SSTO Help

[AeroGav]

In my opinion you're getting further away from a solution.      

The X wing layout seems fine. 

Still no sign that you're using RCS build aid to help with dry CoM - you need to save human intelligence for problems humans are best at (where to put stuff) and let this tool take care of the problem that can be automated.

Fuel tanks - I thought you'd kind of agreed to a short rocket fuel fuselage ahead of cargo bay,  then behind the cargo,  a mk3 to 2.5m adapter feeding into a triple mount ?  This keeps approx the same amount of fuel fore and aft.

Engines - 3 panthers at the rear of main fuselage will lighten back end of the ship.       In between your pair of X wings, mount the rapiers on mk1 size nacelles.   These will be more or less in the centre of mass so their weight won't cause as many issues.

The installation you have at the moment makes for a heavy back end one the front fuel tanks are empty,  and also creates a lot of drag.    Did you see the bit i wrote about unfilled attachment nodes causing flat plate drag,   and how mismatched nodes (1.25m engine directly on a 2.5m mount, with no adapter beteeen) are just as bad?

[MisterKerman]

@AeroGav

I know I know. I understand how frustrating it is to give sound advice and the person who needs help not take it. I'm not ignoring you. (Although I TOTALLY forgot about the 2.5m engine node drag issue!)

Let me explain; I was just playing the game because I was having fun. That was about the third iteration I had come up with straying further from the path. I had randomly created a much more compact variant closer to what was pseudo imagined in this topic which was balanced in all stages of flight, but it ran out of fuel nearly immediately. I wanted to salvage the wings I made so my strategy was to add a bunch of weight around the CoM to make the fuel offload shift less and got carried away trying to make it look right because the wings appeared to be too far forward and it's not the silhouette I was hoping for so I was trying to partition fuel in parts forward of the CoM in order to provide ballast to counteract what would eventually turn into a shift of mass mid-flight. It nearly worked. I was only playing for about 2 hours including test flights so it's not intentional spite-building or anything like that.

I was just excited about possibly having come up with my own design my own way, only obviously there's still problems. (Among those, some which I might not have had I specifically set out to apply what you suggested instead of playing the game and being optimistic in my own ability to create something that works.)

I'll be better next time I play. It was just a chain of events misleading me into thinking I was on to something reasonable. I'll give an update later on if I play again tonight.

Edit: About the Mod you'd like me to install. I'm not a modder. I try not to. I know KSP is a great foundation for adding improvements and literally almost everyone does it, but I've always just played games vanilla and to install a mod for a single craft sort of makes my skin crawl a bit. If it's just a build aid for helping decide where to place parts in regards to wet and dry CoM shifts then maybe I'll give it a shot. Sounds useful I guess. I just hate to think I'll start becoming dependent on it and literally not be able to play the game otherwise. Some people's games play themselves.

Edited February 3, 2019 by MisterKerman

[GoatRider]

21 hours ago, MisterKerman said:

Do you mean the cutaway view to look at the interior of craft? That's stock.

Look at the face-cams of the Kerbals on board in the bottom right of the screen. To the left of those there are some options you can click. Some allow a way to view the Kerbals inside of your craft from the exterior.

Thanks, it's a tiny button with a hover text of "Interior overlay", above the "add slot" and "remove slot" buttons.

[AeroGav]

42 minutes ago, MisterKerman said:

 

Edit: About the Mod you'd like me to install. I'm not a modder. I try not to. I know KSP is a great foundation for adding improvements and literally almost everyone does it, but I've always just played games vanilla and to install a mod for a single craft sort of makes my skin crawl a bit. If it's just a build aid for helping decide where to place parts in regards to wet and dry CoM shifts then maybe I'll give it a shot. Sounds useful I guess. I just hate to think I'll start becoming dependent on it and literally not be able to play the game otherwise. Some people's games play themselves.

I'm trying to limit myself to one cup of coffee a day ATM,   so basically there's a one hour window after drinking coffee where stuff makes sense and i can build an object in KSP,  after that i turn back into a moron,   that's why i like mods like this because you can use that hour more productively.   Without RCS build aid i've only managed to solve fuel CG issues by keeping vessels really simple, with a minimum number of tanks - 

this one is basically 4 rocket fuel tanks arranged in a  "+" sign shape around a central liquid fuel tank.

 As for the mod,  no it doesn't add any new parts, it just gives extra info in the assembly building.

So you've now got that red ball showing the dry CoM of this lander,   as well as torque.    Means you'll be able to iterate through designs much faster , without having to stop to empty and fill tanks - the floor of SPH looks like a real mess when i've been working in there from all the discarded stuff. 

Unfortunately it's not just a single download these days,  there are two dependencies - just like his other mod,  CorrectCoL.      Neither CorrectCoL nor RCS Build Aid appear to be on CKAN,  but the dependencies are at least (i had issues due to googling and installing out of date versions)

[MisterKerman]

41 minutes ago, AeroGav said:

Unfortunately it's not just a single download these days,  there are two dependencies - just like his other mod,  CorrectCoL.      Neither CorrectCoL nor RCS Build Aid appear to be on CKAN,  but the dependencies are at least (i had issues due to googling and installing out of date versions)

I'll consider it, but I've never considered installing mods a solution to a problem to someone who simply wants advice for how to play the actual game. The torque indicator is very appealing however...

[MisterKerman]

UPDATE:

The craft is now perfectly balanced in all stages of flight due to your suggestions. I'm not sure if I need more lift or thrust. It seems to get better each time I add more Rapiers...

I'll keep tinkering when I get a chance. Thoughts?

[AeroGav]

8 hours ago, MisterKerman said:

UPDATE:

The craft is now perfectly balanced in all stages of flight due to your suggestions. I'm not sure if I need more lift or thrust. It seems to get better each time I add more Rapiers...

 

I'll keep tinkering when I get a chance. Thoughts?

The pictures aren't telling us very much that we need to diagnose,  but i'll do my best

1. appears you are running out of oxidizer just short of orbital velocity.    That's with no cargo aboard, correct?

2. You are pitched up more than 5 degrees above prograde.  Optimum lift drag ratio (as produced by wings) above mach 1 is at 5 degrees, so i'd normally say hold the nose at exactly 5 degrees once you go to close cycle mode and keep it there no matter what.    You are pitching up more than that, creating extra drag,  but aren't climbing very steep and are only reaching 40km.   This suggests your design is a bit low on lift.   I reckon on about 6 pairs of big S wings for a 150 ton takeoff weight...   what's your ssto grossing ?        

That said,  the small amount of extra drag should not affect a design with 6 rapiers.    Each of those suckers is doing 180kn in close cycle mode - three times as much as the nerv engines i use on my sstos.   An extra 40 or 100kn of drag isn't going to matter much.  

Sounds like you've either just got to add a little bit more fuel,  or have an incorrect ascent profile.    What speed are you reaching before switching to close cycle burn ?  You should be able to get at least 1500 on something with 6 rapiers like that.   Are you levelling off at 17-21km for a "speedrun" before going close cycle mode?

[MisterKerman]

1) The Cargo is a Jumbo64+X200-8 still on board full. If I can't bring them up full there's no point.

2) Good to know. I have no idea at what height/speed engines operate most efficiently at so I desperately climb to about 25,000m and pray I can achieve enough speed to push my AP to 50K for a circularization burn to LKO. Basically my ascent policy is if I notice acceleration in a thinner atmosphere I point prograde as long as it's above the horizon line on the navball. (Usually the faint blue atmosphere AKA the third chunk.) I achieve nothing with my craft trying to dive and climb like I'd do with other smaller SSTOs so I stopped trying. My original thoughts were that I don't have enough lift, as you mentioned.

I don't remember right now so I'll get back later, but my speed when I switch to closed cycle is usually about 900mps... will verify later! I'll also make sure to include the wet mass as well.

I'm hoping the ascent profile change and added lift will make a difference. It feels like I'm getting close and my craft still looks cool.

 

Many edits...

 

Just want to mention that even if it seems like I'm being a little bullheaded about certain design choices, I really really appreciate the help. I'd never get this thing into space without you guys; you especially @AeroGav . I've already learned a few things I never had a reason to try and this will open up a lot of doors for me, so thanks once again.

 

Edited February 4, 2019 by MisterKerman

[Aegolius13]

A couple thoughts on your latest design (have not followed 100% of the thread, so apologies if something is already covered). 

• Those nose cones on the back of your Mk 2 nacelles could be used for an engine.  This would allow you to remove one of the mk 1 nacelles out on the wing, saving some drag.
• As mentioned above, Mk 2 parts are draggy, but this is compensated for by extra lift.  However, I con't think you get that lift if the Mk stack is sticking up in this manner.  So might be worth trying to get it horizontal. 
• That BigS fin on the back is probably overkill.  You need very little (or no) yaw control authority, and that thing has a bunch.  This can actually lead to yaw instability, plus extra weight.
• When fins/wings go on at an angle (dihedral/anhedral), they lose a portion of their lift.  So you could get more lift out of the diagonal BigS wing by straightening it out (could shift it upward to to clear the engines if needed.

2 hours ago, MisterKerman said:

I don't remember right now so I'll get back later, but my speed when I switch to closed cycle is usually about 900mps

The #1 most important thing with your ascent profile is to maximize HORIZONTAL speed before your jets cut out.  Altitude is really not very important towards getting to orbit - nor is vertical speed (because the lift from your wings should keep you up).  Since the jet engines are 10x more efficient than rocket engines, every extra m/s you can squeeze out really pays. 

Rapiers start losing steam pretty fast around 20km, and die completely by the mid-20s.  This means that from 10km altitude or so, you usually want to be going almost level (maybe 5 degree climb max unless your plane is really overpowered), so that you gain as much speed as possible before the Rapiers die.  Any old plane should be able to get over 1,300 m/s.  Ideally you'd be pushing near 1,600, which is pretty much the limit of the Rapier thrust curve. 

[AeroGav]

33 minutes ago, Aegolius13 said:

A couple thoughts on your latest design (have not followed 100% of the thread, so apologies if something is already covered). 

• Those nose cones on the back of your Mk 2 nacelles could be used for an engine.  This would allow you to remove one of the mk 1 nacelles out on the wing, saving some drag.
• As mentioned above, Mk 2 parts are draggy, but this is compensated for by extra lift.  However, I con't think you get that lift if the Mk stack is sticking up in this manner.  So might be worth trying to get it horizontal. 
• That BigS fin on the back is probably overkill.  You need very little (or no) yaw control authority, and that thing has a bunch.  This can actually lead to yaw instability, plus extra weight.

The problem is he wants the CoM in the middle of the cargo bay - which is why the heavy rapiers are clustered around the CoM.    Up front,  there is only the cockpit to serve as counterweight for all the engine mass - about enough for the relatively light panthers he's got at the back.

Putting rapiers on the back of the mk2 tanks would cause dry CoM issues as that's further back, also the drag of the mk1 nacelles they are currently on is negligible compared to the mk3 and mk2 parts on his design.     

The lift from mk2 parts is negligible.  About 100th of what a big s wing makes.   Meanwhile they have over 3x the drag of a mk1 of same capacity.   If he's min maxing he should swap for 2.5m tanks on the sides.    But he's already said he cares more about looks.      Also,    at this point he is not drag limited.

The incorrect flight profile means he needs twice as much rocket delta V 

2200 orbital velocity minus 900 m/s airbreathing =   1200 m/s required from close cycle mode

if he got to 1500 on air breathing engines,   =  he'd only need 700 m/s 

I have concerns that Boeing 707 tail fins will melt when he starts flying corect profile.    But it would be a straight swap to shuttle one if this happens, no big deal.

Quote

That BigS fin on the back is probably overkill.  You need very little (or no) yaw control authority, and that thing has a bunch.  This can actually lead to yaw instability, plus extra weight.

No, no , no!   You are conflating vertical stabilizer surface area, which always improves lateral stability ,  with excessive rudder use/overcontrolling.  Most players never touch the rudder controls and indeed there is little need to if you've got enough vertical stab area to give the necessary passive stability.

If you think SAS might overcontrol the rudder,  you can always right click and limit authority on the rudder. 

As for drag,  turn on the F12 display and see how little it generates compared with the cargo bay and various rocket fuel  tanks being lugged around.  Also this airplane has over 15 tons of engines and a 4 ton cockpit...   not sure the weight is notable either.

[bewing]

I agree with Aegolius about the BigS tailfin. It adds mass and drag and contributes nothing, if you are steering with a keyboard and not a joystick. Drag is the big killer, because it eats your thrust and fuel in nickels and dimes. For the future: I understand that you hate canards, but they are aerodynamically far superior to trailing-edge elevons -- so if you ever want to graduate to building spaceplanes or seaplanes that actually fly, you are going to have to change your position on canards.

No, the FAT tailfins won't melt. Even if you try to bring the cargo back down, the heating will all be on the wings and the nose. But if you are going to bring the cargo back down, then you should burn off the fuel in the tanks for a huge reentry burn and then you don't have any heating problem at all.

Yes, the problem is in the ascent profile. Probably too much steering involved. You need to learn the proper use of the "F" key -- it lowers your nose a little without steering if you have the plane balanced properly. The best ascent with a keyboard involves touching no key except F all the way to orbit after you've gotten the spaceplane into level flight above the water after takeoff.

You need to keep an eye on where the rapiers are putting out their maximum airbreathing thrust during your ascent. Maybe it's a little below 20km. You want to make sure that you already have the plane flying close to level when you hit that altitude -- which means you have to start gently lowering the nose (a little at a time) before you get to that point. But as said above, if you can't get the plane above 1300 m/s in airbreathing mode, then you have a serious problem ... most likely with drag.

The one place you may need to protect your tailfins from overheating is in the 25km to 38km altitude range, if you can get your speed up into the 1800 m/s range. Protecting them from heat may involve climbing fairly rapidly through this altitude range. You do that by not lowering the nose during those altitudes, but only if you have a heating problem. If you are not experiencing overheating and part failures, then you want to keep the nose as close to prograde as you can. And, in fact, set SAS to prograde hold as early as you can while still making it to orbit.

 

Edited February 4, 2019 by bewing

[MisterKerman]

To clear a couple curiosities up that I can answer without being at my computer.

I have very stable flight. It holds a heading solidly at all times with SAS engaged.

I usually let the nose drop if I need to by rapidly fluttering the T key. (Didn't know about the F key!)

The authorities for the control surfaces are limited to about 25 on the tail for yaw, and about 40 for pitch/roll control surfaces on the wings.

Very interesting suggestions for the flight profile btw. It makes a lot of sense reading through this and will definitely give me a few things to look for instead of flying heavy, blind, and confused, like I've been doing thus far.

[Aegolius13]

3 hours ago, AeroGav said:

No, no , no!   You are conflating vertical stabilizer surface area, which always improves lateral stability ,  with excessive rudder use/overcontrolling.  Most players never touch the rudder controls and indeed there is little need to if you've got enough vertical stab area to give the necessary passive stability.

 

I nothing against a big vertical stabilizer, and I often put a big static triangular wing connector on the back bigger places.  The reason I the Big S is overkill is that it has so much moveable area (can't remember the exact in-game term).  If i recall, moveable area has around double the weight penalty of regular wing area.  And I do think it has the potential for overcorrection.  You can always turn control authority down, but then you're effectively paying the mass penalty without a corresponding benefit. 

[AeroGav]

7 minutes ago, Aegolius13 said:

I nothing against a big vertical stabilizer, and I often put a big static triangular wing connector on the back bigger places.  The reason I the Big S is overkill is that it has so much moveable area (can't remember the exact in-game term).  If i recall, moveable area has around double the weight penalty of regular wing area.  And I do think it has the potential for overcorrection.  You can always turn control authority down, but then you're effectively paying the mass penalty without a corresponding benefit. 

Yeah fair enough,  i just hate seeing planes with diddy little vertical stabilizers and then have people struggle with preventing roll excursions / sideslip on the flight to orbit.     Not only does yaw instability make an airplane horrible to fly,  the drag from the fuselage while it is sideslipping more than offsets what you saved by fitting a smaller surface.

If you're min maxing ,   i like big S strakes turned vertical like this - gives you a bit of fuel too

[MisterKerman]

Also another thing worth noting is I used the BigS tail specifically because it looked better than the other ones and I'm really struggling to find a way to make a decent looking tailplane mounted to the MK3->2.5 adapter... ×m×

[MisterKerman]

sumteimz u maek plaen dat goez tu spaec in a singul stege lulz
I had to use up most of my "bonus" X200-16 fuel intended to deliver just a little more fuel each trip in order to complete my rendezvous, but I managed to deliver a full Jumbo 64 to my space station in 130,000 LKO as I originally set out to do. I'll consider this a success.

I think I'm going to mark this topic as "Answered" now.

I couldn't have done it without you guys, so thanks!