New Rocket Jockey SSTO Problems

[TheDarkMechanic]

I've been playing since the tail end of like .24 or so. Decided to finally get on the forum. Have lurked for a long time and you all (and the youtube guys) have taught me a whole lot.

Intro made, now...

Never been all that great with spaceplanes but until now I've always been able to at least get something working. So what the heck is wrong with my gorram SSTO?

She doesn't want to do much more than a 5-10 degree ascent without losing thrust or knocking off velocity. Around 15,000m and past mach 2 she starts losing a lot of thrust on the air breathers and I engage the rocket engines. She will make space but I never have the fuel to circularize. This is modeled after many working designs, both mine and ones I have seen in videos. Am I overlooking something? Or am I just a crap pilot?

[Vanamonde]

Welcome to the forum, TheDarkMechanic.

(Your thread has been moved to the how-to subforum, by the way. Good luck with it.)

[robertlong13]

Could you post a link to the craft file? That way we can play around with it and see what's wrong.

[GoSlash27]

DarkMechanic,

Judging by the looks, not a lot wrong here. I would get rid of the structural intakes; intake spamming is no longer helpful. You might consider replacing the Swivels with a single Aerospike.

Also, what's up with the odd- looking circular intakes?

Your main problem seems to be the ascent profile. You should be well over 1km/sec and 20 km altitude before you switch to rockets.

Best,

-Slashy

Edited August 23, 2015 by GoSlash27

[Kelderek]

Looks to me like you may be lacking air intakes. The structural mounted intakes are pretty bad, as they barely give any intake air and also have high drag. You should have a couple larger intakes on the nacelles in front of your engines. Shock Cone intakes have good air and low drag.

Other than that, it would be hard to speculate on issues without seeing more numbers and the centers of lift and mass.

Edited August 23, 2015 by Kelderek
meant to use "structural" instead of radial

[Nothalogh]

I've been playing since the tail end of like .24 or so. Decided to finally get on the forum. Have lurked for a long time and you all (and the youtube guys) have taught me a whole lot.

Intro made, now...

Never been all that great with spaceplanes but until now I've always been able to at least get something working. So what the heck is wrong with my gorram SSTO?

http://img.photobucket.com/albums/v304/tdm/x1.1%201.jpg

http://img.photobucket.com/albums/v304/tdm/x1.1%202.jpg

She doesn't want to do much more than a 5-10 degree ascent without losing thrust or knocking off velocity. Around 15,000m and past mach 2 she starts losing a lot of thrust on the air breathers and I engage the rocket engines. She will make space but I never have the fuel to circularize. This is modeled after many working designs, both mine and ones I have seen in videos. Am I overlooking something? Or am I just a crap pilot?

Drop us a link to the craft file, I'll see what I can do with it

---

In the mean time I'll try to replicate your design, but i make no promises, as I have never succeeded with turbo+rocket designs, I rapier+nuke for my stock SSTOs

---

So I put this together, I call it the Flintstone

I build and fly with FAR, so I can't exactly guarantee performance, but I went supersonic in a 30deg climb, pitched down to about 15deg at around 9km, ran up to 1250m/s on the turbos until 19km or so, then switched to the rockets, maintained a 20deg pitch from there on out until my apoapsis hit 71km or so, then set my SAS on prograde and coasted to apoapsis.

Had about 300m/s dv after circularization.

Javascript is disabled. View full album

Oh, and it has a little satellite in it with scientastical things, an engine and an Oscar-B worth of fuel, just make sure it has fuel in it before you let it go because the jets use from all tanks.

Edited August 24, 2015 by Nothalogh

[TheDarkMechanic]

Thanks, everyone, for getting back to me. Included is a pic of the relevant centers. Where/how would one host a craft file? I am playing science mode this patch because I wanted to progress a little faster than career. Still, I haven't built far enough down the tree for the aerospikes, rapiers, or shock cones. As soon as I do you can be sure I'll incorporate them. But I'm very tired of launching a rocket every time I want to go to my space station (old habits from conserving money) so I'm hoping to make something work.

DarkMechanic,

Judging by the looks, not a lot wrong here. I would get rid of the structural intakes; intake spamming is no longer helpful. You might consider replacing the Swivels with a single Aerospike.

Also, what's up with the odd- looking circular intakes?

Your main problem seems to be the ascent profile. You should be well over 1km/sec and 20 km altitude before you switch to rockets.

Best,

-Slashy

I assume you mean the engine nacelles? I am not very familiar with them but I read the stats and figured they accomplished a lot at one time so I slapped them on. Should intakes go in front of them?

Also, I fitted a version with a srb assist stage which produced only marginally better results.

[satnet]

I put together a quick and dirty approximation of your plane. I did swap the structural intakes for ram-air intakes right off the bat since as others have pointed out the structural intakes aren't well suited to spaceplane use. The design isn't half bad, a few tweaks and I could get it to a roughly circular 75 km orbit. I did swap out the long MK2 liquid fuel fuselage for a short LF and a short LFO MK2 fuselage. My first attempt at an orbit had roughly 490 units of LF without any oxidizer. I would also recommend swapping out the engine nacelles even though I didn't try this. They are rarely worth the weight compared to what you can use in their place.

Using my usual ascent profile I peaked at around 1.2 km/s and maintained over 1 km/s up to about 25 km. The short version of my ascent profile is accelerate to about 270 m/s at a modest pitch, then climb to 10 km with as high a pitch as you can while maintaining about 270 m/s (just don't pick up too much vertical speed, keep it under 100 m/s). At 10 km start to level out by setting SAS to point prograde and letting it track prograde down to just below 10 degrees pitch and return to stability. Usually I level out at around 15 km. You should already be going about 1 km/s at this point. As you observed you'll start losing thrust, but you should still have enough to continue accelerating. For TurboJets you'll probably start losing velocity around the 20 km point, but I usually stay with the jets until they either cut out or I drop too close to 1 km/s. At this point you switch to rockets and you can basically ascend like you normally would at this point if you were using a rocket. I'm changing my ascent profile all the time and there is probably someone out there with a better one, but this is the one I like and it worked for my version of your plane at least once. Jet engines are pretty sensitive to the operating conditions, airspeed, altitude, air intake, and angle of attack all factor in and you need to maximize them all for a good ascent with a spaceplane.

The margins were pretty thin, though that may be because my piloting skills aren't tuned for this plane. I barely had enough fuel to de-orbit, but once you're on the right track it should just take a few tweaks to get some more delta-V.

[Streetwind]

She doesn't want to do much more than a 5-10 degree ascent without losing thrust or knocking off velocity. Around 15,000m and past mach 2 she starts losing a lot of thrust on the air breathers and I engage the rocket engines. She will make space but I never have the fuel to circularize. This is modeled after many working designs, both mine and ones I have seen in videos. Am I overlooking something? Or am I just a crap pilot?

While the others are talking about plane design, I'll quickly go into how airbreathing engines work.

In old versions of KSP, air-breathing engines had a thrust curve based on velocity and an Isp curve based on altitude. This changed drastically with 1.0.

Nowadays, air-breathing engines have two velocity curves - one based on velocity and one based on altitude - but no Isp curve. Your Isp stays constant at all times, but if you're operating outside the engine's rated optimal altitude, you're getting less thrust for that constant fuel usage, so that's basically a "stand-in" for the old Isp curve. Except that additionally, it affects the way you fly the plane.

Both the Whiplash and the RAPIER are modeled mainly as ramjet engines, but with turbine assistance at subsonic speed. Ramjets only work properly when flying supersonic; their velocity based thrust curve starts low on the runway and then increases with speed for a while until finally it starts dropping off again with even more speed. As a result, a spaceplane equipped with these engines may have trouble hauling itself off of the runway on turbine thrust alone; and when you're already struggling to maintain airspeed, climbing is even harder. Which is kind of a catch-22, because if you could only climb a little, you could accelerate better with less drag... But, if you're investing your thrust in climbing, you're not accelerating right now, which means your engines don't spool up to higher thrust levels... And so on and so forth. It can get tricky.

But if you somehow manage to push through the sound barrier and pass 350 m/s, the ramjets finally get going and suddenly the plane starts pushing. What now happens to newcomer pilots is that in their elation, they pitch the nose up sharply and try to ride that massive ramjet thrust tsunami up and away towards the edge of the atmosphere. And then the other thrust curve, the altitude-based one, comes into play and the engine thrust evaporates all of a sudden, even if there's still air in the intakes. This is especially true since the high tech engines are quite optimized for high altitude flight, with their atmosphere curve all but plateauing for a while... and then they suddenly fall off a cliff at the end of that plateau. Since you report that you're only at mach 2 while going past 15km and losing thrust, I'm fairly sure that this is exactly your issue. You're going too high too early without accelerating first.

What this means is that you need to learn in which regimes your engines can be operated in the most favorable manner. Find the "cliff edge" of the atmospheric thrust curve, right before the engine starts rapidly losing thrust, and level off at that altitude to accelerate as much as you can. At some point, the engines will again begin to rapidly lose thrust, this time due to sheer speed, and that's the point where you have maxed out their potential. Then you pitch up into your actual climb into space. After your pitch maneuver, you switch the airbreathers off and the rocket motors on.

Judging from the Whiplash's config file, you want to stay above an atmospheric density of 0.16 while accelerating before your final climb. I'm not exactly sure what height that translates to, maybe some of the more experienced pilots can weigh in on that. You can probably get to mach 4 before you need to start pitching up.

[DCWarHound]

A few things to keep in mind:

You want to keep the amount of dead weight you're carrying to a minimum,things like air brakes,monopropellant and rcs thrusters (if you're not going to dock),rocket engines,reaction wheels,wings and control surfaces.

The rocket engines are dead weight in the atmosphere,try and get a combination of "Whiplash" engines and "Rapier" engines.The extra thrust from the rapier will allow you to take more weight to orbit.Your airbreathing stage should get you at least up to 40km.

Airbrakes aren't really needed,tilting your plane upwards will expose its belly which will create allot of drag and slow you down.

If you're only taking up cargo to space and not docking,the monopropellant in the cockpit and 2x single direction rcs thrusters mounted on the top will be enough to get the cargo out of the bay (switch to docking mode and turn on RCS and hold "S" or "W" to fire both rcs thrusters,remember to turn back to staging mode once done)

You don't need more reaction wheels,unless your plane is excessively large,the reaction wheel in the cockpit should be more then enough.

Wings and control surfaces create drag,only use as many control surfaces as you need.To find out if you have enough lift,fly in a straight line and then look at the prograde marker,if the prograde marker is above the direction you're heading then you have too much lift (aka drag which slows you down),if it is just below the direction you're heading then you have enough lift and if it is far below the direction you're heading and you're losing speed then you don't have enough lift.

Calculate how much jet fuel you need by disabling the fuel flow on your fuel+Ox tanks and then doing a standard SSTO SP ascent:

Fly up to 10km at 45 degrees,flatten out to 0 degrees once you reach 10km,once you reach around 800m/s start pulling up to 25-45 degrees (25 makes your rocket stage more efficient but you might burn out)

You need about 2500 of Delta/V for your rocket stage to get in orbits between 72km and 200km with enough left to do maneuvers.

[robertlong13]

Where/how would one host a craft file?

Dropbox is a great free option for file hosting.

[KerikBalm]

In old versions of KSP, air-breathing engines had a thrust curve based on velocity and an Isp curve based on altitude. This changed drastically with 1.0.

Nowadays, air-breathing engines have two velocity curves - one based on velocity and one based on altitude - but no Isp curve. Your Isp stays constant at all times, but if you're operating outside the engine's rated optimal altitude, you're getting less thrust for that constant fuel usage, so that's basically a "stand-in" for the old Isp curve. Except that additionally, it affects the way you fly the plane.

I'm pretty sure that fuel usage is *not* constant for these engines, though I can't check now, I'm pretty sure I remember correctly.

When those engines are at their peak thrust (mach 3-ish), they use much more fuel than on the runway, and when they are barely putting out any thrust, they use much less fuel.

Incidentally, the old version did have a Isp curve based on velocity too, it just wasn't called as such. The old version used Isp to scale fuel consumption.

The velocity curve scaled thrust, but didn't alter fuel consumption.

So while an engine may have been said to have been getting 1000 Isp, if it wasn't at the speed at which it got a 1.0 thrust modifier, it was actually getting considerably less (well, not counting the Isp being 16x too high because it was counting air mass as if it was fuel, and the engines used 15 tonss of air mass for 1 ton of fuel).

This meant their Isp was actually zero if you ran turbojets at 2,400 m/s or above (but on the way there, you could get up to 40,000 Isp)

[ajburges]

I was going to reply yesterday, but forgot to post.

First things first I suspect that vessel is short on dV. Whiplash SSTOs want about 1.6 km/s dV on rocket power. Even better if it can get that while full. Whiplash engines peak at 1 km/s so don't plan to go faster than that on jet power. If you do, save mass by reducing engine count.

Remember, once you load the requisite dV and TWR, drag is king for space planes. High drag means you need more jet power/fuel which feeds back to the rocket equation. Wings are really draggy. You only want enough to climb with 15° AoA while full and land comfortably while empty. You have a ton of wing for such a small craft.

Drop the structural intakes and top the side stacks with intakes; Preferably shock cones, but any intake is better than an exposed stack.

- - - Updated - - -

The reason you don't want structural intakes is because they have crazy high drag for the pitiful air they give. You need over ten to match the air from two stacks with intakes covers, but that has at least twice the drag of the two stacks. Squad just won't make them work using.

[A_name]

I've been playing since the tail end of like .24 or so. Decided to finally get on the forum. Have lurked for a long time and you all (and the youtube guys) have taught me a whole lot.

Intro made, now...

Never been all that great with spaceplanes but until now I've always been able to at least get something working. So what the heck is wrong with my gorram SSTO?

http://img.photobucket.com/albums/v304/tdm/x1.1%201.jpg

http://img.photobucket.com/albums/v304/tdm/x1.1%202.jpg

She doesn't want to do much more than a 5-10 degree ascent without losing thrust or knocking off velocity. Around 15,000m and past mach 2 she starts losing a lot of thrust on the air breathers and I engage the rocket engines. She will make space but I never have the fuel to circularize. This is modeled after many working designs, both mine and ones I have seen in videos. Am I overlooking something? Or am I just a crap pilot?

Right off the top of my head:

-You have too much wing. You can get probably away with just a pair of delta wings. The Mk2 parts give plenty of body lift on their own, they really don't need that much wing.

-Too many air intakes. Avoid the radial intakes whenever possible, they are heavy, terrible for drag and give very little air. In fact, the engine nacelles you have there should give plenty of air, although replacing them with spike cone intakes would be even better. This is no longer like in pre-1.0 where you could spam intakes and get better performance at high altitudes. Now your jets will flame out at a certain altitude regardless of how much air your intakes say they're providing.

-You don't need 2 LV-T's. Go with a single one. Also, I can't tell if they're 30's or 45's but I would go for the 30's. You really don't need the thrust vectoring in atmo since you have control surfaces, nor in space since the cockpit provides enough torque for a plane this size. Alternatively, try a single aerospike instead, which is lighter and more efficient than the LVT's, although I'm not sure it'll give you enough thrust since it's a smaller engine, but try it. Remember that in the rocket phase of a SSTO spaceplane ascent you really don't need that much thrust. You don't even need a TWR>1. By the point you switch to the rocket engine you should be going fast and high enough that a TWR>.5 or so will do the trick.

-Place the cargo bay further back, closer to the center of mass, which will help you avoid the center of mass moving when you release the payload.

Edited August 24, 2015 by A_name

[Nich]

I agree with pretty much everything said and want to add I also feel like your plane has a bit too much wing and switching to aerospikes or terriers on your LFO engines.

[Vegetal]

Come on guys, he already said he is on a science game and doesn't have aerospikes or rapiers.

I'll toss a guess that what's wrong is your ascent profile. It is king for spaceplane SSTOs.

[Bloody_looser]

You have open nodes on the nacelles. Open nodes produce some serious drag.

[ajburges]

Right off the top of my head:

-You have too much wing. You can get probably away with just a pair of delta wings. The Mk2 parts give plenty of body lift on their own, they really don't need that much wing.

-Too many air intakes. Avoid the radial intakes whenever possible, they are heavy, terrible for drag and give very little air. In fact, the engine nacelles you have there should give plenty of air, although replacing them with spike cone intakes would be even better. This is no longer like in pre-1.0 where you could spam intakes and get better performance at high altitudes. Now your jets will flame out at a certain altitude regardless of how much air your intakes say they're providing.

-You don't need 2 LV-T's. Go with a single one. Also, I can't tell if they're 30's or 45's but I would go for the 30's. You really don't need the thrust vectoring in atmo since you have control surfaces, nor in space since the cockpit provides enough torque for a plane this size. Alternatively, try a single aerospike instead, which is lighter and more efficient than the LVT's, although I'm not sure it'll give you enough thrust since it's a smaller engine, but try it. Remember that in the rocket phase of a SSTO spaceplane ascent you really don't need that much thrust. You don't even need a TWR>1. By the point you switch to the rocket engine you should be going fast and high enough that a TWR>.5 or so will do the trick.

-Place the cargo bay further back, closer to the center of mass, which will help you avoid the center of mass moving when you release the payload.

Stay with the Swivels. For the fuel mass and ratios most SSTOs operate with, the higher Isp is a better option. They are also easier to replace with aerospikes once you get them. Just lock the gimbals.

[TheDarkMechanic]

Valentina and Robert did not deserve what I did to them. By the time anyone reads this I will have just launched another pod to my station out of frustration. Today's variant is angering me. Craft here:mad:

How it's going: Crap for thrust or ability to climb up to 8000m. Much fuel wasted here. I'm getting much better thrust performance from there to about 15000-16000m (7-800+ Kn). I can get past 1km/s now but all that thrust craps out long before 20000m and I cannot get a more severe ascent angle without killing my thrust. In the upper atmosphere she WILL NOT pitch up more than about 25 degrees.

Things that helped: Had too much wing surface, big change when I knocked some off. Changing out big LF tank for smaller one and a larger LFO tank. Moving LF tank forward (even though the cargo bay is not actually being used for deliverable payload (no worries, all I need it to do is carry passengers). Ditching nacelles.

A couple things about my design philosophy: Some of you are probably looking at me crazy about the dorsal and ventral tail planes. I had a lot of luck providing more stability with this idea in earlier versions, not sure how well it's translating. Same with the upturned wingtips (obvious). The airbrakes have no priorities at all. They are linked to an action group and I only use them to slow down during decent through the upper atmosphere.

Questions:

Why does my plane randomly roll/yaw without any controller input? I frequently find myself off course port or starboard (mostly starboard) and when I try to correct this I lose inclination, roll unpredictably or it just nudges and then goes back to the previous or slightly worse attitude. TL;DR: HOW TO KEEP THA THING STRAIGHT!?

Is there an advantage to mounting the RAMs pointed slightly downward?

So I put this together, I call it the Flintstone

I build and fly with FAR, so I can't exactly guarantee performance, but I went supersonic in a 30deg climb, pitched down to about 15deg at around 9km, ran up to 1250m/s on the turbos until 19km or so, then switched to the rockets, maintained a 20deg pitch from there on out until my apoapsis hit 71km or so, then set my SAS on prograde and coasted to apoapsis.

Had about 300m/s dv after circularization.

http://imgur.com/a/yw1Jx

What are you using that gives you that red trajectory in atmosphere. I see that on youtube and am jelly.

Thanks everyone for helping and making a new member feel welcome. I will be launching for Duna soon and upon return I should be able to pick up some better spaceplane tech. If someone doesn't fix me by then I'll just scrap this thing and use the better parts.

[Nothalogh]

What are you using that gives you that red trajectory in atmosphere. I see that on youtube and am jelly.

The Trajectories mod, by Youen http://forum.kerbalspaceprogram.com/threads/104694

[GoSlash27]

DarkMechanic,

This looks like a really good revision!

The reason you're having stability issues is because you've got too much active control surface. A proper spaceplane should fly like a sled, not a fighter. Twitchiness is bad.

Try disabling all aft control surfaces (especially the rudders), have roll and pitch on the canards and see if it flies better. If it's a little too heavy that way, try disabling roll on the canards and enabling it on the elevons.

I pitch my ram intakes down a bit because I'm usually flying with 5° angle of attack by the time I hit 20km altitude and that gives me a little more air.

Still not happy with the rocket engine. You don't need that much rocket thrust for that sized plane and the Isp isn't very good.

Still... it looks to my tired eyes like this plane should do the job as it sits. I think your ascent profile may need fine- tuning.

Best,

-Slashy

- - - Updated - - -

*Edit*

Just downloaded it and played around with it.

#1 too many struts. Struts kill your coefficient of drag.

*edit2*

Yeah, that's your problem. I yanked off all the struts and it took off like a raped ape. Got to orbit no problem. 147 m/sec DV on orbit, where before it was about 400 m/sec short.

Edited August 24, 2015 by GoSlash27

[TheDarkMechanic]

Still not happy with the rocket engine. You don't need that much rocket thrust for that sized plane and the Isp isn't very good.

Think I can get away with a terrier?

[Jett_Quasar]

Add two more engine pods like the ones you have on the wings but replace the turbojets with Rapiers. Set up a hot hey to toggle each type of engine separately and one to toggle the mode for the Rapiers, then follow the same trajectory.

JR

[GoSlash27]

Think I can get away with a terrier?

A single terrier is kinda dicey for a ship of this size. I'd personally go with a Poodle or 2 Terriers.

Not really necessary though. It makes orbit as- is. Just get rid of the struts and you'll be good to go.

Also, you should ditch the monoprop. You don't need all that just to dock. I usually run about 15 units on a ship of this size. You've got like 350.

Finally, you've got about 150 more units of LF than you need. You could rebalance (or just ditch it) and save more overall DV.

And again, thin the herd on the active control surfaces. It flies much better without so much authority.

This should set you straight. Nice design!

Best,

-Slashy

Edited August 24, 2015 by GoSlash27

[GoSlash27]

*edit* Round 2 cleanup

Softened up the controls as described earlier. No active controls except the forward canards and no active rudders. Drained the monoprop to leave 15 units. Drained 100 units of LF. Arrived in 75x75 orbit with 444 m/sec DV and another 100 units of excess fuel. Still more cleanup to go, but this is plenty capable of doing whatever you want in LKO. This is already more DV than I design mine to have for ferry duties.

By way of comparison, here's my high- efficiency low tech spaceplane:

This illustrates how little is actually necessary to do this job.

Best,

-Slashy

Edited August 24, 2015 by GoSlash27