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Sir Derpykins

How to make an SSTO in 1.0.5?

Question

I've been trying to design a SSTO space plane to ferry crew and fuel to the small station I recently put in orbit around Kerbin. I can do it with rockets, but it just seems much more viable to me to use a reusable SSTO. I've read most of the good guides out there and I watched all the up-to-date youtube videos on the topic (More or less up to date, most of them were in version 1.0.4). Following all of the tips and instructions given to me I made a basic space plane with ~9km of DV.

However, no matter what I try this thing won't get into orbit. My TWR is good, I accelerate fine, but when my jets cutoff, I don't have either the DV or the thrust to get into an orbit with more than 100 m/s of DV to spare

Here are some screenshots of the craft, with kerbal engineer readout included

?id=680511486


?id=680511463

 

My flight pattern is as follows

  • Take off
  • Immediately point to a 45 degree angle and climb
  • O2 gets low at about 22-25 km and I switch over to rocket power
  • Burn straight prograde
  • 1 of 4 things happens after this.
  • A. I run out of fuel and never make it above 40 km
  • B. I burn up due to the extreme speeds I'm going at while still in the atmosphere
  • C. I make it out of atmo but on a sub-orbital trajectory without enough fuel to both stabilize the orbit and then come back.
  • D. (RARE) I make it into a stable orbit with enough fuel to come back, but not enough to perform an orbital rendezvous.

No matter what flight pattern I try or what I tweak about my engines, wings, fuel, and cargo, I either can't get high enough, can't get fast enough, or just can't get up off the runway in the worst cases. If anyone should care to give me some pointers or better yet, a complete, up-to-date guide with the new heating and aerodynamics I would be very appreciative.

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First: your plane is very pretty. I'd put some little canards on the front, but you seem to have plenty of attitude control.

BUT, the deal goes like this:

1) MK2 parts (despite being supposedly "higher tech") have significantly higher drag than the equivalent MK1 parts. So by using MK2 you are already starting at a disadvantage.

2) To have lots of fuel left when you get to orbit, you need to get through all that atmosphere with not much drag. So you need to pay attention to the way drag changes with speed and nose inclination. Drag basically goes with the square of your speed -- so you don't really want to be hitting 600 m/s until maybe 8km altitude. 1200 m/s at 22km or so. If you're going faster than that before then, you're going too fast. Drag increases rapidly as your nose rises from a 2 degree inclination, to 10 degrees, and then to 30 degrees. Then it really goes crazy. In the lower atmosphere, keep your nose to maybe 5 degrees above prograde, max. In the upper atmosphere, don't let your nose go over 30 degrees. 45 degrees is a very bad idea.

3) You have a lot more engine than you really need, and engines weigh a lot. The whole point of RAPIER engines is that you do not need any other kind -- admittedly they are not as efficient as Whiplashes (or Panthers on afterburner).

I like those shock cones myself, and I understand that you need something to stick them on -- but you may just want to try a couple of the radial ramp intakes, get rid of the nacelles, and the whiplashes, and see what happens when you just run on the two RAPIERs.

4) Heating. If you launch properly, your speed will be between 1300 m/s and 1500 m/s (surface) between 25km and 35km alt. This is the danger zone. My answer is to climb through it as fast as you can (this is where I say you should let your nose go to a 30 degree AoA). There are other answers for how to get through this zone -- throttle back, minimize drag, etc.

5) Don't overuse the controls. Every time you hit a key, it slows your plane down. A lot. I like using the SAS system, and just alternating between "prograde" and "stability" to control the nose angle.

Edited by bewing

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Is it feasible for me to still use mk2 parts if I want something reasonably efficient? I was using mk2 mainly b/c the crew cabin is much better and it has a cargo bay.

EDIT: Are B9 parts better if I want to retain functionality without producing so much drag?

Edited by Sir Derpykins

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Reasonably efficient, yes. MK2 parts are much more forgiving on reentry, which helps. You can also try replacing the Whiplashes with NERV engines. Light them around 22km altitude. There are many experiments to try that may work reasonably efficiently.

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I made the changes you suggested, removed the nacelles and hit the speed limits set, but I still end up running out of fuel with an apo of 80 km on a sub-orbital trajectory. I'm doing much better than before, and I can make a reliable flight now, but I don't know if my problem is enough fuel or my ascent profile.

Should I be adding drop tanks for more DV before I hit 22 km or should I go increase my AoA steadily from 5 degrees up to 30 at 22km instead of going 5 all the way to 22 and then pointing to 30 immediately?

EDIT: I added the drop tanks, and only got a marginal increase in DV. Maybe 50 m/s at best.

Edited by Sir Derpykins

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4 hours ago, Sir Derpykins said:

I made the changes you suggested, removed the nacelles and hit the speed limits set, but I still end up running out of fuel with an apo of 80 km on a sub-orbital trajectory. I'm doing much better than before, and I can make a reliable flight now, but I don't know if my problem is enough fuel or my ascent profile.

Should I be adding drop tanks for more DV before I hit 22 km or should I go increase my AoA steadily from 5 degrees up to 30 at 22km instead of going 5 all the way to 22 and then pointing to 30 immediately?

EDIT: I added the drop tanks, and only got a marginal increase in DV. Maybe 50 m/s at best.

FUEL - I'm doing well if i manage a 50% fuel fraction on a spaceplane.  For a rocket 80% is the norm, but i guess we like our wings, engines and stuff.

 

20160507083201_1_zpsvx918t36.jpg

After six months I am finally a convert to Mechjeb, it just makes things so  much more repeatable. You can see the mechjeb window on the left.  The "Pit" box is the angle of attack the autopilot will try to hit.  At the top, Kerbal engineer gives our apoapse, periapse, mach number and critical thermal percentage (very important!).  On the right, is something built into the stock game - Alt f12 debug menu, physics, aero, display aero data gui.

I adjust my pitch angle in mechjeb to try get the best lift:drag ratio all the way to orbit.  I also look at total drag vs total thrust to make sure i am actually going up and not just treading water.   

Spoiler

 

Case in point - when should i go through the sound barrier?  Drag is high between .85 mach and mach 1.3,  and even once you crossed the bad transonic region,  you'll never see sort of lift:drag ratio numbers as you did when subsonic.   During my initial climb, I get best lift drag ratio at 3-4 AoA.  But as we get higher, air thins, we have less lift and drag, and our lowest drag climb speed gets faster and faster.    As it starts to near the transonic region, i pitch up more than i normally would, to stop us accelerating.    Eventually though, i'm at 13km+ and need AoA of 10 degrees to get enough lift when going so slowly in such thin air.   This high AoA is bad for our Lift:drag ratio, it's now little better than a supersonic one.  Worse, as the air gets thinner, our engine looses power. Normally the whiplash gain a massive surge in power from going over mach 1, which compensates, but we're not letting it do that.

So,  I notice I have 80kn drag at this point and only 100kn thrust.   Most of our power is just "treading water".   Time to go sonic.  I drop the nose to 3 or 4 degrees and briefly light up the NERV to boost us through the sound barrier. 

 

I increase my pitch angle according to the following schedule :

After passing 170 m/s increase to pitch 4.

Pitch of 5 degrees at mach 3.

Pitch of 6 degrees at mach 4.5

Pitch of 7 degrees at mach 5.6 until main engine cutoff.

The above plane is vulnerable to heat having wings, cockpit and crew cabin  with only 1200 heat tolerance , but this is mitigated by the large amount of lift relative to mass.   This means you are higher for any given speed.  The higher you are the faster you can go without burning up.  Above mach 5.5 there are moments when i have to throttle down because critical thermal percentage is heading over 95%(!)  but as we continue to gain altitude the ship cools down and i can hit full gas again.

So no, in my case no sudden pitch up maneuver .  That said,  I have 120kn thrust from 2 nerv at this point, in a ship weighing 30 Tons launch mass.  You have like 360 , 3 times as much, in something probably not all that much heavier.  You will therefore climb much steeper.  But yeah, your gas will run out fast.  I'm hypermiling every gallon of fuel at 800 ISP and you're hot rodding at 305 ISP, it's a big difference.

I'll upload some climb to launch video footage in this ship on wednesday night/thursday morning, unfortunately i gotta work right now.

https://kerbalx.com/AeroGav/Astrojet-Citation

Edited by AeroGav

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6 hours ago, Sir Derpykins said:

increase my AoA steadily from 5 degrees up to 30 at 22km instead of going 5 all the way to 22 and then pointing to 30 immediately?

With SAS, your nose will rise slowly all by itself. Without SAS, yes, a slow rise is best, I think.

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