SSTO "Rules of thumb"

[RizzoTheRat]

I'm sure I've seen a bunch of "Rule of Thumb" comments for SSTO design but my search-fu is failing me and I can't find them.

Are there some general rules of thumb people follow for:

• Amount of lift per aircraft weight?
• Thrust or number of engines per aircraft weight?
• Intakes per engine (last time I built an SSTO the Resources tab told you how much air was available, 1.05 doesn't seem to do that but with 4 shock cones and 6 Rapiers right clicking the engine always showed it had 100% of requirement if that's what that display means)?
• Target speed/altitude before hitting the rockets?

Also, and I know this may sound silly, but has anyone tried a plane with a Mk3 cargo ramp at the front?  It flew fine right up to the point it exploded with absolutely no warning.  Does anyone know if the aero model correctly models it going backwards?  It looks fairly aerodynamic but can't understand the config file well enough to know if it really is.

[Val]

These are my rules of thumb.

38 minutes ago, RizzoTheRat said:

• Amount of lift per aircraft weight?

Anywhere between 1:10 and 1:2 wing-to-mass ratio will be fine. Adjust Angle of Incidence to suit.

 

38 minutes ago, RizzoTheRat said:

• Thrust or number of engines per aircraft weight?

In my experience 15-20 t per engine seems the most efficient with regards to payload fraction. At least for RAPIERs.

 

38 minutes ago, RizzoTheRat said:

• Intakes per engine (last time I built an SSTO the Resources tab told you how much air was available, 1.05 doesn't seem to do that but with 4 shock cones and 6 Rapiers right clicking the engine always showed it had 100% of requirement if that's what that display means)?

Intakes are complicated. The info shown in the editor doesn't show you all you need to know.

Shock cones are best suited for RAPIERs and high speeds. A single Shock cone can easily supply 3-4 RAPIERs at all speeds.

But you can also use 2 Radial Ramp Intakes per RAPIER, though I'd only recommend that on single RAPIER crafts.

Or you can use a Precooler per engine.

Regular Ramp Intakes are good for both all high speed applications, but needs 1 per engine if you plan on going faster than Mach 3.

That is based on my interpretation of this graph.

 

38 minutes ago, RizzoTheRat said:

• Target speed/altitude before hitting the rockets?

Highly dependant on craft TWR and engine types.

Generally I'd say 20-22 km. But I have some designs that go to 29 km (to avoid overheating) and some at 18 km (LV-Ns and low TWR craft)

 

Haven't really played with the Mk3 Cargo Ramp, so I don't know what's happening there.

[Starhawk]

Most spaceplane designers seem to like 0.2 to 0.3 lift rating per tonne.  There is quite a bit of variation outside that range.

If using Rapiers, about 20 tonnes per engine is common.  I've seen some people go with quite a bit more mass per engine than that.

One shock cone can feed two Rapiers.  Or one precooler per Rapier.

It's nice to get over 1600 m/s before lighting the rockets, but only the lighter of my spaceplanes get that fast on airbreathers.  I'm probably not nearly as efficient as I could be.  It gets very difficult to manage heat at speeds like that with altitude around 20 km.

Not sure about the cargo ramp.

Also, this thread about creating the most efficient SSTO's has some pretty good info.

Hope this helps.

Happy landings!

Edited February 8, 2016 by Starhawk

[RizzoTheRat]

Thanks.

I assume the precooler doesn't actually do anything funky in terms of cooling like getting extra power from the engine, and is just an intake with the parameters of that graph?

My smaller design was hitting about Mach 4.5 (1350m/s at 20km) before I switched to closed cycle so it looks from that graph as if the shock cone is a no brainer but I've been trying to fit too many of them, but my ascent profile doesn't seem too bad.

So for a 100 tonne aircraft 2-3 shock cones and 6 rapiers should be fine, but I probably need to double the lift from a single pair of Big-S wings?  I think the lift is probably my main issue then.

Edited February 8, 2016 by RizzoTheRat

[Starhawk]

10 minutes ago, RizzoTheRat said:

I assume the precooler doesn't actually do anything funky in terms of cooling like getting extra power from the engine, and is just an intake with the parameters of that graph?

My smaller design was hitting about Mach 4.5 (1350m/s at 20km) before I switched to closed cycle so it looks from that graph as if the shock cone is a no brainer but I've been trying to fit too many of them, but my ascent profile doesn't seem too bad.

So for a 100 tonne aircraft 2-3 shock cones and 6 rapiers should be fine, but I probably need to double the lift from a single pair of Big-S wings?  I think the lift is probably my main issue then.

The precooler is just an intake and doesn't do anything funky.  I believe it has fairly good thermal emissivity.

Yeah, the engine count should be fine and I agree with adding more lift.  I find reentry and especially landing limit how little lift I can get away with, at least on a moderate TWR craft.

I would probably try to put three pairs of Big-S Delta Wings on it if I were designing it, but you should be able to get away with two.

Happy landings!

[Vegetal]

I made a plane with a ramp at the front and back and it worked fine, but it was just an atmospheric, subsonic plane, not an SSTO...

[RizzoTheRat]

Yeah mine flew fine at low speeds but exploded at probably around 20km and Mach 4+

Had a bit of play last night with heavy space planes and managed to reach a point where my contraption broke up on the runway when it loaded, so I don't think I was going to have a lot of success landing it   I think maybe I need to start a bit smaller rather than go straight for a 150 tonne behemoth with a detachable cargo bay...

[Evanitis]

I only have two rules of thumb: if it struggles to go where it should, add moar boosters.

If it falls to pieces when it gets there, add moar struts.

(note: it's not good advice, but a fun and functional one)

Though it's sad that the cargo-ramp is such a party-crasher. It'd look so nice as a nose-piece. Struts -might- not be the solution here. I have a deep mistrust towards radiators, but who knows, they might would enjoy a ride.

[Val]

On 2/8/2016 at 4:03 PM, RizzoTheRat said:

Also, and I know this may sound silly, but has anyone tried a plane with a Mk3 cargo ramp at the front?  It flew fine right up to the point it exploded with absolutely no warning.  Does anyone know if the aero model correctly models it going backwards?  It looks fairly aerodynamic but can't understand the config file well enough to know if it really is.

I made a test craft to see if there's anything suspicious about using the Mk3 Cargo Ramp as nose cone.

I didn't find anything out of the ordinary.

Craft file for anyone interested.

Spoiler

 

It's not a great SSTO. The payload capacity is only a few ton. Probably 5 or less. But it might make a passable seaplane with the rocket pods removed.

Ascent Profile:

1. Takeoff and hold 5° above horizon until 450 m/s.
2. At 10 km pitch up to 10° above horizon.
3. At 15 km pitch up to 15°.
4. At 18 km engage Poodles (Action Group 1)
5. From 20 km to 25 km follow Prograde down to 10° above horizon.
6. Shutdown Jets when they flame out.
7. Hold 10° above horizon until AP is above 45 km.
8. Follow prograde rest of the way.
9. Throttle down when AP a few km above target orbit altitude.

Action Groups:

1. Poodles On/Off
2. Jets On/Off
9. Cargo Bay Ramps

 

 

Edited February 9, 2016 by Val
Video added

[RizzoTheRat]

That looks about half the weight of mine for the same payload space

I would have been going quicker than that at about 20km so it's probably as much down to my flying as anything structural.

Edited February 9, 2016 by RizzoTheRat

[FyunchClick]

1 hour ago, Val said:

I made a test craft to see if there's anything suspicious about using the Mk3 Cargo Ramp as nose cone (...)

I actually really like the way that looks.

Edited February 9, 2016 by FyunchClick
spelling

[GoSlash27]

My rules of thumb:

 1 jet per 5-10t of payload. This should work out to about 1 jet per 20-40t of spaceplane.

 400 units jet fuel per engine.

 1 unit of lift surface per side per 5 tonnes of spaceplane. There's a lot of leeway here.

.001m² *effective* intake area per engine. I recommend the precooler (1 per engine), but the surface mount variable ramp is good also (2 per engine).

Rocket fuel varies with type of air breathing engine.

2 Terriers per jet engine or 1 Poodle per pair of jets. Rockets should be vacuum engines.

 Wings should have enough incidence to give zero angle of attack (prograde marker aligned with nose) at Mach 1.

Generally speaking, you can expect about 4t of total spaceplane per t of payload (varies with engine type).

Most importantly: Design for minimal drag instead of high thrust. You can make crazy- efficient SSTO spaceplanes that just barely break Mach 1.6 in level flight. The less of your total mass is engines, the more you can dedicate to payload.

Best,
-Slashy

 

 

Edited February 9, 2016 by GoSlash27

[Evanitis]

34 minutes ago, GoSlash27 said:

[numbers]
-Slashy

It's scary how accurately those numbers and relations fit my planes from the small orbital cabs to the huge beasts of hundreds of tonnes.

[Edax]

SSTO rule of thumb.  People on the forums will try to guilt you into designing efficient SSTOs.  Don't let them!  High-thrust, moderate efficiency is no sin!  Some of us like having our SSTOs reach mach 4.7 in 30 seconds, it's wicked awesome and we don't have all minute!  You don't have to worry about your carbon footprint on Kerbin and gas is cheap and certainly not going to burn itself!  If it's K-Prize capable, then you've succeeded in my book!

[Geschosskopf]

On 2/8/2016 at 9:03 AM, RizzoTheRat said:

• Amount of lift per aircraft weight?

Depends on the size of the spaceplane and also the local gravity, atmospheric density, and (due to local gravity) TWR.  Spaceplanes have to be designed for their operating environment and Kerbin ain't the only place you can use them.  An SSTO spaceplane that works on Kerbin will USUALLY work on Laythe but not always, and nowhere else.  One that works on Duna will hardly fly at all on Kerbin, and one that works on Tekto will just fall off the end of the runway on Kerbin.

However, no matter where you go, there's always the basic choice between thrust and wing area.  The whole thing about SSTO spaceplanes is to get to orbital velocity before running out of fuel.  This means that there's a premium on getting as fast as possible as soon as atmospheric effects permit.  The faster you go, the more lift you get out of a given wing area.  The greater the wing area, the more the drag.  But OTOH, if you have insufficient wing for the mass you're lifting and the speed it's capable of reaching, then you have to fly at a high angle of attack to maintain altitude.  High angles of attack create lots of drag, too, and also divert a significant fraction of your thrust into maintaining altitude instead of increasing speed, so you run out of fuel faster at a lower forward velocity.

Therefore, you always need enough wing to keep your AoA low, like no more than 5^.  That seems to be about the break-even point on wing area vs. thrust.  Any more AoA and you need to add wing.  Any less and you could probably take some wing off, unless you intend to land elsewhere than the KSC runway, in which case short-field performance has to be taken into account.  But either way, the 1.25m size of stock jet engines means that large spaceplanes can't have as high a TWR as small ones without a ridiculous number of engines and the fuel for them, which soon hits a wall.  Thus, in general, a spaceplane built around 1.25m parts will need less wing area per unit mass than a Mk 3 spaceplane.  Mk2 spaceplanes tend to be closer to the Mk 1 than the Mk 3 in terms of wing loading, due to the lift from the fuselage, but they still need more wing per ton than Mk 1 spaceplanes.

On 2/8/2016 at 9:03 AM, RizzoTheRat said:

• Thrust or number of engines per aircraft weight?

See above.

On 2/8/2016 at 9:03 AM, RizzoTheRat said:

• Intakes per engine (last time I built an SSTO the Resources tab told you how much air was available, 1.05 doesn't seem to do that but with 4 shock cones and 6 Rapiers right clicking the engine always showed it had 100% of requirement if that's what that display means)?

NOTE:  I expect a change here in 1.1 because 1.0.5 is pretty silly.  So if you build spaceplanes with the minimum intake area that works in 1.0.5, don't expect them to work in 1.1.

However, even with that potential change, the underlying general rule will probably still be in force.  That rule is that stock jet engines (at kerbin ONLY) now simply lose all their thrust at about 23-24km, REGARDLESS of intakes, due to hardwired thrust/altitude curves.  Thus, all you need is enough intake area (forget about IntakeAir capacity, that's meaningless in practical terms) to keep the engines from flaming out due to air starvation below 23-24km.

So here's a test you can do while tweaking your designs.  When you get to 23km, don't switch to rockets, just keep using the jets.  They won't have any appreciable thrust but they'll still be running and your plane will (hopefully) still be climbing.  So let it keep going up until the engines flame out from lack of IntakeAir.  If this happens above 24km, then you've got too many intakes.  Remove excess intakes until the jets flame out at 24km.

@GoSlash27 did some very detailed analysis of required intake area a while back.  Basically, in 1.0.5 you hardly need any intakes at all.  You can run an engine on a single precoooler no problem.  No need for shock cones, rams, or radial/structural intakes.  The precooler is preferred because it provides sufficient IntakeAir up to about 23-24km with the least amount of drag.  This is what I expect will change in 1.1.  But the basic rule of paring down intakes until you flame out at 24km will very likely remain in effect.

On 2/8/2016 at 9:03 AM, RizzoTheRat said:

• Target speed/altitude before hitting the rockets?

The purpose of jets is to get as fast as you possibly can (given atmospheric heating) before they just die at about 23-24km from their hardwired thrust curves.  Because you can't get any higher than that on continuous jet power (although you might coast higher with dead engines), that's where you typically switch to rockets.  So the only real question is your speed when you reach that altitude.  The slower you're going, the more dV the rockets need to achieve orbit, which means the more fuel mass the jets have to lift, which means the lower your jet TWR, which means more wing, which means more drag, etc., etc.

In general, at Kerbin I like a minimum of 1500m'/s at 23km.  That means the rockets need about 1100m/s more to reach orbit and about 500m/s on top of that to rendezvous with something up to about 200km LKO and then deorbit without refueling.  However, big spaceplanes can't usually reach 1500m/s at 23km due to the limited size of stock jet engines,   1100-1200m/s is more their speed,   Which means you need more rocket dV for them.