Spaceplane W/ FAR advice

[Av3ry]

Hello!

I've played KSP for probably two years now, and I guess I've been lame and un-social and avoided the forum, until now. Seeing as I put a lot of hours into KSP and it's not favourite topic of the dinner table or friend circle, it makes sense to engage with others who have interests that match mine!!

Reason for my question : When 0.90 came out I started again in career/megagrind/face-to-the-cheese-grater mode, and while it's not my favourite thing in the world, that is a whole 'nother topic.

Important mods I have : Remote Tech, Deadly Reentry, KW, B9, FAR, and lots of other goodies but these are the ones that count in my question.

SO I BUILT A SPACE STATION. I really wanted a shuttle to service it! I've probably spent 10 hours reading guides/reworking my stupid space plane and I just can't seem to make one that doesn't suck at an unacceptable level! I have read a lot of guides, I know how to read all the FAR numbers in the analysis screen, I just want some tips on what I have and how to make it WORK!

The main body is pretty unchangable, it needs extra crew cabin, docking port, cargo bay, anything else can be changed... I want it to be as efficient as possible and stable enough to allow for a moderately easy ascent and descent.

I do not have RAPIER yet or AEROSPIKE, the tech you see is the most current I have... I am "grinding" to earn the 4.7mil to unlock the 500+ science boxes but i'm only at 1mil and the poor payout of the contracts warrants a more reusable vehicle!!

I hope I havent typed up too much of a storm and any help, pictures, ideas, ANYTHING would be greatly appreciated! I don't know where else to turn! Thank you

Problems with current design : Minor instability, very heavy (slow ascent to 24,000m), difficult to achieve orbit with rocket engine, AOA's most efficient L/D rating is only 8.5*, and all wing parts have been tweaked to 0.5 Strength/Mass

[Alshain]

Despite all the bellyaching people have, plane building in Stock and FAR are the same in principle, as long as you don't do completely ridiculous flying boxes type things. Now, all that said, Stock planes don't work in FAR and vice versa because FAR makes those CoL markers be placed in different places. However the principle is still the same, CoM slightly ahead of CoL.

Key things that are different are drag related. For example, nose cones are needed to prevent too much forward drag, your air intakes need to be AT or BEHIND the center of mass, or they have the potential to pull the front end of the craft around with their drag. Your plane there looks pretty good, though without the CoM/CoL markers I can't tell for sure.

Minor instability: I need to see CoM and CoL to troubleshoot this

Very heavy (slow ascent to 24,000m): Nothing wrong with weight as long as you have the lift and the thrust to accomodate. Slow ascent usually means your missing one or the other, or both. You do have a lot of wing there so I'm guessing thrust. I will say you have a lot more fuel than I would use on a Mk2 plane, it shouldn't take that much to just get to orbit. Get rid of the dedicated LF tanks. The tiny amount of fuel your jets use isn't worth hauling that weight. I also suggest choosing your planes purpose. Either passenger or cargo, not both. You can always build another plane.

difficult to achieve orbit with rocket engine: This could actually be a problem with your intakes. The way the game works for the next week (1.0 will change this), if you place your engines and intakes in symmetry one engine gets all the initial air and if there is spillover the other engine gets that. Once the spillover is gone, one engine flames out but the other still has plenty. To fix this place Intake -> Engine -> Intake -> Engine without symmetry or use the Intake Build Aid mod to correct it so each engine has intakes dedicated to it. In your case, place 2 intakes, one of each kind and one engine, then do the other side. Once you have done this you will find you can get further higher before turning to the rocket.

Also, that engine isn't the greatest of high atmosphere engines (I think, LVT-45?, hard to see). The Aerospike is a wonderful choice, RAPIERS, for smaller planes (not yours) the LV-909 works well. Sometimes the Poodle works good too.

AOA's most efficient L/D rating is only 8.5*: This is also affected by thrust and/or lift.

and all wing parts have been tweaked to 0.5 Strength/Mass: In most cases that should be fine as long as you don't plan on hard turns, which an SSTO doesn't really need to do.

Edited April 21, 2015 by Alshain

[Wanderfound]

Check out the first few posts of the Kerbodyne thread for building and piloting guides, and the Youtube channel for demonstrations. Both linked below.

The Kerbodyne thread also includes a hundred+ example designs. Most RAPIER based designs will also work (less efficiently, but they'll work) if you swap out the RAPIERs for a combination of Turbojets and LV-T45s. You'll see with a few of the later three-engine airframes on the thread that they've been done in three versions: RAPIER/Turbo/RAPIER, RAPIER/Nuke/RAPIER, and Turbo/LV-T45/Turbo.

I can't actually see the image accompanying your post, so I can't give more specific advice. But if you can post screenshots of the ship from above and the side in the SPH, with CoM/dCoM/CoL markers on, we can have a shot. FAR analysis screens at 25,000m/Mach 4 and 500m/Mach 0.5 would also help.

Edit to add: can see the pic now...

The basics look okay. You have more wing than you need, so you could shave a fair bit of weight by trimming that down. I'm not sure what engines you're running due to the non-stock appearance; is it two Turbos plus an LV-T45? Anything lower thrust than that is going to struggle.

What do you have the control surface settings at? You want the canards to be set to influence pitch only, with a low maximum deflection, and the trailing edge wing surfaces to be set as elevators near the centreline, elevons near the wingtip, with a moderate maximum deflection. Rudders set to yaw only, low deflection.

You might get better results if you swap the two pairs of wingtip extensions for a single pair angled at a 45 degree dihedral. Pulling the vertical stabiliser as far back as possible on the fuselage should also be helpful. If that vertical stabiliser is an all-moving control surface, you have massively more rudder authority than you need. Yaw is the axis that needs the least control influence (followed by roll, then pitch).

Edited April 21, 2015 by Wanderfound

[Wanderfound]

A demonstration build with similar layout and capabilities:

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Craft file at https://www.dropbox.com/s/sp9hqzpi5mudpox/Kerbodyne%20Demonstrator.craft?dl=0

It occurred to me afterwards that you probably don't have shock cones; swapping them out for ramscoops would not significantly affect the design.

[Av3ry]

Hey thank you everyone and especially Wanderfound for the advice! That is fantastic I am going to give it a shot an I appreciate the knowledge so I have something to go on. Gonna test it out now

I wanted to ask what mods are you using?? Notably the black plane in the top right and the bottom right helper panel, thanks!

[Crzyrndm]

Black plane is Pilot Assistant (link in sig), bottom right is blizzy's toolbar, and the info readouts next to the navball is DaMichaels Kerbal Flight Data

Edited April 23, 2015 by Crzyrndm

[Wanderfound]

Hey thank you everyone and especially Wanderfound for the advice! That is fantastic I am going to give it a shot an I appreciate the knowledge so I have something to go on. Gonna test it out now

I wanted to ask what mods are you using?? Notably the black plane in the top right and the bottom right helper panel, thanks!

As mentioned, the plane is Kerbal Pilot Assistant (highly recommended as a PID tuner even if you don't need the autopilot; stock SAS is horrid) and the bottom right panel is Blizzy's Toolbar (for those mods that don't use th stock toolbar). The mods showing on Blizzy's thing are mostly just shortcuts to my most commonly used Mechjeb features: the ÃŽâ€V readout, manoeuvre planner, Smart A.S.S., etc.

Off the top of my head, the current modlist:

Kerbal Flight Data

Kerbal Flight Indicators

Kerbal Pilot Assistant

Ferram's Aerospace Research

Deadly Reentry

Environmental Visual Enhancements

Mechjeb

Raster Prop Monitor

Enhanced Navball

Kerbal Alarm Clock

Goodspeed

Distant Object Enhancement

Planetshine

Chatterer

Kerbal Engineer Redux

TAC Life Support

Scansat

Final Frontier

Stage Recovery

...plus another dozen or so small mods (enhanced IVAs, single-part packs, contract packs, etc) that tend to come and go. The first four on the list are absolute essentials for anyone into spaceplanes, IMO.

The demo plane was designed to fly with FAR and KPA; it has enough control authority that SAS will make a mess of it if you don't use a PID tuner like KPA.

- - - Updated - - -

Also note: I have a strong design bias towards fast, low-drag ships. That's good for going to orbit, but it does impact low speed handling a bit. Keep it above 100m/s unless you want to fall out of the sky.

[Crzyrndm]

The demo plane was designed to fly with FAR and KPA; it has enough control authority that SAS will make a mess of it if you don't use a PID tuner like KPA.

I'd recommend phasing out that bit of advice post-1.0. This deals with the SAS shake more effectively (took a plane from runway to 300kPa and back with no wobble) and more intuitively (if it shakes, reduce the % @ Q (once I add a Q readout that is)). Just got to add a few bits of polish (namely defaults, I don't think I'll add multiple presets to this one).

Tuner still has it's place, but it will be the "advanced" tool going forward

[Wanderfound]

I'd recommend phasing out that bit of advice post-1.0. This deals with the SAS shake more effectively (took a plane from runway to 300kPa and back with no wobble) and more intuitively (if it shakes, reduce the % @ Q (once I add a Q readout that is)). Just got to add a few bits of polish (namely defaults, I don't think I'll add multiple presets to this one).

Tuner still has it's place, but it will be the "advanced" tool going forward

Ooh, nifty. Is this aimed at FAR, NuStockAero or both?

[Crzyrndm]

Anything. It can make flying planes at high Q manually a whole lot easier, solve control wobble for SAS/PA, etc.. It's about as configurable as you can get with the very small amount of information it uses (that said I haven't actually tested it with NEAR or stock at all...)

[Av3ry]

Grr... I am so ready to flip the whole computer sideways! I have spent over 10 hours trying to make this silly plane get somewhere, yesterday, we made it to orbit! 74,000. Remaining dV m/s = 0.

I took your advice, added Pilot Assistant, reduced wing area, got rid of the liquid fuel tanks for rocket fuel only, and the Wildcat (KW), LV-T30, and LV-T45. But no success.

I have the stock turbojet that is good up to ~Mach 3.0 (more like 3.3), I don't turn on the rockets until 30,000, I have the intake build aid so that I can really stretch the turbojets up to 30k by reducing throttle to keep the intake air above 100%.

What the heck is wrong? Why does it weigh so much? What's it's deal? I need way more dV to get into a polar orbit (because thats where I placed my space station, that needs rescue xD).

It really grinds my gears that I can't get this, I feel like a Kerbal noob but I have designed amazing rockets with super high efficiency launchers... I just cant seem to wrap my head around it.

Maybe i'm piloting the SSTO incorrectly? I see the provided information about a PID tuner, but I'm not sure how to use one or what the adjustable PID options in Pilot Assistant do... I have just been changing max AoA for the pitch control as 10* is too low when you're up at 20,000. I followed Wanderfounds storyboard type advice which was very useful for pilotting tips, unfortunately I cant try out the attached demonstrator as I don't have shock cones or wingstrakes. But I do have most of the C7 wing parts which can mimic them... I have all of the 300 science boxes.

I am attaching photos (with more useful info, like COM, COL, dCOM ) of two versions, the 2nd of them where the crew cabin (which weighs 2.0t) has been moved closer to the centre of mass.

Please let me know what you think -- Lastly, could be piloting problem, or SSTO design flaw, but in high altitude (passing through 25,000 , and especially after engaging rocket engines and closing intakes etc) the plane has HUGE difficulty staying in a straight line and it takes all my might and RCS to keep the bugger straight! When using RCS build aid it informs me that the main engine is applying torque to the COM for whatever reason, I assume it's because the plane isn't perfectly symmetrical and there is greater weight on one side than another.. I tried using the new smaller cargo bay (just for holding batteries, RCS, essential to flight things, and moving different weight around to try to offset the torque number but it's still very hard to fly straight..

Anyways, thanks again for all the help, I really appreciate it and I'm thankful again in advance!

Oops, I forgot dCOM, and i'm aware the front gear on 1A is too far back xD Will fix now...

I am linking the ALBUM instead of posting the pictures in the post as it is often screwy.

http://imgur.com/gallery/2RwZt

[NathanKell]

If you're only switching over at Mach 3.3 (~990m/s) then you're missing out on another few-to-five hundred m/s of horizontal speed you could be getting, that's probably part of it. Note that you shouldn't actually shut off your jets when you light the rocket engine; only shut them off when the first one flames out. As for staying in a straight line: more tail. Your vertical tail is too short.

Finally, you should probably right-click on the wing parts and reduce their strength/mass, that will lower your dry mass a fair bit. 0.5 should be sufficient unless you plan to do some high-AoA maneuvers.

[Crzyrndm]

If you're only switching over at Mach 3.3 (~990m/s) then you're missing out on another few-to-five hundred m/s of horizontal speed you could be getting,

That is unless he has B9 or the SETI FAR pack which (IIRC) both nuke turbo's even further than FAR does (Mach 3.5 sounds right, but it's been ages since I've use B9)

@Av3ry

If you have problems with Pilot Assistant, just stick to the stock SAS (or alternatively, only use the wing leveller and use trim to control pitch). It's not always the easiest thing in the world to use.

Edited April 24, 2015 by Crzyrndm

[Wanderfound]

The plane is asymmetrical for the same reason as all planes are: it has a tailfin. The CoM is likely a tiny bit above the CoT, resulting in pitch-up torque. You can zero this out (almost, anyway) by just angling the rocket motor a tiny bit.

However, from your description it sounds like you might be getting yaw wobbles as well.

The yaw stuff is aerodynamically caused, rather than being an offset thrust issue. As you ascend above 30,000m, the air becomes thinner. As the air thins, the tailfin has less to push against, reducing its power to stabilise the ship.

With sufficient aerodynamic polishing you can eliminate those wobbles, but for now: vectored thrust, torque, piloting. A Vernor or a couple of linear RCS ports either side of the nose provides a lot of stability, and if you keep them toggled off except when needed they don't use that much fuel. Adding more torque can also help, as can ascending with a flatter profile so as to be moving at a faster speed at a lower altitude (using speed to compensate for thin air; the strength of the tailfin's stabilisation effect is all about how many air molecules hit it over a given span of time).

I'd recommend having a look at my build videos. These are probably the most relevant:

For the PID tuner: before takeoff, open up Kerbal Pilot Assistant. Change it over to Stock SAS mode, then open up the SAS presets. You should see something like this:

Change Kp to 5,000 for pitch and yaw, 3,000 for roll. Change Scalar to 500 in pitch, roll and yaw. The purpose of this is to reduce the sensitivity of the SAS so that it doesn't overreact, forcing the plane into continuous pitch wobbles (as it tends to do when left untuned).