine.

Both test vehicles were given 15.8 seconds of fuel,  however the 2 into 1 splitter design had lower dry mass so had to be ballasted with excess oxidizer.

Final launch weight 14602 and14604kg respectively. 

Both carried 362 units of liquid fuel, and had 4 swivels.

The one with 3 pods made it to 23, 543M.

The one with 2 pods and the 2 into 1 splitter reached 24, 144M.

This looks like a narrow win for the splitter after all then?

One thing though,  I don't very often find myself wanting two engines on my design.    

One fuselage mounted motor with two pods is a very flexible config that lets you use two RAPIERs and one NERV   (or two NERV and one RAPIER) on a mark 2 fuselage spaceplane, without getting into asymmetric thrust issues as you bring dissimilar engines online.    I was going to say, NERVs are for spaceplanes with interplanetary intent, a pure low orbit cargo truck or kerbal bus should only have RAPIERs , but looking at the payload mass challenge thread,  folks are using NERV for their final stage there too.    If you use big-s delta wings and strakes, you just tend to end up dragging unused liquid fuel to orbit otherwise....

 

[Aethon]

 

AAAARRRRRG!  So here's where I am now.  Even with the wing rotated, it won't stay on the prograde.

 

 

The last iteration had the short LF tank in front of the docking port but was way short on fuel to make orbit.  It has a pre-cooler inline and everything aft of the docking port slid over it with the offset widget (at this point I don't care about clipping, I just want it to work).  Should I rotate the wing further?  I also tried the shorter version with just one RAPIER and It didn't have enough thrust to make it.

There doesn't seem to be a good way to do this with the MK2 parts.  There isn't really an inline intake that works and isn't too draggy.  At this point I'm going backwards.  The craft in the OP made orbit, though just barely.  

What say ye.

 

 

[GoSlash27]

Aethon, you're going way too slow to be at that altitude. What do you mean when you say "it won't stay on the prograde"?

 If you can fly that high and slow and still be climbing, then you've got way more thrust and lift than you actually need.

 If you tuck the nose down a bit in the lower atmosphere (say around 6km)to get it past 360 m/sec, it should go all "John Cena" on you almost immediately..

Best,

-Slashy

Edited December 5, 2015 by GoSlash27

[Aethon]

Roger that.  I was trying to get to a higher alt before I started my speed run to avoid heating.  

I mean it seems like I have to pull up above the prograde marker too high with the nose to stay climbing.  Then when I switch modes and kick the nose up, the prograde marker stays waaaaay below where the nose is indicated as being.  I'll try it again with better piloting and see what happens.  At this point I just want to smash the darn thing.

[Wanderfound]

22 minutes ago, Aethon said:

Roger that.  I was trying to get to a higher alt before I started my speed run to avoid heating.  

I mean it seems like I have to pull up above the prograde marker too high with the nose to stay climbing.  Then when I switch modes and kick the nose up, the prograde marker stays waaaaay below where the nose is indicated as being.  I'll try it again with better piloting and see what happens.  At this point I just want to smash the darn thing.

Excessive AoA means one of three things:

1) The air is too thin; you're too high for the speed you're doing.

2) Too much wing loading; lose some weight or add some lift.

3) Too much stability. Shift some lift forwards.

 

Edited December 5, 2015 by Wanderfound

[GoSlash27]

56 minutes ago, Aethon said:

Roger that.  I was trying to get to a higher alt before I started my speed run to avoid heating.  

I mean it seems like I have to pull up above the prograde marker too high with the nose to stay climbing.  Then when I switch modes and kick the nose up, the prograde marker stays waaaaay below where the nose is indicated as being.  I'll try it again with better piloting and see what happens.  At this point I just want to smash the darn thing.

Aethon,

You're struggling to keep climbing because you're too slow for the altitude.

 Punch through Mach 1 at a lower altitude with denser air and magical things will happen. More speed means more lift and more thrust, which means more speed.

That speed allows you to climb at a faster rate while keeping a lower pitch and accelerating even faster. You want to ride the effect until it starts petering out around 18 km, then keep a clean profile to get all the speed you can (without overheating) before switching to rockets.

You'll know it when you do it; it's like somebody strapped a booster to you.

Best,

-Slashy

[Aethon]

Well IMHO the above pictured craft should make space.  This time I was at 900m/sec at 14,000m, 1300m/sec at 17,000m, aaaaaannddd ran out of fuel before getting a periapse.  I'm sure it's just poor piloting.  

I'm done with sandbox testing it for a while.  Back to career and rockets, which I'm much better at.  Thanks for all the babysitting guys.

[Val]

@Aethon Try the ascent profile I use on the slightly larger B-1 Polo. It's not super efficient, but it is very simple.

Quote

Ascent Profile

1. Full throttle, SAS on, Stage.
2. Let it use the full length of the runway, then set the nose 5° above horizon.
3. When speed reaches 300-350 m/s lift the nose to 10° above horizon.
4. Don’t touch anything until it reaches 24 km altitude.
5. At 24 km press 1 to switch Rapier Mode
6. At 27 km set the SAS to follow Prograde.
7. Throttle down when AP is above 80 km.
8. Coast to space and circularize.
9. Plan a Rendezvous with your station.

Though since your craft is lighter you should go directly to 10° in step 3, right after take-off, or only 2/3^rd throttle until around 8 km. Maybe both.

My rule of thumb, for thermally vulnerable crafts, is to be around 1000 m/s at 10 km, while being in a 10°-15° ascent.

Hope it helps.

[Aethon]

My rapiers seem to asymmetrically flame out at around 18-19km on air breathing.

[GoSlash27]

10 minutes ago, Aethon said:

My rapiers seem to asymmetrically flame out at around 18-19km on air breathing.

Aethon,

I can't tell from that pic what you're using for intakes. It could be you're still too slow for the altitude (900m/sec is slow, believe it or not) or it could be that you don't have enough intake area to feed 2 RAPIERs.

Best,

-Slashy

[Aethon]

It has a pre-cooler inline and everything aft of the docking port slid over it with the offset widget (at this point I don't care about clipping, I just want it to work).