Need help with SSTO's

[AeroGav]

2 hours ago, zeekkay said:

It's usually best to do a mock-up of the craft first using kerbal engineer mod. This is my method.

1) Create a mock-up of your craft. Include all command modules and test cargo mass. Then add some rocket fuel and engines, as well as your jet engines. Your goal is to find out how much rocket fuel you need to haul to 12km to start your run. Make sure your rocket engines have a TWR of at least 1, or you're not going to make orbit. You should want at minimum 1800m/s dV.

2) Now that you know how much rocket fuel you need to reach orbit, begin assembling and laying out your aircraft. Also add some fuel for your jet engines. The trick is to keep that center of lift just behind the center of mass. (Try to keep your fuel towards the middle of the ship if you can)

3) Lastly make sure your jet engines also have a TWR between 1-1.5. It makes things a lot easier. SSTO's are more late game, panther engines might be able to get your craft into orbit if the ship is very lightweight.

That part about starting with the rocket section and working back from there is a valid approach.   The delta V requirement of the rocket section does depend on how high and fast the airbreathing part can boost it.     Kerbal Engineer system delta V calculator doesnt take into account lift and drag but i suppose it's reasonable enough for the closed cycle part.     I'd disagree about the TWR requirement though - an airplane does not need high twr.

I'm in a career mode atm.

My first spaceplane - and no, it's not single stage to orbit - had a cockpit, ft800 tank , some wings and a terrier engine.  A panther jet with an intake and some fuel , as well as a small wing to counteract the centre of gravity effects of all this mass - were held on to the back via a stack separator.   After dropping the airbreathing stage, we're at mach 2.5 and 18km or so.   The upper stage has 2500dV and 0.81 TWR,  it just barely makes orbit.

My second spaceplane was based around the "upper stage" of the first, but with a mark 1 crew cabin added on.   This brings the delta V down to 1800 and TWR to just 0.6 .   The lower stage however, is two pod mounted Whiplash engines under the wings. They can take you higher and faster than the panther, also if they're still producing some thrust but no longer enough to climb or accelerate you can fire up the terrier while still keeping the whiplashes burning - it's not a case of either/or like the first design.  When they finally flame out, the jet engine pods are separated off.

This one makes to orbit very comfortably with 630dV remaining.

Dread to think what the TWR of this is - one NERV pushing all that - doubt it's good !

Check out the AeroGUI menu on the right side of the screen, I'm so glad I found this.   Press f12, go to the Physics tab, Aero sub tab, and check the "Display Aero Data Gui".  Two things you really want to watch - 

Total Drag and Total Thrust.   If thrust is higher than drag you'll be climbing or accelerating or both, this means you're good. If it's not, then you can only be gaining in altitude at the expense of airspeed or vice versa, which means you won't be going to space today.

Also note Lift / Drag Ratio.   In the lower reaches of the atmosphere, you can kind of work off the rule of thumb that so long as your lift drag ratio is in the good to a greater degree than your thrust:weight is bad,  you can go to space today.         For example,  if you have a lift/drag ratio of 4:1  then so long as your TWR is not worse than 0.25  you should be able to climb.       As we get closer to space, it all gets less accurate because orbital effect is supporting more and more of your weight and you need less lift to stay up there.    However, this effect is working on your favour, so don't get too upset.

In the above picture I came very close to overheating so had just yanked the nose up. We were showing 10 degrees AoA plus 5 degrees of wing incidence so the lift surfaces were at 15 total AoA, which is not optimal for Lift:drag.

At low altitude and speed,  eg. 1000m and 100 m/s,  I get 7-10 lift:drag ratio in most of my spaceplanes, seems to peak at AoA 3 if the wings are mounted to the fuselage with 0 incidence.         At 400m/s and 13500m,   I get 4 or 5 lift:drag ratio with best angle of attack about 4 degrees.      Hypersonic,  6 degrees seems best but I'm only getting L/D of 2 or 3 by that point, but orbital effect is also helping out.  

 

Going back to my career spaceplane,  if I was shooting for a TWR > 1 on the upper stage, that means swapping the terrier for a swivel - taking the mass of the upper stage from 9.5 t to 10.5t and reducing the isp from 345 seconds to 320 seconds.   

Edited April 24, 2016 by AeroGav

[Red Iron Crown]

18 hours ago, Vanamonde said:

I've been playing this game for about 4 years and still can't consistently get SSTOs to orbit, but I have learned one bit of advice to pass along. Building a plane that can can do it is easier than figuring out the ascent profile for the plane to fly. I would suggest downloading a design someone else has made, so that you know that it works, and practice flying it to orbit. Once you get that technique figured out, making a plane of your own that can do it really isn't all that difficult. 

I have to agree with this. For an airbreathing SSTO the ascent profile is absolutely critical to success. Too fast down low and you'll burn up, too slow and you'll need to much rocket fuel to complete the ascent.

For me, I found it best to start by making planes that had no payload other than the cockpit and a generous amount of thrust, this makes it easier to figure out the ascent profile. Once you get a feel for how to fly them you can start adding payload and using less engine. Oh and start out with Rapiers, using other engine combinations is certainly possible but more difficult.

Sample high thrust/low payload spaceplane:

Spoiler

 

 

The other thing I found quite helpful was adding some angle of incidence to the wings. Place you wings and balance the CoM and CoL, the add a small amount of angle to the main wing so that the leading edge is a bit higher than the trailing edge.

[Cipher_077]

I use rapiers and LVN Nuke. All ny fuel is liquid fuel except for one tank on each side, which is LFO. I use two rapiers usually and a MK2 fuselage. I go on rapier power climbing slowly up to 6000m, then pitch down 10° to accelerate past 450m/s, which is when the Rapiers get stronger. then at ~23km I turn on the Nuke because the ISP there is high (~750) and switch the rapiers to closed cycle until the oxidizer ends. Then my Apoapais should be between 65 and 75km and the Nuke is enough to achieve orbit and do other stuff.

[Jarin]

If you're not using rapiers, you really have to build your craft as much like a rocket as like an airplane. Conservatively, your jets are only going to knock 1000 dV requirement off of reaching orbit, so build with that in mind (you can get more, certainly, but build with margin when learning). So you still need 2000+ dV in your rocket engines to reach orbit.

[sumrex]

I am kinda in the same boat for me though the small ssto's I loved are now the most difficult. The most basic small model I built is the following.  You can build flying lawnchairs, but I prefer a degree of functionality.  Here's a basic science ssto I am fleshing out.

It's basically getting fleshed out as a dropship for laythe but it makes orbit around Kerbin. She's so basic the only parts not in the pic are the ram air intakes and landing gear. She flys out at a 20 degree climb.  when it cuts over to rocket mode watch your apogee to 75-80 k cut engines, coast then circularize.  

 

Of course always fiddle around this tiny ssto is a pain to get off the ground and more so to fly, but can actually hit orbit. 

Edited June 15, 2016 by sumrex
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