Am i too slow?

[Octa]

I only found quite vague tips or guides for that on the forum, and i don't know if you guys were referring to FAR/NEAR or stock aero.

I try to make this this design fly to space for quite a while, with so much changes and flights that i lost track of it (i once started with X1 and now i'm at X7...with all the changes included it'd be X30 i guess)

Now, i've finally documented a whole flight to get some feedback from others. The numbers stay roughly the same on every flight/design, i start my kick-out burn at 1000-1200m/s surace speed because of the lack of intake air or fuel for the jets, is that too slow? I hope all necessary information for this is in the screenshots.

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At least, i managed to land on the ground (intact! )

It's stock aero, or soupero. Are there any major flaws that i can improve?

Edited March 31, 2015 by Octa

[Rhomphaia]

As you start to run out of air you should reduce the throttle. You should be able to get much higher and faster, looks like you have plenty of intakes.

Also you seem to be going very fast while still deep in the atmosphere. Just wasting fuel at that point.

[Tweety]

Your right about not going fast enough before gunning it for space. I personally would strip all the intakes and jets off and then put one shock cone intake on one side the a jet on that side and then place another shock cone intake on the other side then the second jet. This will reduce the tendency to flameout early on by tricking the game mechanics. You ideally wanna be doing at least 1500-1600m/s at 30km or faster. You can do it with less but it just makes your life harder.

Make your initial climb up to 15-20km as quickly as you can then level out so you are climbing at about 10-20ms and just throttle back as your jets start to struggle for air until you get a flameout at under 1/3rd throttle then hit the rockets on.

Hope this helps.

Tweety

[eddiew]

If you want to do spaceplanes, getting NEAR/FAR should be a high priority, tbh. Right now you seem to be fighting stock aero, which is unintuitive and not designed for planes.

Also, kicking the rocket in at 18km is at least 10km too low... nurse the jet throttle as long as you possibly can, keep the angle of ascent low, build up as much horizontal speed as you can muster before you finally kick in the rocket.

If you're relatively new to SSTOs, dump the docking port and the crew tank, and make a simpler design. It's not a thing you can pick up on first try, and the crew tank particularly is a lot of weight that doesn't contribute to thrust or fuel. IMHO, it's a crew tank, OR a cargo - because the crew is the cargo in the first instance.

[Zephram Kerman]

Make sure your wings are all oriented correctly. A wing attached sideways still creates drag, but no lift. SM did a report about this: http://youtu.be/GvWzB81vXgM?t=2m49s

At trans-orbital speeds, drag is the dominant factor. So if you can get by with less wings, so much the better. (Consider the Skylon; the wings on that one look tiny in comparison to the rest of it.) In fact, every little bit of stuff subtracts from performance. So remove as much stuff as you can. Do you have any excess fuel that could be subtracted?

Also, if that were my plane I would add another LV-909, or use two or more of the 48-7S instead. My reasoning is that that phase of the flight requires a relatively large boost of thrust to be efficient. Since the 48-7S has such a nice thrust-to-weight ratio, a few of those would dramatically improve the transition phase without adding much more weight.

Finally, your ascent profile can change everything. Experiment with different ways of approaching the transition. Can you start the rockets before the jets shut down? (The extra speed means more intake air.) What about crossing over at a faster or slower rate of ascent?

Hope this helps too. Rest assured, building a SSTO is a tough challenge, and all those iterations you've done are not unusual. You've almost got it!

[Captain H@dock]

I my experience it's all about the profile. I used to need 50% of my fuel to get to a nice orbit, and now i'm doing it with less than 20%. As to a starter, i'd say go to 18K in a little time as you can (almost vertical climb), then level off and accelerate from there, with as little climb as you can. Try to see where you design stands, but you'd want to be at least 1600 m/s before reaching 23K, where you'll need to throttle down to prevent flame out. Only switch to rocket mode once minimum throttle is still is too much to keep the engine from flaming-out.

Also, instead of making a multi-purpose plane (yours seems to be both cargo and transport), make separate versions. You probably won't do multi purpose mission anyway, so make everything you bring to orbit count.

[RexKramer]

Couple of suggestions.

1. A plane that size can be flown with a single turbojet engine. The advantage to that is you eliminate asymmetric flameout, you just keep the engine powered up until it stops making thrust. Perhaps try that vessel with a single jet engine and two small rocket engines.

2. Climb profile: The climb profile varies with your ship, but here's what works very well for me, with similar ships.

- Climb at 45-55 deg. until 10km altitude. That's 45-55 degrees attitude, although down this low your attitude and prograde vector should be very close.

- Between 10km and 20km, gradually lower your pitch. Shoot for about 20 deg pitch. More important is where your prograde vector is, this should be down to about 10 deg above horizon by 20km. 5 degrees above horizon is fine at 20km also.

- Above 20km, try to keep your prograde vector about 5 degrees above horizon. This will keep you climbing, and also allow you to build horizontal speed.

- Above 36km, you should aim for level flight, with a slight climb. Prograde vector should be very slightly above horizon. Usually, you will need a pitch attitude of about 15 degrees at this point.

- Your speed should reach over 1900m/s surface, or 2050 orbital. Ideally, you can reach 2300-2400 m/s at about 40km. Horizontal speed is more important than vertical speed.

- When the jet stops working, switch to rockets. A low thrust setting is all that is required to give you the final push out of the atmosphere. Use rockets burning prograde, or perhaps 10 degrees nose up if required. Again, using rockets to maintain horizontal speed is more important than gaining vertical speed.

Your design looks like it should reach orbit easily, with two jet engines or a single jet.

[Starhawk]

In my spaceplane ascents using stock aero, I shoot for 700 m/s at 20 km, 1600 m/s at 30 km, and 2175 m/s at 36 km. These velocities are quite attainable with turbojets and a moderate number of intakes (two ram intakes per turbojet is what I usually use and it's overkill). I limit my craft to about 15 tonnes per turbojet and try to keep the lift/mass ration close to 1:1.

For details of an ascent profile which only requires about 30 m/s from rockets for circularization, take a look at this album.

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Hope this helps.

Of course, stock aero (and other aspects of building spaceplanes) will completely change with 1.0 and we'll all need to relearn how to design and fly.

Happy landings!

Edited March 28, 2015 by Starhawk

[Zephram Kerman]

Oh, I forgot to mention one more thing. I notice you have MechJeb on this plane. If you're using MechJeb's "prevent flame-out" utility, be aware it is overcautious. Even with the safety margin set to 0%, MJ will cut throttle too much. If you're trying to coax the turbojets to the edge of the envelope, don't use that utility. Instead, manage throttle manually. (I like to keep an eye on the yaw indicator. Once it starts sliding over one way, I know flame-out is near.) MJ flame-out prevention will rob too much performance.

[GoSlash27]

I agree with all the others here. You are going too fast in the lower atmosphere, then transitioning to rockets too early and climbing too steeply.

That is also way more engine and intake area than you actually need for that size airframe.

The best way to avoid asymmetric flameout issues is by using a single engine. This is adequate for up to 20t of aircraft, though 10-15t is more common. Also note how little rocket is actually needed for this job.

Try this profile:

At takeoff, pitch as steeply as you can to maintain 100 m/sec airspeed.

Keep increasing pitch as necessary to maintain this speed until your climb rate hits 500m/sec.

Maintain a maximum climb rate of 500m/sec until you hit 20km altitude.

Reduce climb rate to 100m/sec, and maintain that to 27km altitude.

Reduce climb rate to 50m/sec, and maintain that to 32km altitude.

Reduce climb rate to 10m/sec, and maintain that until your angle of attack is 15*

At this point, your engines will begin to flameout. Throttle back incrementally to keep them lit and maintain 15* pitch.

You should hit a suborbital trajectory around 40-45km altitude. Switch to map view and throttle back as necessary to maintain a 72km apoapsis.

Eventually you will be unable to keep the jets lit. Switch to rockets at that point and throttle just enough to maintain a 70km apoapsis.

When you have drifted up to 70km altitude, complete circularization.

Best,

-Slashy

Edited March 28, 2015 by GoSlash27

[Deutherius]

One thing to add here - if you experience assymetric thrust/flameouts, alt+right click both the turbojet engines (like you would when transferring fuel in tanks) and tweak their individual thrust limiters to keep symmetric thrust, while slowing overall throttle (ctrl) to prevent flamouts.

As you can see, I had to throttle the left engine to keep it from smothering the right one

It made all the difference - when I just switched to the rockets at the first sign of flameout, I barely managed orbit with no fuel left. Using this technique I was able to milk the turbojets a lot more, resulting in more than 660 m/s dV remaining after circularising orbit.

[GoSlash27]

One thing to add here - if you experience assymetric thrust/flameouts, alt+right click both the turbojet engines (like you would when transferring fuel in tanks) and tweak their individual thrust limiters to keep symmetric thrust, while slowing overall throttle (ctrl) to prevent flamouts.

As you can see, I had to throttle the left engine to keep it from smothering the right one

http://i.imgur.com/IVyPKmN.png

It made all the difference - when I just switched to the rockets at the first sign of flameout, I barely managed orbit with no fuel left. Using this technique I was able to milk the turbojets a lot more, resulting in more than 660 m/s dV remaining after circularising orbit.

haha "Apollo Kerman"

Best,

-Slashy

[Octa]

Ok, guys, had no time to play during the weekend, but i kept an eye on this thread. Thank you for you feedback, today managed to implement your suggestions

If you want to do spaceplanes, getting NEAR/FAR should be a high priority, tbh. Right now you seem to be fighting stock aero, which is unintuitive and not designed for planes.

I use mods to add new parts, mostly. Mechjeb for convinience . Stuff like Interstellar, FAR or Kethane just modify the gameplay itself too much for me.

Make sure your wings are all oriented correctly. A wing attached sideways still creates drag, but no lift. SM did a report about this: http://youtu.be/GvWzB81vXgM?t=2m49s

[...]

Finally, your ascent profile can change everything. Experiment with different ways of approaching the transition. Can you start the rockets before the jets shut down? (The extra speed means more intake air.) What about crossing over at a faster or slower rate of ascent?

Hope this helps too. Rest assured, building a SSTO is a tough challenge, and all those iterations you've done are not unusual. You've almost got it!

The Wings were not rotated in SPH. I know about the Lift issue if the wigs are rotated. So much theoretical knowledge about KSP from reading in the forums, the wiki and watching Scott Manley play, but not enough skill to actually use that knowledge

About the last paragraph...read below.

Also, instead of making a multi-purpose plane (yours seems to be both cargo and transport), make separate versions. You probably won't do multi purpose mission anyway, so make everything you bring to orbit count.

Done that in the X8. Kerbals were supposed to take a nap in the crew module if there's no maneuver to fly. Well, since my kerbals already managed to fly to Duna with the tiny and uncomfortable 3 man capsule, any development for increasing the quality of comfort during flight was therefore ditched due to resolve the now conflicting goals (managing to get into orbit vs. a nice flight wasn't much of a choice for me)

Oh, I forgot to mention one more thing. I notice you have MechJeb on this plane. If you're using MechJeb's "prevent flame-out" utility, be aware it is overcautious. Even with the safety margin set to 0%, MJ will cut throttle too much. If you're trying to coax the turbojets to the edge of the envelope, don't use that utility. Instead, manage throttle manually. (I like to keep an eye on the yaw indicator. Once it starts sliding over one way, I know flame-out is near.) MJ flame-out prevention will rob too much performance.

Machjeb was only used to control keep pitch, heading and roll steady. Also, i used this intake mod to assign the intakes to the engines, so there are no suprises or assymentric flame outs. all left intakes are aligned to the left engine and vice versa.

Now, here's what i did today:

- removed the crew module

- added another LF-Tank for the Jets

- re-did the wings, as CoF/CoM was messed up

- re-aligned the RCS-Thrusters

The the rear landing gear was also moved a bit forwards and i fixed the action groups, because a few intakes where not toggled. These 2 things never bothered me me very much and i kept forgetting about it in the SPH.

This time, i used the thottle to adjust my vertical speed, something i never did before. I always kept the throttle at 100% and used the pitch for this, which was not very accurate. Now, I had an eye on my ascend rate, acceleration and intake air and adjusted the trottle to keep it all in control. I was _so_ surprised by this: 1600m/s at 30 km with 0.20 intake air left iirc.

I did 2 mistakes during the kick-out burn and quickloaded, but then i managed to get into an 260km circular orbit. YAY!!!!!!

In space, i transferred a bit fuel back to the jets to have reserve, because i thought "you certainly mess up the timing to land!" I did. Re-Entry was somewhere above the desert west of KSC, during the flight back to the runway, fuel was about the run out, so i throttled up and down to keep my consumption rather low. After a while i thought "Well, now you don't need that much fuel....but your speed is as slow as your consumption!". Just before the runway, i got a surprise flameout on one jet and i landed nearby. I just 50m away from the runway. Cause of the Flameout: Assymetric fuel transfer. The other engine had 0.95 LF left, the flamed out one was dry. Who thought that this tiny amount of fuel might get important anyway? But hey, all survived and the did not end up at the other side of the planet!

1 flaw i still have: I needed to transfer fuel from my rocket to the jets during ascend, but before overhauling the design once again, i'll just experiment with more agressive flight profiles. I think i kept the vertical speed to 10-20 m/s to early.

But once again, thank you for you tipps . God knows which revision of my SSTO would have made it into space if i wasn't asking you... the X40 could've become a christmas '15 present from me to myself

[Octa]

I just did it! A more agressive flight profile did the job. No more fuel transfer mid-flight, and now i have 1300m/s left. There's plenty of fuel left for the jets, almost 1 tank for each.