OPT very heavy spaceplane, plz help getting to orbit

[agrasyuk]

Hello  , forum. i'm a longtime board reader, decided to join to pick at collective wisdom getting my very heavy spaceplane to orbit. Steam says I have almost 350 hours clocked in KSP so far. i'm proficient enough with vertical launchers and can send stuff up there acceptably well,  but  never dabbed in planes and spaceplanes to much. when i came across OPT Parts (https://kerbal.curseforge.com/projects/opt-space-plane-parts-v1-9) I knew I just have to build the wide K body liner. And it had to be VTOL with a rear hatch to drive a jeep into!
My Craft “K-Lifter” features 3 cargo sections with a back ramp. 4 J-deployment bays house pair of aerospikes each  For atmospheric propulsion I use four OPT monstrous J-92 scramjets, four S3 KS-25 Vectors for Vacuum. With Dry craft Mass 150 ton, Fueled 370ton vertical takeoff from kerbin is impossible its more of a landing stage fit for Mun or Duna, luckily runway is long and awesome. Takeoff config adds pair of 50 ton drop tanks holding 5000units of LF and 3500 of OXi each. Rocket takeoff assist stage comprised of 8 Large Boosters for a total take-off mass 668 Ton before any payload. To achieve CoF behind CoM wings have 1degree of AoA . Handling of this contraption is not very simple so using Infernal robotics hinges and a probe core I created a working “fly-by-wire” stability augmentation device that allows me to finely adjust trim. 

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Takeoff config with droptanks and SRBs attached

 

Underside, RAT boosters, J-Deploy bay with VTOL Aerospikes visible. Infernal Robotics "fly-by-wire" probecore   gimball  on the fuel tank.

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Mission  profile:
1.    On craft load parking brakes set to on, X to set 0 thrust, switch craft control to stability probe-core, Sps to launch.
2.    Action group 1 deactivates VTOL engines, shuts of bay lights and closes the bays
3.    Action group 4 to deactivate KS-25 Vector engines
4.    Throttle set full, as scramjets spool up deactivate parking brakes

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5.    At third mark activate RAT stage. fly off the edge of runaway at ~120M/sec pitching up VERY slowly. I manage to keep the low hanging Boosters most of the times. well its possible - proof: 

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6.    Pitch to 35-40 degree climb. Boosters shut down and ejected at ~4600-4800Meter of altitude after accelerating the craft to 280M/sec. moment before shutdown KS-25 Vector rockets ignited to help getting altitude fast. 
7.    Slowly leveling off at 10K (at which point craft breaks mach1 )and begin acceleration. Drop tanks jetted at 1.6 mach. Very shallow climb.

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8.    at  2.7 Mach rockets are shutdown as scramjets reach their peak performance and can accelerate the craft without assistance. pitch up 5-15Deg. to keep speed below Mach3.  seeing more and more flames

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9.    switching to rockets again at about 23Km, maintain prograde which is slightly above horizon pushing that Apoapsis to 75Km. 
10.    enjoy the rest of suborbital flight since there is not enough fuel to Circularize even when test payload is only a measly 4.5 ton. After few test flights I tweaked LF to LF/ Ox fuel tanks so at this point leftover fuel component is below 100 units.
11.    Gliding OPT stuff is super easy (unless critical AoA is exceeded on descent), easy touchdown without any badabooms (most of the times )
Flying that craft is not easy. Yaw was driving me crazy until I rebuilt the engine adapters without symmetry.  While CoM both dry and Wet is in in front of CoF (even if just a bit) the controls are twitchy transonic and faster, very easy to throw tumbling away, keeping heading is hard (usually can within5deg of desired) Infernal Robotic actuated probe-core makes pitch and yaw control manageable. I
so far I attempted MOAR engines, added 1 more scramjet on each side which resulted in better dynamic, but still needed rocket assist to get to full scramjet thrust speed. still achieved only suborbital flight and even if it would have made an LKO it makes craft haul 25 ton of useless for space travel engines. Thinking to try bigger drop tanks (Mk.3) and add 4 more SRBs on belly in front for a “RAT stage two”.  next I would try total of 4 per side Scramjets , but make that structure removable (large docking port) on orbit to be replaced with perhaps nuclear propulsion module . obviously will need refuel to get anywhere further.
what can I improve? opinions welcome. 

PS that feeling when the original post you typed up is gone before it was posted and you need to retype knowing for sure the intial one was better.
 

[seaces]

I like this mod too and I almost build spaceplanes only with this parts.
How many tonnes of cargo do you wan to carry and where? I am not proficients with VTOL but maybe I can help you get this in orbit at least but first I must know what do you wish to do with it?

[Streetwind]

Keep in mind I'm not a plane kind of person, but looking at your craft, I can't help but feel like you need more wing. It's almost all lifting bodies (which are inferior to wings in terms of pure lift) or even non-lifting parts. If you could get more lift, then perhaps you could carry more payload to orbit.

[bewing]

I do a fair amount of stuff with planes and spaceplanes. You are definitely on the extremely low side for the amount of wing you have, as Streetwind says. Adding more will definitely help your ascent -- especially with payload. And I'm not familiar with the efficiency profile of your scramjets. What is their in-atmosphere Isp? What is their thrust level at Mach 1.5 and 1km altitude, compared with 10km altitude?

However, I can definitely say that if you want to lift any payload, then your climb rates are way too fast, and your plane is going way too slow in the lower atmosphere, and you are using up all your rocket fuel in the atmosphere, rather than above 70km where it can do you some good. Liquid-fueled, air-breathing engines are extremely fuel efficient when compared with rockets. You need to get those scramjets lit early and keep those rocket engines off.

So, I think your biggest problem is in step 6. You need a much more horizontal launch. You need the boosters to get you to mach 1.5 minimum before they burn out. Altitude is not important -- speed is. From 1km to at least 25km you want to be flying entirely on your scramjets. And there is no real reason to keep their speed down below mach 3 when you are above 12km, I don't think. Mach 4 is pushing it, but the only important consideration is overheating. Add that LF back in that you took out. Your droptanks contain Ox -- very bad. They should be pure LF. If you have trouble accelerating and shallowly climbing just on the scramjets, then you need to lower your drag. Primarily by adding wing, and reducing your nose AoA. Or maybe add in another pair of airbreathing engines.

I think you have 4 vectors? Consider dropping the number to 2, to save weight and drag. Can 2 get you to orbit? Or maybe replace 2 of the vectors with extra airbreathers, if you need more thrust in the lower atmosphere. Do you know the inverted nosecone exploit for your rocket engines?

You need to understand that a spaceplane launch to LKO takes 15 to 20 minutes. If you rush it, it costs you immense amounts of fuel. Let the airbreathing engines do their job, and get you up to the maximum speed they can -- because their fuel cost (in terms of thrust over time) is so low compared to the rockets.

 

 

 

[agrasyuk]

Many thanks for your replies guys.

Seaces,
intended payload is a small mobile mining operation or a colony base. Destination Mun or Duna probably will be doable (after refueling in orbit), maybe further Dv permitting. I really like that K-wide cargo-hold can fit two Feline rovers side by side and for now its my chassis of choice for drill rig, mobile refinery and a tanker. i'm still working on those so exact weight is unknown. am i hoping for impossible if I say ~40 ton (its a big hold, lots of stuff can be stuffed there )?

Streetwind, bewing,
More wings does make sense. I guess this craft being a good glider (dry and unpowered) gave me the impression of sufficient surfaces.
will attempt to add surface but keeping that CoL to the back of craft will be hard.

bewing,
thanks for the in-depth answer! OPT J-92 scramjet is basically four stock ""whiplash" jets combined so its as if I have 16 ramjets total. from what i can tell ISP and height/speed characteristics are exactly the same or at least very comparable.
I don't insist on steep climb at Step 6 and will be again trying a shallow one as you suggested (speed is indeed good). the reason for me being in a hurry to get to 10K is that during previous test flights i found that flying lower I fail to accelerate past Mach2 even with rockets on for to long, i suspect due to drag, scramjet hits the sweetspot to late and flight never leaves the atmosphere altogether. hopefully more wings and less AoA as you suggested will get it done (getting to 25Km on only airbreathers does make lots of sense).

At this point what i'm really after is learning. If mission control says this object absolutely has to be in orbit right NOW no matter the cost i'd just strap it to a rocket and launch vertically , but i really want to do it the proper way. SSTO is a pipe dream, I'm ok shedding boosters (SRBs have chutes on them and are recoverable) and drop-tanks, but rather not jettison stages with engines if i can help it.
the most enjoyable factor of this game for me is the engineering challenge. VAB and now SPH is where bulk of my game time spent.

 

best regards

Anton

[seaces]

Do it milestone by milestone. You have complicated design and mission for first time. Build first a payload, and try to make payload lightest and smallest and around it build your plane. Then first try to lift it with simple K parts plane (no VTOL)0km orbit and land it. See how it flies back to KSC often they fly nice on the way up but bad on the way down.
Step by step improve the craft until you get to the Mun with VTOL craft and back to KSC, the key is simplicity. Also try to minimize the amount of stuff hanging from the plane especially big parts like J bays around your craft and drop tank etc. I believe the give a huge amount of drag also I believe you will need more of those jet engines 2-4 to climb with nice performance and speed.Here is my craft that lifts full loaded orange fuel tank to station around Kerbin at 600km orbit to refuel station fuel tanks ten it gets ack to KSC directly to 20-15km Pe without any issues. It has 4 jets from the OPT mod, docking port, 3 service bay for payload (I have a version with 2 bays only that does the same thing too like this one) and linear rocket engine for space. It took me 10 concept crafts to create a working version and every version had its own milestone to complete and I believe you should try also like this and tweek your craft from version to version until it meets your mission parameters.
P.S I still did not remove these Mk2 intakes that I do not need.

Edited March 9, 2017 by seaces

[agrasyuk]

thanks for the pictures Seaces.
looks very interesting indeed. I am gravitating to conclusion that i'm trying to bite much more then i can chew here, but not ready to give up. I did practice with a J-body test airframe to get a feel how pair of OPT J-92's behave at various speeds for atmospheric flight. did make it to orbit (had rocket motors on it as well) and landed at KSC but it had no payload to speak of and I would guess i didn't fly it in the most efficient way. i can post the design when back at home PC, but doubt anyone will find it interesting.

Are you saying you have eight scramjets in there? i can only spot 4, did you attach them one after another and "collapse" them together with gizmos? which brings me to question on how game aerodynamics (im pretty sure i'm using FAR) considers "occluded" parts? to stiffen my structure I used hidden struts: moved J-nose cones away , strutted side consoles to wings and the pylons, moved nosecone back. do those occluded struts count towards drag? will pushing landing gear gondolas further into the body improve drag?
any idea if a forward facing tail section produces more drag then a nose-cone(i have one in that the docking port bulge on top of cockpit)?

bewing, a followup Q if i may.
i looked up the reverse nose-cone topic. neat trick and indeed looks good on the RAPIER but i can't figure out if it improves drag characteristics on any other engine. will attaching those at the end of Vector motors and then pushing them in improve the drag?
with that it does give me an idea of attaching them on stack decouplers and stage/jettison on Rocket motor ignition

 

[bewing]

2 hours ago, agrasyuk said:

i looked up the reverse nose-cone topic. neat trick and indeed looks good on the RAPIER but i can't figure out if it improves drag characteristics on any other engine. will attaching those at the end of Vector motors and then pushing them in improve the drag?

Yes, a bit. It's not a huge effect, but can make the difference between go/no go.

(And I agree that some version of the whiplash is a good choice for engine.)

Edited March 9, 2017 by bewing

[seaces]

2 hours ago, agrasyuk said:

thanks for the pictures Seaces.
looks very interesting indeed. I am gravitating to conclusion that i'm trying to bite much more then i can chew here, but not ready to give up. I did practice with a J-body test airframe to get a feel how pair of OPT J-92's behave at various speeds for atmospheric flight. did make it to orbit (had rocket motors on it as well) and landed at KSC but it had no payload to speak of and I would guess i didn't fly it in the most efficient way. i can post the design when back at home PC, but doubt anyone will find it interesting.

Are you saying you have eight scramjets in there? i can only spot 4, did you attach them one after another and "collapse" them together with gizmos? which brings me to question on how game aerodynamics (im pretty sure i'm using FAR) considers "occluded" parts? to stiffen my structure I used hidden struts: moved J-nose cones away , strutted side consoles to wings and the pylons, moved nosecone back. do those occluded struts count towards drag? will pushing landing gear gondolas further into the body improve drag?
any idea if a forward facing tail section produces more drag then a nose-cone(i have one in that the docking port bulge on top of cockpit)?

bewing, a followup Q if i may.
i looked up the reverse nose-cone topic. neat trick and indeed looks good on the RAPIER but i can't figure out if it improves drag characteristics on any other engine. will attaching those at the end of Vector motors and then pushing them in improve the drag?
with that it does give me an idea of attaching them on stack decouplers and stage/jettison on Rocket motor ignition

 

Please do not quit push your technology to the limit, I am just advising to improve design and its capabilities one at a time. First orbit, then go for the mun orbit with return then VTOL at mun with return.

My mistake I have 4 engines only attached on Mk2-Mk1 rocket adapter. As for the gear I have moved them mostly for aesthetics and to make my craft stand on ground horizontally or slightly tilted backwards for easier take off. But I would not be surprised if the reduce drag in this way. I do not use struts I use Kerbal Joint Reinforcement mod.

I was thinking maybe that you should try to make a plane that can land vertically like and other lander on mun and avoid VTOL construction and complications from it? I believe you can find some mods with nice big lander legs in addition to that you would need good reaction wheels and RCS to keep it nice and steady until you land there. Or least challenging but it is close to your original plan is to just orbit mun with plane that would drop its payload to the surface. The payload would only need small engines and little fuel how much is required to land it, or you can download mod that adds retrorockets that would make smooth landing of your payload you would only need to make it fall vertically and of course tune the retrorockets in the VAB for optimum force so that they do not overshoot. I think in this way since I see that VTOL equipment is huge and taking a lot of load of your plane while creating extra drag there. Now if you find to optimize VTOL equipment and not make it bulky then go with it, ultimately it is up to you. Wish you good luck and send us pics if you make it I am interested to see how that machinery would look like.

[agrasyuk]

Placeholder. 

Guys, any recommendations as far as good looking wings? Anything other then B9?

[AeroGav]

I've been doing the Shuttle Challenges,  in order to comply with the spirit of the challenge some of my horizontally launched designs had kickback boosters like the OP's.

Honestly, they are not great things to hang off an airplane.   On rockets the massive short term thrust boost from zero velocity is great I am sure,  but the 26 ton mass and large drag does not suit them to airplanes.  

A spaceplane most needs a thrust boost to get through the sound barrier.  After that , the ramjets start going nuts and you're away.    With kickbacks, you either light them on takeoff, in which case you're still in the really thick part of the atmosphere and can't get over mach 1 or stay over it once the boosters run out,  or you carry them up to altitude and don't light them till you're ready to punch through mach 1.   Except that if you got enough jet power to carry those heavy damn things up to altitude, you don't need their help to get past mach 1 anyway.

Instead of kickbacks,  use  whiplash or panther jets on drop pods.   One whiplash per 60 tons of launch weight will be a huge help.   Combine those booster pods with extra air intakes, and supplemental liquid fuel tankage.  No need to run oxidizer consumers below 20km !

 

[agrasyuk]

Thanks Aerogav. That is indeed more efficient to have airbreathers on pods. Perhaps I will need to ditch my idea of not dropping engines. 

But I think I found the biggest culprit. Not sure why I didn't try F12 overlay sooner. my rear side consoles ( the J part ) were producing immense amounts of drag! I guess Something there didn't snap right (sometimes OPT stuff refuses to connect to snap points) and the game probably calculated an area as flat. It was quite interesting experiment when I gutted the deployment bays of everything , replaced scrams with inverted nosecones to try and pinpoint the source of this drag and even in that config the red line was tripple the drag of wide middle K section. 

 Rebuilt from scratch, drag is now comparable to the rest of components and the dynamic is much better. Will not get to orbit without boosters but step in right direction )

[AeroGav]

5 hours ago, agrasyuk said:

Perhaps I will need to ditch my idea of not dropping engines. 

Whiplash engines are only 2000 funds each ,  so don't feel too bad about dumping them.  Compared with NERVs at 13,000 funds, Rapiers at 6k etc.   Cheaper than kickbacks too.

For the Shuttle Hubble telescope challenge, i built two space shuttles, one used two whiplash, two terriers , two nervs to get to 650km orbit.  The whiplash and terriers were on structural pylon decouplers.  Even so this variant of the shuttle was way, way cheaper to launch than the one with kickbacks and an external tank.

[agrasyuk]

Revisiting this thread.
after several trials i just couldn't make any good looking B9 wing configs. All i added is just another set of OPT large pylons, anything more was just not looking right. However what really helped with lift is adding incidence to all wings
Went through several iterations of dropped aerial boosters and instead of loads of whiplashes i ended up with AE-4 Valkyre from Mk.4 plane mod. crazy powerful engine, not as much top speed power as OPT Scramjetsbut opening to best thrust much sooner (much lower service cieling too). for the rockets I kept the Vectors (T30's were just not cutting it). test payload 72t. - two full orange tanks

accend profile is just a ted cheaty for a luck of better word. the plane will klimb on just the airbreathers to 10K but accelerating past 1.2 Mach is not happening due to extra drag - 5Deg or so AoA has to be keep to stay level. as it was said in some other thread on here the solution to high AoA is either more wing and-or more thrust. fire those Vectors!!! it does not take a long pulse at all from those monsters to kick the plane into ~1.55Mach at which point the AE-4 Valkyres have enough thrust to keep the craft accelerating. scramjets are slowly waking up and at 15Km go absolutely crazy. pods are dry of fuel at about 20Km, AE-4 drastically dropping in thrust at this point so away they go. at 26 or so Km Vectors are fired again helping the rapidly loosing scramjets (they keep firing till 32Km or os) pushing that apoapsis out.  by the time Ap is at 100Km, Pe is already above the surface since the burn is nearly horizontal, does not take much to circularize. this is one hot ascend however and i'm sure alot Dv lost to friction. but there is some leftover fuel.

 

At the runway displaying cargo.

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initial climb

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Pods drop, things are heating up

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coasting to the edge of the Atmosphere, outrunning the daylight. solar panels hidden in aft service bays extend to catch some energy

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Circularized at sloppy 119x105.5Km orbit , emerging from night side. opened the Baysto showcase the VTOL Spikes 

 

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most forward K 6-Meter fuel tank is locked and hence still full after the acsend. if all remainder fuel also pumped forward reentry can be made at perpendicular (radial out) attitude for excelent breaking. if fuel not pumped entry is strictly prograde, smallest deviation and it tumbles uncontrolably    just hitting the edge while over the dessert:

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deeper in the Atmo. (took the screen on another flight). the craft has to be very nose heavy to keep this attitude, to heavy to keep horisontal in normal flight , so another fuel transfer needs to be made at about 30-25Km.

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Edited March 21, 2017 by agrasyuk