SSTOs to and from Laythe

[bewing]

2 hours ago, MitchS said:

I was wondering if the LF nosecones ahead of the in-line Mk1 fuel tanks (and their accompanying canards) were causing too much drag too far forward.

No, those NCS adapters have a small fraction of the drag of your main fuselage. MK2 parts have triple the drag of MK1 parts, so if you want to find the drag, look there. But your fuselage is well-designed, so it's best to stick with it.

2 hours ago, MitchS said:

I can't really tell where my CoP is, or where my CoL is when body lift is taken into account (which, why would it ever NOT be, Squad?!)

Because it's non-linear with speed, so you can't place a dot without knowing the exact speed. So it would need graphs and printouts and analysis -- and can't be nicely displayed in the editor.

2 hours ago, MitchS said:

Also, isn't one of the symptoms of my craft that it's overmaneuverable as it is?

Not exactly. One thing you need to understand is that pretty much all planes will swap ends on you if you push them too far off prograde. All planes have a region around prograde where they will fly straight. So it's not that yours is overmaneuverable -- it's that your dynamic stability region is too small. All you need to do are some tricks to add a bit of stability, to make that zone bigger. (Yes, there are designs that are inherently stable, but I don't think that is a smart design goal, considering the tradeoffs.)

With your CoM and CoL right on top of each other, you are guaranteed to have a plane that is extremely sensitive to the controls. So that's good for SAS, which can use fine control deflections to steer your plane. So it doesn't need to do much work (and create a lot of drag) to keep you flying straight. But it's bad for manual control -- especially if you are using a keyboard like me, where all you have is either 0 or max deflection (turning on soft-control mode helps, of course).

2 hours ago, MitchS said:

Why do you think they don't give enough bang for the buck?

I usually find that they hit max deflection while I'm still within my stability region -- so I could have remained stable if the darn things would just respond more. Maybe I should play with my authority slider more. *shrug*

2 hours ago, MitchS said:

I would normally place them farther forward too, but I was trying to limit the torque moment of their drag at high AoA

Understood. Maybe turn the authority up to 150%, instead, though. During reentry, you may be wise to pump fuel forward to enhance high-AoA stability. Move the CoM, rather than the canards.

2 hours ago, MitchS said:

What does the rest of the community think about it as far as you know? Are you alone in your opinions, or am I in the minority?

I don't think there are enough hardcore spaceplane designers here to form a statistical base. So you're stuck with individual opinions that you have to try one by one.  But basically, to get optimal performance out of your spaceplane, you launch it with 0 degrees of wing incidence. Then you see how far your nose is above prograde, on average, while you climb -- and especially during your speedrun, which is when it is the most vital. During my check, it looked like it was 2 degrees to 5 degrees, with an average of 3 -- and it was about 2 degrees during the speedrun. So 5 degrees of incidence would put you nosedown during the speedrun, which is bad.

But in any case, you were complaining about how the plane assumed a nosedown attitude with SAS off, so wing incidence is just one more thing you need to mess with to see if it's causing you trouble. Like I said in my above post, I'm not sure if each of my tweaks was necessary or not -- but it seemed like something to check out.

2 hours ago, MitchS said:

Did you notice that there are two Big-S wings, one above the other, with the bottom wingtip clipped gently into the top? I DO have double the wing already.

Nope. I failed to notice that from the pic. So that's OK.

2 hours ago, MitchS said:

 ...I've noticed other SSTO guys stacking wings, and while I was a little opposed to it at first, I feel I can justify it because I'd be carefully designing the profile of the wing on a real spaceplane in a way that a single board-wing on KSP doesn't really allow or reflect. Does KSP aero dislike stacked, gently clipped wings?

KSP is just fine about wing clipping. Just so long as it's only wings. The amount of wing area you get from unclipped wings is unreasonably small, most people agree. Doubling it up makes it come out about right, visually, I think.

2 hours ago, MitchS said:

Okay. I'll remove some oxidizer and try to accommodate a shorter rocket burn in my already-nearly-underpowered-but-otherwise-efficient ascent profile. Probably should have done that to start with...  

I think you'll find it bumps your orbital deltaV up quite a few notches. Yeah, it takes longer to get to orbit, but it's worth it. If you rush an SSTO spaceplane to orbit, you'll be sorry.

 

[AeroGav]

@MitchS Your flight profile for going supersonic sounds bang on,  as is the 22km speedrun.   Personally I'd start the nukes before switching modes on the RApiers however,  that many enable you to carry less Ox and not run closed cycle mode for so long.    Or,  leave the Rapiers running  longer in air breathing mode to milk the last little bit of 3200 ISP thrust.  On three engine ships , if i switch mode below 29km, it's on the middle engine only, since that gives me 180kn of rocket power which ought to  be plenty.

Completely agree with @bewing about using a proper bicoupler though, though i like what you did to offset the nuke forwards from  a cosmetic and CoG standpoint.

As regards moving the batteries and reaction wheels to the back,  I can see why you'd want to leave them fwd.    The engines are at the rear and very heavy, you don't want anything that increases their distance from CG.  Putting this stuff behind the cockpit etc. pushes the cockpit further forward and gives it more lever arm to offset the engine mass.

RCS Build aid is always nagging me to add more fuel to the rear of my planes to match all the stuff going at the front (otherwise CG will shift as fuel burns off). So the limited space between CoM and the engines gets used exclusively for fuel.

As for the handling problem you're actually asking about, i'd rather not bombard with advice without having actually flown your ship.  Other than Mechjeb going HAL2001 on you , i was slightly concerned to see the CoL moving forwards with increasing pitch.  Ideally it should be going in the other direction.   Remember your first spaceplane thread, where me and Slashy got into a spat over canard stability?

I recommended on a plane with incidence, to use the coarse tool to angle everything (canards, strakes, main wing, tailplane) at 5 degree, then to use the fine rotation tool to add an extra degree or two to the canard and to take a little incidence off the tailplane.

That way, at high angles of attack, the front of the plane gets into diminishing returns first with further increases of aoa,  and the rear of the plane will gain lift faster.  Puts a soft limit on max aoa.     Have a play with this thing to see how it feels

https://kerbalx.com/AeroGav/Firefox-II

I strongly recommend the mods  CorrectCoL and RCS build aid.

[MitchS]

@bewing,

"All you need to do are some tricks to add a bit of stability, to make that zone bigger."
Interesting... to increase that zone of stability, that means just moving the CoP further back, right? I'd never thought about that stability zone before, because it was large enough not to matter on most of my previous spaceplanes.

"...launch it with 0 degrees of wing incidence. Then you see how far your nose is above prograde, on average, while you climb -- and especially during your speedrun..."
WOW. How did I not think of this?! Of COURSE 5 degrees incidence isn't going to be one-size-fits-all! Stupid of me to overlook. I love this advice, and can't wait to apply it. That may be a core flaw with this design. Will build, test, and report back on this one with enthusiasm.

"If you rush an SSTO spaceplane to orbit, you'll be sorry."
At 23 minutes to orbit, this is one of my slowest designs that I've considered passing off as complete. Hahaha! With previous SSTOs, if I took more than 20m on a launch, it usually meant I screwed up my ascent profile and (usually) barely survived the heating of the end of the speed run. Hahaha. This one is... well, it's heavy for its size, and has close to minimum allowable engine count as far as my tests have shown. It brings a lot of delta v to orbit, but I think the process to get it there is anything but rushed!

[MitchS]

@AeroGav,

"Personally I'd start the nukes before switching modes on the RAPIERs..."
I do! But once the combined thrust of airbreathing engines and nukes starts to equal the backward force of drag... well, doesn't matter the Isp, because you aren't going to reach orbit.  That's when I toggle modes.

"though I like what you did to offset the nuke forwards from  a cosmetic and CoG standpoint."
Thanks... compliments on aesthetic choices mean a lot on things you spend so many hours designing! hahaha. Too bad it probably doomed the whole design. Ah, well. Looked nice while it lasted.

"I was slightly concerned to see the CoL moving forwards with increasing pitch."
I think you may have misread it, or I may have mistyped it.... It moves forward with increasing atmospheric density to a point, i.e., as I descend from the tenuous upper atmosphere, and remains the same from, oh, hypersonic at 30km down to sea level. Which... makes sense, I think... right? Because CoL moves backward as aircraft exceed supersonic speeds? I guess I actually have no idea how low-pressure aerodynamics differ from the more familiar stuff. Huh.

"I'd rather not bombard with advice without having actually flown your ship."
I know, I know, I'm itching to get home in an hour and get that to you guys! First priority when I get home.


 

[bewing]

39 minutes ago, MitchS said:

Interesting... to increase that zone of stability, that means just moving the CoP further back, right?

Well, I'd say it more involves moving drag from the front to the rear, or increasing drag at the rear.

39 minutes ago, MitchS said:

but I think the process to get it there is anything but rushed!

You can always spend another 15 minutes in the atmosphere, going halfway around Kerbin again, before climbing to the Ap and circularizing.

 

[MitchS]

Here it is!!

www.wrongcraftfile.com

That should be totally stock. Just removed some scanners, lights, and KER unit from the front cargo bay. Shoot, KML just revealed two hidden omni lights that I missed. Stand by.

Okay, HERE it is.
https://www.dropbox.com/s/q7qzf5lyfvq4ixb/SSTL Mk_ I Stock.craft?dl=0 

Edited January 11, 2017 by MitchS

[MitchS]

...I switched out that rear tank for a bicoupler, and rebalanced CoL and CoM (adjusting draggy parts rearward as applicable), and have SSTL Mk.1 v2 up and flying. It behaved nicely on takeoff and in the climb, although I did use about 95% of the oxidizer I was carrying (didn't reduce any of that yet...) because there's a point around 30km where I just start to catch up to my apoapsis and, despite still accelerating, can't raise my prograde marker even with up to 5 degrees AoA. For that section of flight, oxidizer has been pretty crucial for me. Always has been, for just about all my SSTOs. I think I may be messing up some post-speedrun aspect of my ascent profile. Any advice?

(Note: Ascent was better than Mk.1 v1's ascents. Before, it was taking 17-20m powered flight, orbit at 27m with 3200m/s dv left. This time, it took 15m powered flight, orbit at 24m with 3500m/s dv left. I think that rear fuel tank and clipped nuke setup was clearly a major drag culprit!)

Unfortunately, the reentry problems weren't solved. It was MUCH more stable on reentry, and even held a 20, 40, 50, and even 60 degree AoA in the mid-upper atmosphere just fine. However, as I was having troubles bleeding off enough speed to set up for final approach to KSC at around 10km, I rolled 180 degrees and aimed for 10 degrees AoA via SmartASS (not an uncommon maneuver, and one which my SSTOs have all handled just as well as any right-side-up manuever) and the whole thing started spinning again like last time. I recovered at 6km and 300m/s, and landed uneventfully.

It seems like it still has a rather small stability zone around prograde, although it's miles better than its predecessor. I can feel it starting to depart controlled flight when I maneuver.

...It just occurred to me that maybe it doesn't want to change its prograde vector when I pull back on the stick because it weighs so much, all loaded down with fuel. High inertia. If it behaves well after I drain out 80% of the fuel, I'd be prepared to say mystery solved. This is definitely the heaviest craft I've ever reentered on so little wing, because it's still full of Jool/Kerbin insertion burn fuel when I run my tests! Bad testing discipline. Shame on me.

Here's v2's craft file... Has KER, SCANsat, and omnilights mod parts. https://www.dropbox.com/s/wc6b20y56cvqvfc/SSTL Mk_ I.craft?dl=0

Edited January 12, 2017 by MitchS

[bewing]

Yup, you have to fly heavy planes more gently -- or they have little tantrums and throw their wings off, or give other signs that they are annoyed.

[AeroGav]

Alright, this is is really simple.  CorrectCoL shows the plane is not stable from the moment it takes off.

Hand flying it, negative pitch stability is also obvious.   On rotation,  i push the up key till we have 10 degrees of pitch.  The nose then continues to rise by itself, albeit slowly, until we're going backwards.  The main wings  simply need to slide backwards a bit.  Also there isn't enough vertcial stabilizer area in my opinion, that's the next problem i hit when correcting CoL.

Also you want 1.25m cones on the back of those engines to reduce drag.  And replace the rocket fuel fuselage with a short lf fuselage.  And get a conventional bicoupler on the back, but offset the nukes forward as it looks nice & helps balance.

 

Edited January 12, 2017 by AeroGav

[MitchS]

Correct COL isn't on CKAN... I'm out with friends right now, but I'll check to see if I can add the mod tonight. 

Why is there a discrepancy between stock Col and correct Col, exactly? The lift from the mk2 fuselages? 

... Cones on the backs of engines? Like, in the exhaust? 

[fourfa]

Attach a cone (standard, advanced, or circular intake) to the back of the engines (nukes and RAPIERs).  A shroud fairing will appear.  Right click, disable shroud.  Press 2 to bring up the offset gizmo.  Click on the cone, adjust it several notches toward the front of the ship until it's no longer visible.  Now, it won't interfere with the exhaust, it's invisible, it greatly reduces the drag experienced by the ship, and generally makes the SSTO envelope broader and more usable.  The costs are kerbucks, a bit of mass, and part count, and a little slice of your soul.  Overall it's a worthy trade

[bewing]

While fourfa makes a compelling case, I tend to avoid doing it. Too few slices of my soul remaining to spend them lightly.

 

[MitchS]

Actually, I'm not having much trouble finding the justification for that. The game makes open nodes extremely draggy, and I believe only a fraction of that modeled drag would be generated at the back of an engine nozzle in real life. Personally, I don't mind using cheats or tricks when the problem I'm facing is the game's shortcoming, and in this case, it seems like that open-node drag on SSTOs is a side effect of the way the game allows engine coupling in staged rockets. A happier solution would be to "disable node" on engines you don't intend to attach to, but I'm not sure fourfa's solution is really that much different. I'd like to see the numbers, though... if it makes it too much better than it ought to be in real life, I'd probably put it in the "that's not how I like to KSP" strategy pile next to the "MOAR BOOSTERZ" picket signs. Hahaha

@AeroGav, is that static stability analysis the Correct CoL mod you've been referencing? Looks like FAR, without the revised aero model.

[Spricigo]

24 minutes ago, MitchS said:

Actually, I'm not having much trouble finding the justification for that. The game makes open nodes extremely draggy, and I believe only a fraction of that modeled drag would be generated at the back of an engine nozzle in real life. Personally, I don't mind using cheats or tricks when the problem I'm facing is the game's shortcoming, and in this case, it seems like that open-node drag on SSTOs is a side effect of the way the game allows engine coupling in staged rockets. A happier solution would be to "disable node" on engines you don't intend to attach to, but I'm not sure fourfa's solution is really that much different. I'd like to see the numbers, though... if it makes it too much better than it ought to be in real life, I'd probably put it in the "that's not how I like to KSP" strategy pile next to the "MOAR BOOSTERZ" picket signs. Hahaha

@AeroGav, is that static stability analysis the Correct CoL mod you've been referencing? Looks like FAR, without the revised aero model.

Actually the idea of disabling node seems like easy for someone with basic moding skills. That its not my case unfortunately, but from what I read in the sub-forum "add-on discussion" one would take, say the Whesley and setup the parameter to perform, and require the same tech, as the rapier. Maybe even easier its to to remove the node.

Maybe ask a question about it in the add-on discussion sub-forum,  an one-part-mod to 'fix' a recurring issue you have may be worthy the trouble. Or if it bother other people may even be available

[AeroGav]

https://kerbalx.com/AeroGav/SSTL-gavd

quick mod between work hours.    Shifted wings back as CorrectCoL was asking.   Then added strake tailplanes because it wouldn't stop sideslipping.  Then i thought why have tailplanes they are pushing you into ground when trying to flare for landing at lowest possible speed, so removed them and that gave me room to  push wings back more.   Now it hasn't got enough pitch authority...   added more strakes + canards.   also did the "washout " thing, where slightly more incidence at front than rear.   Cones on engines.   Proper bicoupler for tail.  Got rid of the fuselage oxidizer tanks apart from what's in the bicoupler and the mk1 adapter ahead of cokpits.

hands off it settles into 2 or 3 aoa.  Full nose up gets about 12 degree aoa, full nose down gives you -8.     Too stable and hard to manuver do you think?  At least you can't tailstrike on landing?

Edited January 12, 2017 by AeroGav

[Wanderfound]

22 hours ago, MitchS said:

Here it is!!

www.wrongcraftfile.com

That should be totally stock. Just removed some scanners, lights, and KER unit from the front cargo bay. Shoot, KML just revealed two hidden omni lights that I missed. Stand by.

Okay, HERE it is.
https://www.dropbox.com/s/q7qzf5lyfvq4ixb/SSTL Mk_ I Stock.craft?dl=0 

Still not opening in Stock KSP, unfortunately. Perhaps make a no-mod copy of the game on your own PC (just Ctrl-C/Ctrl-V the game folder and then scrub everything except Squad from the Gamedata folder in the copy) and check to see if you can open it in that?

[MitchS]

@AeroGav,
"Then I thought why have tailplanes they are pushing you into ground when trying to flare for landing at lowest possible speed"
I'm not sure I'm following this logic?

@Wanderfound,
Really? It still won't open?

I'm at work, and won't have access to my laptop all weekend unfortunately, so I'm afraid I can't do that. But I DID convince the work computer to download KML, so I pulled up that craft file and look through the parts. There aren't any mod parts at all. I don't understand why it wouldn't be opening. I'll post pictures of the list below... Any ideas?

Edited January 12, 2017 by MitchS

[Wanderfound]

4 hours ago, MitchS said:

@Wanderfound,
Really? It still won't open?

I'm at work, and won't have access to my laptop all weekend unfortunately, so I'm afraid I can't do that. But I DID convince the work computer to download KML, so I pulled up that craft file and look through the parts. There aren't any mod parts at all. I don't understand why it wouldn't be opening. I'll post pictures of the list below... Any ideas?
 

Could be at my end; updating the patches on my KSP to check.

 

EDIT: yup, it was me. Sorry for the false alarm.

In compensation, see https://www.dropbox.com/s/3brwod8hpjchjhu/SSTL Mk_ I Stock Kerbomod.craft?dl=0 for a work-in-progress variant. Mod-free download; Mechjeb unit visible in the pic for testing purposes.

Edited January 13, 2017 by Wanderfound

[Wanderfound]

First impressions:

* You need way more power generation to run that ISRU

* You don't need that much wing

* Although it'll work with two and two, it'd be a lot easier to fly with more jet and less nuke.

[AeroGav]

5 hours ago, MitchS said:

@AeroGav,
"Then I thought why have tailplanes they are pushing you into ground when trying to flare for landing at lowest possible speed"
I'm not sure I'm following this logic?

 

If a low stalling speed is a design criteria, and it might be, given you want to land off the runway on hilly ground, then bear in mind what happens at minimum speed.

You will be pitched up to high aoa for max lift,  but to get that nose up your tailplanes are pushing the nose down hard.  They get the nose up by pushing the tail down.   With the CoM near the back of the ship (often is, because of engine mass) then you need a lot of downforce to get the nose up, due to short lever arm.  This all subtracts from total lift,  the canards are adding to it in the same situation.

In your case i don't know if i reduced stalling speed any,  due to not being able to make more than 10-12 degrees of pitch in the mod layout.

On this one,  it has a problem with not taking off in 1.2.2  https://kerbalx.com/Val/SSTO-XS-160-Olympus-Mk3

It has a front wing with elevons (basically a canard ) but was only set for controlling roll.   Pitch control was entirely done from the back wing and elevons (by pushing down).    By swapping control assignments around, i was able to lower stall speed enough to get it off the runway.  It is no less stable, because it already had the canard and front wing "destabilizing" the ship, they were already there.

[bewing]

11 minutes ago, Wanderfound said:

First impressions:

* You need way more power generation to run that ISRU

Just run it in bursts. But an RTG or two would sure be handy out around Jool.

11 minutes ago, Wanderfound said:

* You don't need that much wing

Yes you do. Especially on Laythe.

11 minutes ago, Wanderfound said:

* Although it'll work with two and two, it'd be a lot easier to fly with more jet and less nuke.

Jets are deadweight for 99% of your trip. So I think that would be a bad tradeoff.

[AeroGav]

32 minutes ago, Wanderfound said:

 

* Although it'll work with two and two, it'd be a lot easier to fly with more jet and less nuke.

Plenty of nuke can be good though, especially above 20km.   Climb to 20km is slower due to less jet twr, but above it you got more baseline power that doesn't suddenly quit when the oxidizer runs out.

[MitchS]

Busy with real life for the night, but wanted to quickly comment that I haven't fully fleshed out the ISRU section yet, and will have plenty of power generation for it! Just wanted to get the important plane parts designed first. 

[Wanderfound]

1 hour ago, bewing said:

Just run it in bursts. But an RTG or two would sure be handy out around Jool.

Yes you do. Especially on Laythe.

Jets are deadweight for 99% of your trip. So I think that would be a bad tradeoff.

Sufficient wings on Laythe:

 

Switching the ISRU on and off constantly is a nightmare; you're going to have to mine for months at a time. I'd strongly recommend a couple of Fuel Cell Arrays. A single RTG might be useful if you're prone to forgetting to extend your solar, but it won't make a significant contribution to ISRU power.

As I said, it will work with two jets. But it'll be a slow and painful climb to orbit, and you'll arrive with near empty tanks.

If you get a bit more jet thrust, you can make a more efficient ascent that puts you in orbit with enough fuel left to reach Minmus.

1 hour ago, AeroGav said:

Plenty of nuke can be good though, especially above 20km.   Climb to 20km is slower due to less jet twr, but above it you got more baseline power that doesn't suddenly quit when the oxidizer runs out.

I usually try to make my final ascent into a bit of a zoom climb; gain speed to the edge of toasting at 15,000m, pull up just enough so that the apoapsis breaks 30,000m before the jets lose puff, add the nukes as soon as you're over 20,000m and keep the jets in open cycle until they choke at 30,000m. Pop the apoapsis over 70,000m on closed cycle, then shut down the RAPIERs and keep the nose prograde and the nukes burning until you've circularised.

Two nukes gives more room for error on the final ascent, and makes interplanetary burns less tedious, but there's a big weight penalty.

Edited January 13, 2017 by Wanderfound

[AeroGav]

16 hours ago, Wanderfound said:

Switching the ISRU on and off constantly is a nightmare; you're going to have to mine for months at a time. I'd strongly recommend a couple of Fuel Cell Arrays. A single RTG might be useful if you're prone to forgetting to extend your solar, but it won't make a significant contribution to ISRU power.

Agree on the IRSU.    A small drill with a large converter will run fine off 4 RTGs - slow, but you can just leave in high time warp while you run the kettle.  Solar panels are a mare, you have to restart the operation every morning at sunrise , for months.

The smaller converter uses (a lot) more ore to make 1 unit of fuel, but uses less electricity per unit of ore consumed, so you may get away with less RTG.

Quote

 

As I said, it will work with two jets. But it'll be a slow and painful climb to orbit, and you'll arrive with near empty tanks.

If you get a bit more jet thrust, you can make a more efficient ascent that puts you in orbit with enough fuel left to reach Minmus.

 

This is a repeat of another debate that was had elsewhere. 

 

Power falls off a cliff beyond a certain speed.  If you can get to mach 1 you can theoretically get to mach 5.5,  more engines will speed up the time to get there, but no amount of 2 ton rapier engines will take you past mach 6 air breathing.

As for empty tanks - well it depends on your flight profile and design of ship.  Your design and profile works for you so stick with it.   Spaceplanes need either a high TWR or a good lift : drag ratio to avoid gravity losses,  yours has good TWR.  Increasing either adds to dry mass and reduces fuel fraction.

At the other extreme is my first seaplane (which is also an ssto), 2 nukes  1 rapier > 37 tons.   Takes 20 minutes to get to 1400m/s but only another 4 to reach orbital velocity once the nukes kick in.  As you can see from the video it's hardly a stressful process (set the pitch trim so it holds itself at 5 deg AoA and wait), and we've got loads of fuel left.    The acceleration to mach 5 is pretty slow of course compared with yours.   If you're making a journey of a lifetime it might be worth the wait, if you just want to test the RCS system i admit to using the cheat menu.