Need help making a Panther SSTO spaceplane

[Jovus]

Hi all. I have some very specific mission requirements that I've toodled around with and haven't yet gotten to work. So I've decided I need some help from the forums.

Right now I'm using 2 Thuds for rockets and 3 panthers for jets. My intakes are engine nacelles and a circular intake on the front of a Mk1 Inline; I don't have ram intakes yet. I'm using stock aero, not FAR (which is a first for me).

Here, have a link to the craft file: https://kerbalx.com/crafts/13522/

and a picture:

Spoiler

It needs to be able to get at least 2 passengers (preferably more, but I'll fly more missions) into orbit, rendezvous with a station in 125km x 125km (not dock), and re-enter. I'm comfortable with a bingo fuel reentry. Right now it gets into an 85km x -60km suborbital trajectory. My flight pattern involves getting up to Mach 2.5 around 15km on afterburners, then cutting over to rockets for the rest of the flight (keeping jets on until they starve out for the extra savings).

I originally tried a single-core plane without the extra fuel on the front; it behaved similarly, but I had to enter a shallow dive from 10km to punch through the Mach barrier. I'm comfortable doing that, if it works, but with 3 Panthers this is just point and zoom.

 

What modifications do I need to make to make this work? Am I flying it correctly, or is there a more efficient profile? I haven't flown SSTOs since 0.90, and I haven't flown stock SSTOs ever (I'm a FAR aficionado momentarily exiled by my potato computer).

 

Thanks pre-emptively for your help.

[ForScience6686]

My version of a panther ssto used a higher vertical climb rate.  Since you can't get the speed like in traditional ssto, you need more vertical speed to have enough time to reach orbital speeds before you pass your ap.  So my advice would be increase your vertical speed, do not level out to gain horizontal speed.

[bewing]

Well, I have a number of issues with your design. But I'm picky that way -- so don't take any of this as a criticism. Since you are so close to getting to orbit, you may just want to implement one of these suggestions and run with it.

1) After generating the initial speed and altitude, the airbreathing engines are basically dead weight (until after reentry at least). The panther is the lightest decent jet engine, but it's still darn heavy. So you only want ONE in your design.

2) An SSTO must minimize drag and weight. The front end of your plane is very good for this. The back end, not so much. Why two tailfins? At most you need one. And it should be as small as possible to minimize drag and weight. Do you have advanced reaction wheels yet? You could substitute one and get rid of many/all of your control surfaces -- control surfaces add a lot of drag and weight.

3) There is a bug with fuel ducts and struts right now that make them add an insane amount of drag. Like, 5 times as much drag as the rest of the plane, combined. I see two fuel ducts, at least. Are you dead-set against doing manual fuel transfers in flight? Jet engines can access fuel everywhere in the plane without using fuel ducts. So if you put your rocket tanks in front of your rockets, then you don't need fuel ducts.

Do you have any struts on that thing? If so, try hard to get rid of them.

4) Two to four little canard wings would completely substitute for every single control surface you have on that plane, AND give you more lift. Bahfooey on negative control surfaces. (<- this opinion is not universally shared).

5) Terriers are lighter than thuds and are much more efficient. I promise that 120kN of thrust is more than enough to get you to orbit, if your plane is a little lighter and less draggy. So combining with point 1) and 3) -- get rid of the thuds, get rid of the two extra panthers, replace the two panthers with terriers, and move all the rocket fuel into the nacelles.

 

-- Beyond that, panthers need a little help in order to get into their high-thrust regime -- somewhere just over 400 m/s. You can go into a shallow dive at 7 or 8 km altitude (often the best strategy with a panther). Or you can turn on the rockets for a few seconds. The panthers are guaranteed to flame out when you hit about 940 m/s -- as low as 17km altitude. After that it's all rockets, and I advise letting the nose rise by itself to a 30 degree attack.

P.S. in 1.1.2 there is also a bug with "body lift" being zero. It has already been fixed for the next version -- so what that means is that any SSTO you make now will perform a little better in 1.1.3.

Edited May 20, 2016 by bewing

[AeroGav]

I'd be tempted to say the Thuds are not a great solution,  their Vacuum ISP is rather low, and above 10km is effectively "vacuum" as far as ISP is concerned - and a pair of them is over 2 tons added to your upper stage mass.

Perhaps you could try a fuselage mounted Terrier with a panther in each of the side pods.  Or a single panther in the fuselage and a terrier in each side pod.

I've made early career planes with single terrier and 2 panthers in underwing pods, which i dropped when they flamed out.   I could each orbit without dropping them but doing so gives a little bit of delta V margin and is still half the cost of a rocket of the same ability.

Bear in mind, each panther is 1.2Tons that your rocket engine has to accelerate from mach 2.5 to mach 7, so i'd say spamming them for a better zoom climb is a mistake.

BTW if your mission involves going much beyond low orbit, you could make part of the ship capable of undocking for the high orbit excursion, like so - greatly reduces the amount of mass that needs to be taken upstairs -

 *please note i'm using Kerbalism here so have to drag a very heavy food container with me 

An earlier version.  Straight wings much easier to construct but it snags on solar panels and was very hard to undock -

 

However, the drag created by the docking adapter is probably not worth it if you're only going up to 125km.  Munar flyby, that's another matter.

 

Edit - Well Jovus,  I spent 3 hours messing around with your Hermes plane. 

I removed the fuel ducts, replaced the thuds with a terrier, added canards and a few wing strakes, but it ended up being a pig to fly and still wouldn't orbit.     Lift drag ratios were only 60% of what my own spaceplanes normally get,  it also felt under winged and i kept having to crank the nose up to get sufficient lift, or fly faster lower than i would normally like, all of which made the drag issue worse.   The terrier was only putting out 60kn and drag was 40-50 for far too long.

I just gave up on trying to do this purely single stage and reinvented it as a staged airbreather.  Wasn't able to preserve the original look of the ship :-(

https://kerbalx.com/AeroGav/Hermes-1c

Cost to fly is 4331 kredits per launch , assuming it returns to ksc for 100% recovery, it returns empty of fuel and of course the Panther stage is discarded and counted as a total loss.

edit 2 - you could probably not throw away the air intake/jet fuel tank and conceivably still meet performance criteria, that would save substantial cost.   I just put them aft of separator because i didn't want the jet engine to futz with the balance of fuel in the rocket tanks, could lead to mass shifting around in the closed cycle stage of the ascent, which is the worst possible time to be dealing with that stuff.

 

Edited May 20, 2016 by AeroGav

[bewing]

@OP: I just modded your jet to look basically like one of mine.  -- The only big further changes I'd make would be to swap the terriers for nukes (and go to a pure-LF system -- it's actually quite a bit more efficient), and to swap the wings for BigS deltas. AeroGav is right that you would do better with at least 50% more wing area -- and the BigS wings hold fuel. Of course, it's also true that a whiplash works much better for this whole job than a panther does, but I'm assuming that you have tech limitations?

But my modded spaceplane gets to orbit now with 89 oxidizer and 280 LF remaining to do your rendezvous and land with. It still flies OK in atmosphere and reenters pretty well. You might even be able to get away with adding another crew module to it. You could certainly put a probe core in it and do away with the pilot.

If you want a picture or craft file, just say so. 

Edited May 21, 2016 by bewing

[mk1980]

you're probably overthinking the whole problem. you don't make it to orbit because you don't have enough LF/Ox fuel.

here's a simple low tech SSTO i made a while back (as you can tell from the landing gear, it's from version 1.05). got it to orbit, but it only had ~150 m/s left at 75x75 km orbit, so it could use some more fuel if you want to get to a higher orbit.

in retrospect, it would have been smarter to use a single swivel instead of a pair of thuds. that alone might improve efficiency enough to get it to a higher orbit and back (i think)

[AeroGav]

9 minutes ago, mk1980 said:

you're probably overthinking the whole problem. you don't make it to orbit because you don't have enough LF/Ox fuel.

here's a simple low tech SSTO i made a while back (as you can tell from the landing gear, it's from version 1.05). got it to orbit, but it only had ~150 m/s left at 75x75 km orbit, so it could use some more fuel if you want to get to a higher orbit.

in retrospect, it would have been smarter to use a single swivel instead of a pair of thuds. that alone might improve efficiency enough to get it to a higher orbit and back (i think)

Woah, that's the highest fuel fraction i've ever seen on a spaceplane, the most i ever had was like 50% or so.

The OP does have a lot of fuel though

one FT800 tank  800 LFO

three FT400 tanks  400 LFO

total  2000 LFO. Also about 400 jet fuel in the two nacelles.  He must be in diminishing returns at this point, it certainly feels like it struggles  for lift with all that on board.

My version of the ship had one ft800 and two ft 200, total 1200 fuel.  Generally that's about as much as you can put on one terrier anyway.

It's a good point though.  We obsess about efficiency sometmes  and neglect the obvious.  I've deleted perfectly good spaceplanes for under performing, looked at screenshots of them flying and realised they were making orbit easily with a fuel fraction under 30%.   

[mk1980]

oh yes, you're right about the excessive fuel. that was one of my first SSTOs so it's not really optimized in any way and i probably made lots of rookie mistakes in the ascent. 

i guess my point is that you could probably just slap on some more fuel without doing a fundamental redesign and it would still fly reasonably well. 

in the end, it's about getting stuff to orbit. imrpoving overall efficiency is the elegant solution, but in some cases a simple "brute force" approach will get the job done, too.

[Jovus]

Thanks for all your help, guys!

It's nice to see it's not just me being entirely incompetent. I ended up going with a drop-pod solution

Figured it out with your input. I give you: the Hermes 2

https://kerbalx.com/Jovus/Hermes-4

My problem came largely in two parts: 1) I didn't realize I could manually transfer fuel between tanks (thought I needed another building upgrade) and 2) I thought the engine nacelle only had 40 fuel, not 150! (Shows me for just assuming I knew things instead of reading part descriptions).

 

To address a couple specific questions or pieces of advice:

First, the tailplane and control surface design. I'm coming from FAR and I plan to go back to it. The dual tailplane is for yaw and roll stability at higher altitudes (and it's not enough, frankly, but I'm willing to compromise that far). The same goes for the control surfaces - that and I don't have any electricity generation on the craft, and I'm using TAC-LS, so I don't want to drain my batteries just to keep the nose up. That said, I recognize the validity of the criticism, I'm just playing with a handicap.

I started with the Thuds because I started with a single core; once I moved to three cores the Rockomax engines were quickly replaced with LV-909s.

I'm not using anything like nukes or even ramp intakes because I don't have them. I just unlocked the Panthers and don't even have a Mainsail yet. When my Space(plane) Program truly gets off the ground I'll definitely be switching over to LV-Ns, because who wants to bring oxidizer everywhere?

I was frankly shocked at the suggestion to add more wing area. Not only would more wing add more mass (and more drag), but also the plane feels like it flies just fine to me. Does anyone want to expand on why more wing area might be a good idea?

Again, I really appreciate all the help here. Thanks guys!

[AeroGav]

2 hours ago, Jovus said:

Thanks for all your help, guys!

It's nice to see it's not just me being entirely incompetent. I ended up going with a drop-pod solution

Figured it out with your input. I give you: the Hermes 2

https://kerbalx.com/Jovus/Hermes-4

My problem came largely in two parts: 1) I didn't realize I could manually transfer fuel between tanks (thought I needed another building upgrade) and 2) I thought the engine nacelle only had 40 fuel, not 150! (Shows me for just assuming I knew things instead of reading part descriptions).

 

To address a couple specific questions or pieces of advice:

First, the tailplane and control surface design. I'm coming from FAR and I plan to go back to it. The dual tailplane is for yaw and roll stability at higher altitudes (and it's not enough, frankly, but I'm willing to compromise that far). The same goes for the control surfaces - that and I don't have any electricity generation on the craft, and I'm using TAC-LS, so I don't want to drain my batteries just to keep the nose up. That said, I recognize the validity of the criticism, I'm just playing with a handicap.

I started with the Thuds because I started with a single core; once I moved to three cores the Rockomax engines were quickly replaced with LV-909s.

I'm not using anything like nukes or even ramp intakes because I don't have them. I just unlocked the Panthers and don't even have a Mainsail yet. When my Space(plane) Program truly gets off the ground I'll definitely be switching over to LV-Ns, because who wants to bring oxidizer everywhere?

I was frankly shocked at the suggestion to add more wing area. Not only would more wing add more mass (and more drag), but also the plane feels like it flies just fine to me. Does anyone want to expand on why more wing area might be a good idea?

Again, I really appreciate all the help here. Thanks guys!

Well to an extent you're being pulled in different directions here because Bewing believes that oversized control surfaces cause excess drag , whereas i prefer to err on the side of caution, also i think the drag is dependent on deflection angle so a large surface with the authority turned down in context menu is better than a tiny one that's maxed out.  Piloting style comes in there - Bewing likes to lock a pitch angle with SAS,  I use pitch trim or Mechjeb, both methods limit violent G pulls, AoA and control deflection angle spikes that come when flying on keyboard (or with the game's horrible joystick support).

Wings, I prefer large ones.  I've dabbled with FAR myself , I know that in FAR, wing lift starts off lower (so higher takeoff/landing speeds) but declines less at supersonic speed than in stock (so easier to get a fast jet with small wings to high altitude).   Also, in FAR, optimum lift drag ratio at mach 5 is something like 20 degrees.  In stock game, it's only 6 degrees.

So , in this example we're making a lift:drag ratio of 3 (it's carrying fuel tank dorsally as part of a payload challenge),  AoA is 5, we're getting a lift/drag ratio of 3 and experiencing a total drag of 13.549kn, versus total thrust of 59.870 kn.  This leaves us  46kn to accelerate/climb with.   If the wings were much smaller, we might have AoA of 15, and only be getting a lift drag ratio of 1.5, meaning twice as much drag and so there will only be 32.772kn surplus.

On the other hand, as you point out, wings have mass,  so it's a balance you have to strike.  The largest of the modular wing pieces weigh 0.2 tons each, which isn't much though , compared to engines etc.

You can access that aero data gui  by pressing alt f12 and going to the physics tab, aero subtab, and checking the box to display that data.

Edit - taking the above example, if you did have small wings, your options are

a) increase their size so you can get lift at the same altitude and airspeed without flying an inefficient AoA

b) stick to the optimum AoA, but be at a lower altitude for each mach number milestone on the way to orbit due to less lift.  This can cause parts to overheat

c)  brute force it with more powerful engines.   drag going up from 15 to 30kn is less important if you got 120kn instead of 60kn thrust because of having two terriers.  And tbh this is easy to do, Terrier's are only half a ton each so it weighs about the same as doubling your wing area.

Edited May 22, 2016 by AeroGav

[Jovus]

I know about the AeroGUI data, but thanks for pointing it out. What I didn't know was the differences in L/D optimum AoA and how wing area functions differently between Stock and FAR, though it makes sense they'd be vastly different, since that's the point of FAR

[bewing]

2 hours ago, Jovus said:

I was frankly shocked at the suggestion to add more wing area. Not only would more wing add more mass (and more drag), but also the plane feels like it flies just fine to me. Does anyone want to expand on why more wing area might be a good idea?

AeroGav and I have collaborated on spaceplanes previously, so everything he said is correct.

Besides providing weight and drag, wings provide lift -- so they are not deadweight. It's not just about the plane feeling nice to fly. As AeroGav was saying, it's also about the amount of engine thrust that needs to be devoted to keeping the plane from plummeting back to the ground. A plane with more lift needs a lower AoA and less thrust. But it's a bit of a tradeoff with part count, weight, cost, and total drag. Your wings have an effective area of a little over 4. It's easy to try a couple experiments to increase that number by 50% or 100% and see what happens. And what happens is that you end up in orbit with more fuel.

The same goes for the control surfaces - that and I don't have any electricity generation on the craft, and I'm using TAC-LS, so I don't want to drain my batteries just to keep the nose up.

You know that all those control surfaces all use electricity all the time, right?

Edited May 22, 2016 by bewing

[Jovus]

I do now.

 

2 hours ago, bewing said:

Your wings have an effective area of a little over 4. It's easy to try a couple experiments to increase that number by 50% or 100% and see what happens. And what happens is that you end up in orbit with more fuel.

Huh. Stock is weird.

[AeroGav]

Jovus, grats on making the 125km orbit SSTO.    I could do it easily but only by discarding the jet engine.

I tried a 3 core layout like you appeared to have pictured - a central jet, and two terrier pods, but it didn't want to fly much above 4km without going afterburner and wouldn't go much above 10km subsonic.    I got up to 11km, shallow dive, made it through the sound barrier, barely, but by then i'd used up all the jet fuel and was digging into the rocket tanks. Thrust was only slightly above drag.

I'd tried clipping nose cones onto the back of the terriers too (then offsetting them inside the engine so as not to block the thrust path) to lower drag, but no go.

What I could try next is more clipping abuse, a single core design with a terrier and then a panther clipped inside of it.  Less drag than a 3 core design, and if the jet engine robs fuel from the rocket tanks they at least can return the favour when the time comes.   Still,  it's at least 15% increase in dry weight to be pulled to 125km if we bring the engine, you could add more fuel, but if the weight starts creeping up to 20t then that's too much for one terrier. 

4 hours ago, Jovus said:

I do now.

 

Huh. Stock is weird.

Stock has so many deviations from real world physics, but if you look at real world aircraft, those that go fast high up do have a lot of wing area, just like planes that go slow and low.   However the shape is very different for shock and wave drag, and not so efficient at low speed.  Think Concorde, XB-70 Valkyrie.   

Of course if you want to go fast at low level, just put on the tiniest stub wings you can get, minimise frontal area. Even the Skylon has a reasonable wing loading, bearing in mind the whole fuselage is a lifting body, and that all the LH2 it is stuffed with is at least 11 times less dense than jet fuel.

 

[Jovus]

5 hours ago, AeroGav said:

Stock has so many deviations from real world physics, but if you look at real world aircraft, those that go fast high up do have a lot of wing area, just like planes that go slow and low.   However the shape is very different for shock and wave drag, and not so efficient at low speed.  Think Concorde, XB-70 Valkyrie.

You and I have different definitions of fast. Mach 2 won't get you to orbit.
https://en.wikipedia.org/wiki/Boeing_X-51

https://en.wikipedia.org/wiki/NASA_X-43

https://en.wikipedia.org/wiki/North_American_X-15#Specifications_.28X-15.29

 

But even the Concorde has relatively high wing-loading. The Concorde's wings are so big simply because the fuselage is so  heavy.(and, I imagine, because passengers don't like the kinds of landings necessitated by smaller wings).

Bell X-1 wing loading: 463 kg/m²

SR-71a wing loading: 406 kg/m²

Concorde wing loading: 530 kg/m²

X-15 wing loading: 829 kg/m²

Compare the wing loading of a C-130, a quintessential heavy cargo plane: 434 kg/m²

I excuse the X-1 and SR-71 by pointing out those are early experiments, and they wanted the Blackbird to have some staying power instead of just getting up and down.

[bewing]

3 hours ago, Jovus said:

You and I have different definitions of fast. Mach 2 won't get you to orbit.

*snip*

Before you can light your rockets and zoom prettily to orbit, your SSTO jet has to climb from 0km to the lower stratosphere. Not one of your pictured examples was capable of that. And that's the part that you need the bigger wings for. That initial heavy climb.

BTW -- since you mentioned that you barely made your orbit and you might need a better launch profile to save some fuel, maybe try this: take off in dry mode, and get as high as you reasonably can that way (probably about 2km), then light the afterburner, then at 5km altitude level out, wait a few seconds for the panther to hit max speed, then light the terriers for a few seconds to get your speed just above 400 m/s. Then you need to temporarily turn the terriers off again -- this is most convenient with an action group, if you have them. The panther should start accelerating the whole deal all by itself to about 650 m/s in a nice climb at that point. That should save you a lot of fuel in the end.

Edited May 23, 2016 by bewing

[KerikBalm]

3 hours ago, bewing said:

Before you can light your rockets and zoom prettily to orbit, your SSTO jet has to climb from 0km to the lower stratosphere. Not one of your pictured examples was capable of that.

Thats nice and all, but keep in mind what he was responding to:

11 hours ago, AeroGav said:

if you look at real world aircraft, those that go fast high up do have a lot of wing area, just like planes that go slow and low

The examples you point to also need to be able to fly well enough "low and slow". So it comes down to things that fly low and slow need to have a lot of wing area.

Those that go fast up high don't need to have it... if they do, its for going low and slow at a different time in the flight

[AeroGav]

15 minutes ago, KerikBalm said:

 

Those that go fast up high don't need to have it... if they do, its for going low and slow at a different time in the flight

Darpa falcon, mach 6 wave rider concept.  Looks like they never got very far with this one , because two rather different versions on line

 

Now that compression lift is better understood,  i wonder if they actually make more use of lift in the cruise phase rather than just basiclly make a rocket and put the smallest stub wings on you can get away with for landing on a 12 mile strip of lakebed when empty.   They now think you can get 10:1 l/d ratio at mach 6 with a wave rider

THe Xb70 Valkyrie  (above) was designed for mach 3+ at 70,000ft plus.    If it was meant to do mach 3 lower in the atmosphere smaller wings would of course be optimal, because there would be more skin friction drag and simply reducing the area as much as possible helps with that. When the air is rarefied, friction drag is less and wave drag predominates.   Because of careful area ruling and because the large wing area is all behind the nose's shock cone the large wing area doesn't add much drag.  I've done similar in FAR and achieved low supersonic drag, though takeoff speeds were still 70 m/s.

The biggest drawback, in stock KSP, to using large wings is that your plane won't want to go supersonic too low down, which can make Rapier designs very sluggish and waste a lot of their fuel on the slow subsonic climb, due to low thrust weight ratio "gravity losses".   Two workarounds, drop tanks or detachable panther boosters (a small fraction of the cost of a NERV/Rapier long range spaceplane)

that one had two cargo bays and could fly straight to minmus with a small ISRU setup.

[Val]

Here's my attempt at a Panther SSTO, updated for 1.1.x.

• Crew: 1x Pilot + 4x Passengers
• Propulsion: 2x J-404 Panther + 1x LV-T45 Swivel
• Liquid Fuel: 1900
• Oxidizer: 1760
• Dry Weight: 13.4 t.
• Wet Weight: 31.7 t.
• Time to MECO: 7-8 minutes.
• Time to Orbit: 14-15 minutes.

~350 m/s dV in 80 km equatorial orbit.