SSTO Spaceplane Launch Profile

[digger1213]

I have attempted many a spaceplane, but I have never managed to get one to orbit that uses jet engines. (For some reason, I can get ones to orbit that use only rockets, but... not jets.)

I believe that it is my launch profile and ascent. What I currently do is I rise to about 10-12k, then accelerate nearly as fast as I can, avoiding blowing up to reentry. I then try to rise to a 5-15 degree angle, but I never manage to make an apoapsis above the atmosphere before the jets flame out. When they do flame out, my apoapsis is usually below 30km, and I am unable to get out of the atmosphere with whatever weak rockets I added.

When I add heavier rockets, I either do not get enough speed using the jets, or I no longer have enough DV to circularize once out of the atmosphere. Adding more DV means more weight, which results in the first problem again. When I just eliminate the jets entirely, I can just make orbit with about 200 delta V spare, using the standard rocket gravity turn.

What is a good ascent profile for SSTOs, and what other good tips are there for building them?

EDIT: I should probably add that I haven't unlocked Rapiers yet in my Career save.

Edited April 22, 2017 by digger1213

[Razorforce7]

The first order of business is you giving a screenshot of the vessel. In the SPH preferably with a good complete view of all the attached parts.

2 minutes ago, digger1213 said:

I then try to rise to a 5-15 degree angle, but I never manage to make an apoapsis above the atmosphere before the jets flame out. When they do flame out, my apoapsis is usually below 30km, and I am unable to get out of the atmosphere with whatever weak rockets I added.
 

 

A spaceplane SSTO design for beginners is usually a design with a combination of jet engines and rocket engines. You need both in order to get to orbit. Although you can also do it with jet engines and LV/N's.
The rapier engine has both a jet and rocket mode. So it might be a good idea to use the rapier engine. It's usually a little easier to design ssto's with rapiers.
 

2 minutes ago, digger1213 said:

When I add heavier rockets, I either do not get enough speed using the jets, or I no longer have enough DV to circularize once out of the atmosphere. Adding more DV means more weight, which results in the first problem again. When I just eliminate the jets entirely, I can just make orbit with about 200 delta V spare, using the standard rocket gravity turn.

What is a good ascent profile for SSTOs, and what other good tips are there for building them?

I think the screenshot will tell us more about what to tweak. But based on what you say it seems you have to much aerodynamic drag.
I'm unsure about what jet engine your using.
But the recommended jet engine for ssto's besides the rapier is the whiplash.
You should be able to get to about 1200m/s at around 19km altitude.
This is where you'll want to change to rocket engines and start pitching up.
The jet engines themselves are not supposed to get your Apoapsis above the atmosphere. Although I'm sure there are some designs where this is the case.
The most important thing is horizontal velocity.

 

[digger1213]

Since I do not have the Rapier unlocked, I use the Whiplash. I am unsure which rocket engine to use. I started with the LV-909, which I realized was a bad idea, changing to an Aerospike then an LV-T30. At 10km altitude I can reach 1200m/s. Should I gradually climb to 19km altitude, gaining speed, or should I level out at 19km and gain speed there?

I think my other problem is that when I pitch up I lose a lot of horizontal speed to my wings and drag. The LV-T30 does not have enough TWR to lift the rest of the SSTO out of the atmosphere in time.

I'll quickly try some experiments using the ideas you told me, but here's the plane I've been using. I've changed the engines around the couple times, but here is the LV-T30 combo.

Should have mentioned this earlier, but I am trying to get a probe of about 2 tonnes into orbit.

Thanks for your help.

 

EDIT: I just managed to make orbit. *just*. Using the LV-T30 engine, and a modified, extended version of the above picture, I made it to orbit with 30ms of DV left. Of course, this is not very good, but it works. I would still like tips on how to do this better.

 

Edited April 22, 2017 by digger1213

[Reusables]

The mk2 cockpit is fairly draggy, so it could the performance. Though I think your ascent profile was the major problem, as you suspected.

Any SSTO spaceplane is too draggy on low altitude, under 15km. So you should be going over there to enter level flight and gain speed. The optimal altitude should be in 18km~20km with Whiplash. You can gain speed rapidly till design limit of Whiplash, as drag is very low there. Expect speed of 1.2km/s~1.3km/s. (With the speed comparable to )

After that, raise AoA to 5 degrees and turn on the rocket engines to accelerate more and coast to space. If I remember correctly, 2 terriers should be enough for the gradual ascent.

[ForScience6686]

Well you've done, so now to refine it.  You should fly hands off for the most part, any direction change adds a lot of drag and reduces speed.  You should begin your speed run at 10km, and let the nose naturally rise off the horizon.  Engage the rockets before your acceleration stops and continue to climb until above 35km, then follow prograde.

What I see as most people's failure comes from excessive builds.  Using many surface attachments, over building engine and fuel supplies.  Bringing rcs when no docking well be performed.  Not adding static incidence to wings.  Basically, purpose build it, have a goal and build to accomplish it and nothing more.  Start small to get a grasp of the basics, you haven't even gotten into the re entry portion, and that can ruin what you think may be a good design.

[bewing]

As Abastro said (but didn't emphasize enough) MK2 parts are pretty, but are extremely draggy. If you redesign your plane with MK1 parts, it will zip to orbit with immense amounts of dV to spare. Climb gently until you are at about 15km before lowering your nose and really starting to pour on the speed. Let it continue to accelerate and climb gently until 20km.

As ForScience6686 said, don't pitch up. Any maneuver other than lowering your nose wastes immense amounts of speed (that took a lot of work to build up). Your nose will rise all by itself over time. Just set SAS to stability mode and wait for 30 seconds.

And adding one or two degrees of incidence to your wings will probably help a lot, too. But that will move your CoM, so you will need to shift things around a little afterward.

 

[Reusables]

In my opinion, not pitching up at all won't worth it. I didn't experience cost of over 30m/s pitching up 5 degrees, and it means staying longer in low atmo which makes the ship easy to overheat.

Also low TWR efficient engines could be used in this case. Typically it's Terriers, but sometimes even Nukes can be used.

[digger1213]

Thanks for the help.

I would use Mk1 parts, however they probably wouldn't be able to carry any, if much, cargo. I don't remember seeing an Mk1 cargo bay, and even if there was, I'd have trouble putting much in it. The main reason I want to use SSTOs is to carry light cargo to space for cheap.

I have tried using the Terrier, but it did not produce enough thrust and I would fall back down before making it out of the atmosphere. I doubt I could do anything with the nukes.

Using the little to no pitch up method, I managed to make orbit with no cargo, only tourists, with 400 delta V spare. Adding cargo to it meant it could no longer get out of the atmosphere. However, my Whiplash would flameout at about 19km, or produce too low thrust to provide acceleration. I find that after 17km I start to lose speed, and I activate my rockets around there.

I still have not managed to get cargo heavier than about 4 tonnes to orbit, and my Mk3's cant usually make orbit at all.

Thanks for the help.

[digger1213]

I dont feel like making another thread for another problem while this is still open, so I'll post it here first.

Recover debris mission. Debris is a Turbofan engine orbiting Kerbin. First question: how.

Second question. How. The. Hell. Do. I GRAB THIS THING?

 

ITS CENTER OF MASS IS SO FAR AWAY FROM THE ENGINE ITSELF, IT IS NEARLY IMPOSSIBLE TO GRAB. I HAVE TRIED LIKE 30 TIMES SO FAR.

Sorry. Had to vent some steam there.

But really. How do I grab this thing..?

I will post a seperate thread about this if I need to, sorry if I shouldn't have done it this way.

Edited April 23, 2017 by digger1213

[bewing]

1: MK1 & cargo means that you use an interstage fairing as your cargo bay. You can fit as much as you want inside the fairing, provided you have enough clearance on the wheels to get it off the ground.

2: That's not exactly how you use a klaw. To attach to something with a klaw, you don't need to be anywhere near the CoM. All you have to do is find a flat spot, and be perpendicular to it when you touch. That's all. If the Wheesley starts to spin, touch the 5x timewarp to stop it spinning, kill the timewarp, and try again. But the back end of the wheesley is the flattest spot, and you just need to make sure to be perpendicular to it.

 

[Reusables]

1 hour ago, digger1213 said:

Whiplash would flameout at about 19km, or produce too low thrust to provide acceleration. I find that after 17km I start to lose speed, and I activate my rockets around there.

Oops, so you should perform speedrun (gain speed) on about 16km.

You get to 1.2~1.3km/s while airbreathing mode, right?

Also single Terrier won't work on the case, did you used 2 Terriers?

Besides, Aerospike is indeed good choice for spaceplane engine; just land on the runway(launchpad) or you'll lose several bucks.

Edited April 23, 2017 by Reusables

[digger1213]

The Wheesley only took about half an hour worth of attempts to attach properly to it, and then I was too scared to move it into a better position as it was almost sideways. (Oh wait, thats right, you can go free-mode... oh well.) That problem is sorted, so I don't need to make another post about that.

Back to the SSTO issue, on airbreathing my jets can reach 1k-1.25k per second on jets. I have improved and can usually make orbit now, but not with enough DV spare to even dock with a refuelling station.

It's possible two terriers might help more.

The Aerospike just seemed like it didnt have enough Vacuum ISP. Since at 19k-ish the Vacuum ISP seems to matter more, it just seems like vacuum efficient engines might be better there. I could try some more designs. What is also good for Mk3? I've seen that loads of people can get orange tanks to orbit in an Mk3 but the best my SSTO's that size have been able to do so far is about 4 tonnes with about 300 delta V spare.

How much Delta V should I be reaching orbit with spare? As a general amount, anyway, maybe to reach a refueling depot.

[Reusables]

6 minutes ago, digger1213 said:

It's possible two terriers might help more.

The Aerospike just seemed like it didnt have enough Vacuum ISP. Since at 19k-ish the Vacuum ISP seems to matter more, it just seems like vacuum efficient engines might be better there

I don't think 5s difference for spaceplane  would matter much. The drag loss due to relatively low TWR should be bigger in most cases.

7 minutes ago, digger1213 said:

How much Delta V should I be reaching orbit with spare? As a general amount, anyway, maybe to reach a refueling depot.

For LKO station, 300m/s you gained is enough.

[Wanderfound]

6 hours ago, digger1213 said:

Thanks for the help.

I would use Mk1 parts, however they probably wouldn't be able to carry any, if much, cargo. I don't remember seeing an Mk1 cargo bay, and even if there was, I'd have trouble putting much in it. The main reason I want to use SSTOs is to carry light cargo to space for cheap.

I have tried using the Terrier, but it did not produce enough thrust and I would fall back down before making it out of the atmosphere. I doubt I could do anything with the nukes.

Using the little to no pitch up method, I managed to make orbit with no cargo, only tourists, with 400 delta V spare. Adding cargo to it meant it could no longer get out of the atmosphere. However, my Whiplash would flameout at about 19km, or produce too low thrust to provide acceleration. I find that after 17km I start to lose speed, and I activate my rockets around there.

I still have not managed to get cargo heavier than about 4 tonnes to orbit, and my Mk3's cant usually make orbit at all.

Thanks for the help.

If you want to use a Mk2 you can, but you need to build accordingly.

You want to set it up so that the potentially draggy fuselage is always pointed as close as you can get it to surface prograde. The way to achieve this is with wing incidence; angle the leading edge of the wings up a few degrees, so that the wings are at a greater AoA than the fuselage.

17km is way too low to be firing your rockets; you want to be extracting as much speed and altitude from the jets as possible.

Mk2 parts have good heat tolerance; use that. Crank it up to edge-of-melting speed while flying level around 10km, then gently pull into a climb to push your apoapsis above the jet ceiling. The more gentle the pull-up, the less speed lost; also, the lower you start the climb, the more time your jets have to recover the speed lost in the beginning.

[Razorforce7]

1 hour ago, Wanderfound said:

Mk2 parts have good heat tolerance; use that. Crank it up to edge-of-melting speed while flying level around 10km, then gently pull into a climb to push your apoapsis above the jet ceiling. The more gentle the pull-up, the less speed lost; also, the lower you start the climb, the more time your jets have to recover the speed lost in the beginning.

I would indeed recommend Mk2 for hot re-entries. But only when going interplanetary (meaning hot)
Most spaceplanes have lot's of aerobraking drag because of the wings. I always use Mk 1 cockpits and crew cabin on missions to Mun and Minmus.
With a good aerobraking maneuver I can circularize on Kerbin after dropping down from Minmus orbit. Admittedly it gets the heat gauge a pixel away from blowing up, but it works.

The aerodynamics and ascent profile largely decide how fast you'll be going and whether you can maintain that speed of ~1200m/s all the way up to 20+ km or there below.
I personally play on the keyboard. And I find that tapping (S) to pitch the nose up helps in avoiding excessive speed losses. In addition I would also lower the control surface pitch authority while in high seed flight to a much lower setting.
This way you'll be very gently raising your nose, avoiding sudden and abrupt losses of speed due to drag caused by control inputs.
 

[digger1213]

Angling the wings is something I used to do, but I stopped it for some reason. I should try that again.

If 17km is too low to be firing my rockets, when should I do it then? I've seen a lot of different numbers, but I find I start losing speed on jets alone above 17/18km because the jets no longer produce enough thrust or they flameout completely. I find if I wait any longer, I spend more Delta-V trying to regain that speed with my rockets than if I just start rockets at the 17/18km mark.

[fourfa]

I can usually count on RAPIERs to hit their max airbreathing speed at around 24km.  Some hit ~28km with very high-lift, low-drag designs.  The Whiplash gives up earlier - perhaps only 19-20km.  Both of those are for slick, minimal designs with as much as possible shielded from drag, with some amount of wing incidence preventing the fuselage from getting to a high AoA and becoming draggy.

Are you using Whiplashes?  Possibly with too many nacelles and radial-attached junk out in the breeze generating drag?  Maybe with flat wings and too many of them? That could cause you to max out at 18km from drag issues.

[Capt. Hunt]

16 hours ago, digger1213 said:

The Aerospike just seemed like it didnt have enough Vacuum ISP. Since at 19k-ish the Vacuum ISP seems to matter more, it just seems like vacuum efficient engines might be better there. I could try some more designs. What is also good for Mk3? I've seen that loads of people can get orange tanks to orbit in an Mk3 but the best my SSTO's that size have been able to do so far is about 4 tonnes with about 300 delta V spare.

How much Delta V should I be reaching orbit with spare? As a general amount, anyway, maybe to reach a refueling depot.

The Aerospike is kind of a niche engine.  Like the RAPIER, it is designed to work equally well in space as it does at sea level so that you only have to haul one set of engines into orbit.  However, it sacrifices both sea level and vacuum Isp to do so.  It's not the best engine by any stretch of the word, but if you can't afford the weight of both a dedicated vacuum engine and a dedicated jet engine it does the job.  Pairing it with other engines though is a waste of delta V.

Your delta v allowance is going to depend on your intended orbit, the orbit of this refueling depot, your payload to and/or from orbit and of course contingency fuel reserves.  I usually try for at least 300 to 500 m/s of on orbit allowance, but that generally leaves me a considerable safety buffer, and I usually return with most of it still in the tank.

[digger1213]

14 minutes ago, Capt. Hunt said:

I usually try for at least 300 to 500 m/s of on orbit allowance, but that generally leaves me a considerable safety buffer, and I usually return with most of it still in the tank.

My SSTOs reach orbit with that 300-500 delta V spare, but that is when carrying a three/four ton payload in a 100 ton Mk3 SSTO. What is a good payload percentage?

Whenever I add more rocket fuel, the jets can no longer get enough speed, so I add more jets, and then I am back to the first problem.

39 minutes ago, fourfa said:

I can usually count on RAPIERs to hit their max airbreathing speed at around 24km.  Some hit ~28km with very high-lift, low-drag designs.  The Whiplash gives up earlier - perhaps only 19-20km.  Both of those are for slick, minimal designs with as much as possible shielded from drag, with some amount of wing incidence preventing the fuselage from getting to a high AoA and becoming draggy.

That would be great, if I had Rapier researched in my Career save. I am quite close to getting there though, so maybe I'll try to aim for that.

[Wanderfound]

4 hours ago, digger1213 said:

Angling the wings is something I used to do, but I stopped it for some reason. I should try that again.

If 17km is too low to be firing my rockets, when should I do it then? I've seen a lot of different numbers, but I find I start losing speed on jets alone above 17/18km because the jets no longer produce enough thrust or they flameout completely. I find if I wait any longer, I spend more Delta-V trying to regain that speed with my rockets than if I just start rockets at the 17/18km mark.

You want to fire your rockets once the jets begin losing significant amounts of speed. However, you want to plan your flight so that that moment occurs just as you get to the jet altitude ceiling.

How to do that varies with the craft; see 

 

Generally, I'll be using one of two flight profiles, depending upon available TWR.

Low TWR: a low-angle (5° or so) constant-climb ascent, aiming to hit maximum jet speed just as I get to an altitude where the jets begin to fade (around 22km for RAPIERs), adding nukes to buffer the jets through the low-thrust 20-30km zone. RAPIERs switch mode at 29km.

 

High TWR: up to 10km as rapidly as possible, level off and accelerate to 1,500m/s, then pull into a zoom climb to push the apoapsis over 30km. Aim to do this low enough so that the jets can regain the speed lost before reaching 20km. Keep the nose on prograde to minimise speed loss during the 20-30km zone, switch RAPIER mode at 29km.

The profiles are much the same with turbojets, just a few kilometres lower and a few hundred metres slower.

10 minutes ago, digger1213 said:

My SSTOs reach orbit with that 300-500 delta V spare, but that is when carrying a three/four ton payload in a 100 ton Mk3 SSTO. What is a good payload percentage?

About 30%.

That's an extreme example, though; that ship would normally fly with about half that payload (and a much more comfortable takeoff).

Edited April 24, 2017 by Wanderfound

[Capt. Hunt]

15 hours ago, digger1213 said:

My SSTOs reach orbit with that 300-500 delta V spare, but that is when carrying a three/four ton payload in a 100 ton Mk3 SSTO. What is a good payload percentage?

Whenever I add more rocket fuel, the jets can no longer get enough speed, so I add more jets, and then I am back to the first problem.

That would be great, if I had Rapier researched in my Career save. I am quite close to getting there though, so maybe I'll try to aim for that.

such is the tyranny of the rocket equation.

One of the major hangups in real SSTO design is the low payload fraction compared to expendable rockets.

Based on your numbers your payload fraction is about .03 to .04, which is about average compared to real life rockets.  But of course, this is Kerbal Space Program, your results may vary.

Edited April 24, 2017 by Capt. Hunt

[Spricigo]

15 hours ago, digger1213 said:

 What is a good payload percentage?

Based in the result in the current relevant challange 35% can be archieved (with very optimized and well piloted spaceplanes). Based on design shared at https://kerbalx.com/   seems that designers will often be satisfied with much less (~20% seem common), specialy for smaller/lower-tech .

Personally I care about 1st: The rule of cool; 2nd:How easy its to fly; 3rd: How fast I can recovery the cost of it. As such I never calculated the mass fraction of my own vessel (and my spaceplanes are not that good to begin with). Someone else (e.g @Wanderfound, @GoSlash27, @AeroGav) can give you a better idea of what is a good mass fraction (or whatever metric they think you should look at)

[AeroGav]

On 22/04/2017 at 1:29 PM, digger1213 said:

 

Since I do not have the Rapier unlocked, I use the Whiplash. I am unsure which rocket engine to use. I started with the LV-909, which I realized was a bad idea, changing to an Aerospike then an LV-T30. At 10km altitude I can reach 1200m/s. Should I gradually climb to 19km altitude, gaining speed, or should I level out at 19km and gain speed there?

I think my other problem is that when I pitch up I lose a lot of horizontal speed to my wings and drag. The LV-T30 does not have enough TWR to lift the rest of the SSTO out of the atmosphere in time.

I'll quickly try some experiments using the ideas you told me, but here's the plane I've been using. I've changed the engines around the couple times, but here is the LV-T30 combo.

 

I suspect your main issue is poor lift/drag ratio, because the mk2 fuselage is like a barn door, and your craft has quite small wings.

In flight, you need to generate lift equal to the plane's weight.   Actually, this is only true at subsonic speeds -  as you get closer to orbital velocity, freefall effect supports more and more of your weight, which decreases the lift requirement.  But you get the idea.  You need lift.

Lift =  speed  x  air density x angle of attack (AoA).

The optimum lift : drag ratio occurs at an AoA of 5-7 degrees when supersonic, and for the wing parts themselves you are getting > 15kN lift for every 1kN drag created.

Fuselage components generate much more drag than wing parts,  and negligible (though not zero) lift.    

This craft, with huge wings and very short, skinny mk1 fuselage, still has 75% of the drag coming off the fuselage.

Adding an incidence angle to the wings is pretty much essential to get a good performing mk2 design, because a mk2 rocket fuel fuselage - long , has more than three times the drag of a mk1 ft800 tank, despite holding exactly the same amount of fuel.

Adding incidence angle will allow you to keep the body of the plane at prograde, minimising drag, while the wings still work at their optimum 5 degrees AoA.

However, adding more wing, up to a point, is still beneficial as it allows you to get sufficient lift whilst flying higher (which reduces fuselage drag).     However, if you fit too much wing, you can find your plane generating too much lift which sends it zooming out of the atmosphere before it can hit air breathing top speed.   If the wings have built in incidence angle, the only way you can reduce the lift produced is to fly with the fuselage at a nose-down angle, which is very inefficient.

I'd aim for at least 900 m/s at 17000km. Beyond 900m/s the Whiplash's thrust declines rapidly with increasing speed, so it's not really worth packing extra engines (1.8 tons each) or clipping the wings to get faster.         Beyond 17km engine power decreases faster than air density, so even if your design is able to generate sufficient lift without excessive AoA, flying higher won't give you a higher top speed in level flight.

My general rule of thumb is 60kn of close cycle power for every 15 tons of launch weight, and one jet engine for every 30 tons of takeoff weight , though it's a bit sluggish at the top end of these scales (20 minutes to get to orbit).

By my reckoning, a couple of Terriers should have been able to push that thing to orbit if the lift/drag ratio was good enough.

However the Aerospike generates 3x the thrust for only twice the weight and only has vacuum ISP 1.3% less, so it's probably an even better choice, especially as you'll only need one engine mount point.

I'm a bit mystified at how the Reliant succeeded where the Aerospike failed.   It's vacuum isp is much worse,  the only area it wins is gimbal (your plane has reaction wheels and control  surfaces, no need)  and sea level ISP (and you're going to be using rockets > 15km,  where the Vacuum engines perform better).

If you could share the craft file , we could properly diagnose it, in the meantime, consider  minimising the amount of mk2 parts in the design.   cockpit , cargo bays, and engine mount/mk1 adapters only.  For regular fuel tanks, use Oscar Bs or MK1s.

Low TWR close cycle climb videos -

 

 

[Hodo]

The myth of the Aerospike engine being a poor choice for SSTOs is just that a myth.

 

This SSTO is powered by 4 Aerospikes and one small maneuver engine. It took off like a rocket from the KSP launch pad, and lands like a plane.  Kind of like the Venture Star program. Which also uses a aerospike engine.  

The Aerospike has one of the best all around ISPs of any rocket engine in game.  Its biggest drawback is the lack of gimbals.   

 

As for the launch profile, in normal KSP not FAR, is this...

-Climb to 10-12km as fast as possible.  

-Bring your nose down to about 5-10deg to fly mostly level with a slight climb no more than 10m/s till you reach 1200-1500m/s on jet power.

-Switch to rockets and pitch up to 30-45deg (gradually as to not stall your craft), and climb to desired AP, then circularize as normal.

[ForScience6686]

Mk2 parts are perfectly fine to use.  And suggesting that they stop using and switch to mk1 is counter productive.  It just requires more finesse and careful choice of layout, but will produce a better engineer in the end.  They are there to make sleek space planes, so let's help them to do that instead of insisting they change the entire design, looks are important to some people.