Spaceplanes again

[RizzoTheRat]

I've finally managed to get a plane in to orbit, after uninstalling FAR, giving up on my own designs, and basing the design on one I found in a youtube video by Cruzanak

My next step was to increase the size, and after a bit of trail and error I came up with this.

My plan was to run only on the single engine at high altitude to avoid issues with flameouts causing me to lose control, however this still proved tricky as I struggled to find the sweet spot where it would still accelerate on one engine, I had to bodge in a couple of extra intakes to get it working on all 3 turbojets much beyond 20km, but I'm finding I need to keep it going to about 40km to get to orbit without using the rockets until apoapsis.

So in my quest for bigger spaceplanes (I want to use them to ferry fuel in to orbit for longer missions) I have questions.

- To use more engines at 30-40km is there an upper limit on how many intakes I can have before drag becomes more of an issue than airflow?

- How do people generally power bigger space planes but avoid flameout yaw trouble without clipping lots of engines together?

- Is there a way to work out the optimum speed/altitude curve for turbojets/shock cones against aircraft drag?

- Is my approach of getting to 40km and as fast as I can to get the apoapsis out beyoned 100km and then coasting to apoapsis to use the rockets to bring my periapsis up the correct approach?

[Signo]

Hi, it seems to me that your approach is mainly correct.

There are a few tricks you could use to squeeze out something more from jets (e.g. while building in the VAB putting first one single engine then all the intakes related to that single engine and so on).

Usually to airhog my way to a 100Km Ap I need 4 ram intakes for each turbojet, the plane you can see in the pic below weights around 50t at take off.

You could experiment with lift too. There are some wings that are simply better than others (wing strakes, old swept wings, structural type D).

It seems to me that you want to ferry up some 2.5m tanks, you might want to wait till 0.90 to have (afaik) a bigger cargo bay that could hold that kind of component.

I have seen many SSTOs that can bring up that kind of tanks but I never built one myself, you can look for some of them in the K-prize thread.

Hope this can help.

[RizzoTheRat]

Clearly I'd misunderstood something I'd read elsewhere and was putting intakes then engines, I'll try it the other way round and see if that improves things. My early designs had way too much wing, and therefore too much drag, the delta wings seem pretty good but I'll try some of the other ones too.

Is the Ram Air intake or the Shock Cone better at high altitude? I experimented with both on my first design above but didn't really notice any difference.

Can't quite make it out on your picture but are you saying 4 intakes per engine at to top of your flight, so 6 engines and 8 intakes and then shut of engines until you have 2 engines and 8 intakes? My second design had 6 intakes for 3 engines and I struggled in the 20-30km range, where there wasn't really enough air for 3 engines but not enough thrust from 1, so maybe a couple more intakes might do the job.

I think before I go to 2.5m tanks, my next goal will be try something with an orange tank as the body with the aim to lift it to 80-100km orbit with the orange tank still full, however that's a 36 tonne tank which is about the weight of my current spaceplane.

[Alshain]

- To use more engines at 30-40km is there an upper limit on how many intakes I can have before drag becomes more of an issue than airflow?

You really can't airhog in FAR. The engines velocity curve is capped so if your ascending properly, it wouldn't do you any good. You should have enough air to get to 1200-1500m/s @ 20-25km, do that and air hogging is unnecessary because the engines will soon cut out due to speed, not air. It depends on the engine, I've found speed gains are too low for turbo jets around 1300m/s. Rapiers are a bit higher. Don't bother trying to air hog, just get to 20km, level off an accelerate till your engines just can't accelerate anymore and then pitch up. You should get 30-40k apoapsis at that speed before they finally cut out and most importantly, your periapsis should be much higher. Maximize horizontal speed and you will make it.

- How do people generally power bigger space planes but avoid flameout yaw trouble without clipping lots of engines together?

You can do the intake/engine placement trick above. Or you can just set them to an action group and shut them off when you hear them start to die. They make a rather distinctive noise just before flameout and some tiny control may waver. At that point, hit the action group and stage your rockets in.

- Is there a way to work out the optimum speed/altitude curve for turbojets/shock cones against aircraft drag?

The velocity curve is in the engines config section in the far config files for stock engines.

- Is my approach of getting to 40km and as fast as I can to get the apoapsis out beyoned 100km and then coasting to apoapsis to use the rockets to bring my periapsis up the correct approach?

No. As mentioned above, Horizontal speed (periapsis) is the most important. It costs a lot to cirularize, and by going faster before switching off air breathers, you lower the amount you have to circularize in the end. Follow the instructions above to maximize horizontal speed before switching to rockets. Once you do switch to rockets, you can likely throttle way back and still climb as not to burn fuel needlessly. Continue with your nose at 15 degrees till you hit 70km and you will find your periapsis is very close to positive. You will end up with a circularization burn of 10 seconds or less, and finally you will find a lot of fuel left in your tank.

EDIT: One other thing, LFO engines are more efficient at a lower throttle, so rather than powering up at full throttle, climb slow and steady and increase that periapsis at the same time. It seems like your burning longer, but at a lower throttle your actually saving fuel. If you have engineer you can see this fact in the dV estimation. A engine with 30 seconds dV at full throttle may have 3 minutes at 1/3 throttle. That's a lot more than 1/3 gain.

Edited November 11, 2014 by Alshain

[RizzoTheRat]

Sorry, failed to properly describe my relationship with FAR, I've taken it off for now and am back on the stock aerodynamics model. Ascent profile is getting my apoapsis to 100+km with high enough speed that my periapsis is about 25km, then shut off the jets, coast to apoapsis, and a few seconds burn to circularise. It sounds as if a lot more rocket fuel is needed in FAR.

Just read this http://forum.kerbalspaceprogram.com/threads/97615-0-25-KSP-air-intakes-compared/page2 and think I should try adding some radial intakes, I'd assumed they caused a lot more drag at high speeds but it seems not in stock aero.

I wanted to get an idea about what works in stock before I go back to FAR, as I was spending all my time in FAR tweaking designs to be able to get a rocket in to orbit, and not getting out exploring the universe

[Wanderfound]

Whatever works for you is fine, but just as a demo: https://www.dropbox.com/s/lbnz9s8k9h7gwgb/Kerbodyne%20Benchmark%20StockAir.craft?dl=0

That's a non-airhogging stock aero plane designed to fly on a single turbo in the upper atmosphere. It won't get the periapsis very high on jets alone, but it doesn't need to. Flown right, it'll hit orbit with tanks half full.

[Taki117]

FAR and stock are two completely different animals. If you are wanting to use FAR build in FAR and make it look plausible. If you want to build in stock build in stock and make it work. It's going to be very hard to go from one to the other like what I think you are trying to do.

[Alshain]

If you want to learn FAR, start with NEAR until you know how to build and ascend a craft for it. If you do that FAR will be exactly the same except for mach effects on center of lift.

Edited November 11, 2014 by Alshain

[capi3101]

OP, I haven't got the time this morning to delve into your craft's design and give you as much assistance as I'd like to. I'll just point you to DocMoriarty's KSP Space Plane Construction and Operation Guide for pointers on how to build and fly the kind of planes you're wanting to do. The guide's designed for stock craft. If I get some free time later today, I'll revisit this thread. At a very quick glance, it looks like your design is light on the amount of lift it needs to produce for the mass of the craft you've got.

[RizzoTheRat]

Can't download it at work but that guide could well be what I need, will have a look tonight, thanks.

Bit of a red herring with the FAR discussion, I've removed it to make the game easier for now. I was spending all my time trying to get rockets in to orbit in FAR, but haven't been beyond Minimus yet. Plus I tried one of Wanderfounds spaceplane designs in FAR and couldn't even control that so I decided to simplify the game for a while until I'd explored the universe a bit and researched the whole tech tree. I will revisit NEAR/FAR again in the future, but for now I'm just looking at designs in stock.

[KerikBalm]

If you want to learn FAR, start with NEAR until you know how to build and ascend a craft for it. If you do that FAR will be exactly the same except for mach effects on center of lift.

I found FAR SSTOs to be a lot harder than near SSTOs.

My designs kept becoming unstable around mach 5...

[Alshain]

I found FAR SSTOs to be a lot harder than near SSTOs.

My designs kept becoming unstable around mach 5...

Yeah, but that's a challenge. It's just not a challenge you want to be plunged into head first. That's why I say learn NEAR first, then move to FAR later.

[Raptor9]

If you're running stock aero, like me, I would recommend investing in designs utilizing the RAPIER engines. I know a lot of people don't care for them due to their weight and low thrust compared to the turbojets, but I was sold when I realized how much less air they require before flaming out. I currently have a spaceplane powered by two RAPIER engines, with one shock cone and two structural air intakes per engine, and I can usually get going to around 1500m/s around 30,000m before they autoswitch over to oxidizer. It has all the mission-equipment installed: large cargo bay, docking port, full RCS systems, solar panels, and spare batteries. The one downside of the RAPIER engines is they don't have alternators so you'll need to bring enough battery power to run your SAS and/or reaction-wheels until you get to orbit and deploy solar panels.

A couple things to remember, when you do switch to rocket power (conventional rockets or RAPIER closed-cycle mode) make sure you have a hotkey programmed to toggle your intakes closed; this will take away drag, especially if you have a bunch of them. Also, look at tailoring your fuel balance for your spaceplane trajectory. You have two ends of the spectrum: a steep ascent requiring a longer gravity burn at your apoapsis, or a flat trajectory gaining high speed across the ground while in the upper atmosphere requiring a short gravity burn at the end.

1) Steep climb: requires more raw thrust-to-weight ratio and oxidizer for the rocket engines since those will be burning much longer than air-breathing engines. Not very ideal for RAPIERs due to lower thrust.

2) Flat high-speed atmo trajectory: requires a more precise balance between weight and lift, but requires less oxidizer (which is heavier) for the rockets. RAPIERs take advantage of this method.

Manual switching the RAPIERs can squeeze out a little more altitude if you manage your throttle, but you'll be fighting the asymmetric thrust tendency of the stock aero as the engines start getting air-starved. These are just my techniques, so take them as suggestions, not what's right or wrong.

EDIT: some guy did a youtube video testing various engine configs using turbojets, rockets, aerospikes and RAPIERs. I'll see if I can find it, it shows how much higher the RAPIERs push you in the end before switching to rocket power.

Edited November 11, 2014 by Raptor9

[RizzoTheRat]

Comparison vid could be good. I was put off the rapiers by the fuel consumption, I'm getting to 40km and 2000+m/s on turbojets, allowing a circular orbit with under 10 seconds burn from 4 x 24-77's (or a pair of LV-909's on the smaller design). I'm guessing rapiers make more sense in NEAR/FAR where it sounds like you cant get as fast/high on jets.

How much difference does shutting the intakes make? Is there really enough atmosphere at 40km for them to create a significant amount of drag?

Good point on steep vs shallow climb, what sort of TWR are people aiming for in air breathing mode? Think mine was about 1.2 allowing for a very steep climb so I guess I could get a fair bit more weight on a 3 engine design, but in stock aero drag is proportional to weight, so I might struggle when I switch to 1 engine in the latter stages.

[Signo]

Is the Ram Air intake or the Shock Cone better at high altitude? I experimented with both on my first design above but didn't really notice any difference.

TBH I do not like shock cones. They are supposed to do the job twice as better compared to RAMs but I never had the same performances I was trying to match.

Structural intakes OTOH work for me exactly how they are supposed to do like in the case below. (1 turbojet, 8 structural intakes and more or less the same performances you can have with 4 RAMs)

[Hodo]

Comparison vid could be good. I was put off the rapiers by the fuel consumption, I'm getting to 40km and 2000+m/s on turbojets, allowing a circular orbit with under 10 seconds burn from 4 x 24-77's (or a pair of LV-909's on the smaller design). I'm guessing rapiers make more sense in NEAR/FAR where it sounds like you cant get as fast/high on jets.

How much difference does shutting the intakes make? Is there really enough atmosphere at 40km for them to create a significant amount of drag?

Good point on steep vs shallow climb, what sort of TWR are people aiming for in air breathing mode? Think mine was about 1.2 allowing for a very steep climb so I guess I could get a fair bit more weight on a 3 engine design, but in stock aero drag is proportional to weight, so I might struggle when I switch to 1 engine in the latter stages.

I was trying to avoid this question for a while but I have to set some things straight.

FAR- does nerf jet engines, but it does also make the atmosphere more realistic so it balances out. The nerf on the jet engines is about 50% of stock. You can reach mach 5 on Turbojets, it just requires a VERY streamlined craft to achieve it.

If you design a bad aerodynamic design, it will fly poorly. If you over fly the craft, it will fly poorly or just come apart. I know Wanderfounds SSTOs work, and work quite well. If you didn't follow the flight instructions on it you will most likely not succeed with any SSTO.

There is a trick with any SSTO, rocket or space plane, is your flight profile. You can get away with a bit more in stock then you can in FAR or even NEAR. In stock you can put a brick with some wings and enough engines and intakes and you can fly to the mun.

This simple design I made just before .25 came out worked perfectly as a SSTO and was one of the cheapest designs you can make.

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You can mimic the design with the new parts and still achieve the same results with a decent flight profile.

In short with FAR you have to have a basic understanding how aircraft work, if you don't know how a plane works. Look outside at one flying over head or go to Youtube and watch a 5min video on planes, and you will understand enough to get one to fly in FAR.

[RizzoTheRat]

I know Wanderfounds SSTOs work, and work quite well. If you didn't follow the flight instructions on it you will most likely not succeed with any SSTO.

In short with FAR you have to have a basic understanding how aircraft work, if you don't know how a plane works. Look outside at one flying over head or go to Youtube and watch a 5min video on planes, and you will understand enough to get one to fly in FAR.

That was why I tried one of Wanderfounds designs to check if was my design skills or piloting skills that were the problem, turns out its both which is a bit embarrassing seeing as I have a degree in Aeronautical Engineering and about 8 hours solo in gliders The main problem I found was that I was getting a continuous pilot induced oscillation, and find the digital nature of keyboard controls really difficult with aircraft (I can't play driving games on a keyboard either). Plus I clearly needed to spend some time reading up on how to use the tools provided with FAR but just wanted quick gratification for a while hence went back to stock.

[Alshain]

Comparison vid could be good. I was put off the rapiers by the fuel consumption, I'm getting to 40km and 2000+m/s on turbojets, allowing a circular orbit with under 10 seconds burn from 4 x 24-77's (or a pair of LV-909's on the smaller design). I'm guessing rapiers make more sense in NEAR/FAR where it sounds like you cant get as fast/high on jets.

How much difference does shutting the intakes make? Is there really enough atmosphere at 40km for them to create a significant amount of drag?

Good point on steep vs shallow climb, what sort of TWR are people aiming for in air breathing mode? Think mine was about 1.2 allowing for a very steep climb so I guess I could get a fair bit more weight on a 3 engine design, but in stock aero drag is proportional to weight, so I might struggle when I switch to 1 engine in the latter stages.

2000m/s is just wrong. There is no such air breathing engine in existence or even theorized that can go that fast. The top existing one, the SR-71, can go ~1150 m/s. The work in progress Skylon (the basis for the RAPIER engine) is theorized to go 1700 m/s. 2000m/s... that's just shy of Mach 6! The limits in FAR bring a bit of realism to it, which is what most users of FAR would want.

As far as shutting the intakes goes, if I have an action group to shut down engines, I just put them in there too because... why not? If I'm using rapiers I might add an action group if there is one available, but it's not imperative.

When designing, TWR should be enough to get you up to speed. A TWR of 1 gets you a vertical launch, which is a rocket with wings, more like the Shuttle, but it's going to cost you. You don't need that much. Shallow climbs will save you design cost and fuel.

That was why I tried one of Wanderfounds designs to check if was my design skills or piloting skills that were the problem, turns out its both which is a bit embarrassing seeing as I have a degree in Aeronautical Engineering and about 8 hours solo in gliders The main problem I found was that I was getting a continuous pilot induced oscillation, and find the digital nature of keyboard controls really difficult with aircraft (I can't play driving games on a keyboard either). Plus I clearly needed to spend some time reading up on how to use the tools provided with FAR but just wanted quick gratification for a while hence went back to stock.

You can build planes with FAR without ever looking at those tools. All the planes in my signature fly in FAR and I never once checked those tools.

Edited November 11, 2014 by Alshain

[RizzoTheRat]

2000m/s is just wrong.

Agreed, similarly with altitude, the record on earth is about 38km in a Foxbat but that was a zoom climb with his engine flaming out not level flight.

[Ruedii]

If you need to you can undo the FAR stock rearrange of the stock engine power. I personally prefer the B9 setting set, although I still think they could use adjustment.

Personally, I'm thinking of making my own module manager files for jet engines.

[Alshain]

If you need to you can undo the FAR stock rearrange of the stock engine power. I personally prefer the B9 setting set, although I still think they could use adjustment.

Personally, I'm thinking of making my own module manager files for jet engines.

B9's settings go a little too far. You still have to be able to get into space. It's realistic, not 100% real. There is a degree of future tech involved, just like with rocket engines like the LV-N.

[capi3101]

Alright...getting back to the second plane design, here's what I can guess. As best I can tell, here's the parts you've got: 3 Mk2 Bicouplers, 2 Advanced Canards, 6 Shock Intakes, an Inline Cockpit, a Mk2 LF+O Tank, a Mk2-Mk1 Adapter, 4 24-77 engines, 3 Turbojets, 2 Delta Wings, 2 Elevon 2s, a Tail Fin for a vertical stabilizer in the back, 3 Small Gear Bays and 2 Reaction Stabilizers. That sound about right?

Assuming the fuel tanks are all full and my part analysis is right, your plane's mass is just over 18.5 tonnes - it carries 810 units of Liquid Fuel and 990 units of Oxidizer, and has a total lift rating of 5.9.

General rule - you want 1 lift rating for every tonne of aircraft (the actual ratio is 1.15 lift rating per tonne, but the 1:1 relationship is easier to manage and remember). Based on that, you've got about a third of the amount of lift you want for a plane of that mass.

EDIT: I've got that backwards - it's 1.15 tonnes per lift rating, or roughly 2.3 tonnes per Delta Wing (lift rating = 2).

Another general rule - for Turbojets, you want one per 15 tonnes of aircraft. Since you've got only 18.5 tonnes, two turbojets should be sufficient to do the job. If you're worried about asymmetric flameouts, add rudders - I had a two-engine design with four rudders that did an asymmetric flameout and remained controllable - it even maintained its heading. I still throttled back to get the other engine working again, but I didn't have to correct a flat spin as well.

How's the handling on the design? I'm not seeing a large number of control surfaces there, though from what I'm seeing you should have reasonable pitch and roll authority. Yaw authority, not so much...

Pretty sure you've got an overkill on fuel, Oxidizer in particular. Unless you're going to the Mun with the design, you probably need about 530-540 units of Oxidizer tops, and that's probably an over-estimate.

That said, you're making space. How about landing?

Edited November 11, 2014 by capi3101

[RizzoTheRat]

Good eyesight, you're pretty close, the side engine pods are Mk1 fuselage, liquid fuel only, and the tail fin is a small delta and Type E wing with an Eleveon 1 as a rudder. You can't see from the angle but it's a biplane with 4 Deltas each with an Elevon 1.

Total mass according to KER is a shade of 16 tonnes, assuming it ignores the vertical wings I make the lift 10.9, so sounds like I need to add another 6 or so lift.

The excess fuel is deliberate, I want to get heavier designs in to orbit to use as a tanker, it's currently carrying 840 fuel and 660 oxidiser.

ETA:

Added another 6 lift, and changed it to 2xturbojet and 2x909, whole thing is a bit under 16 tonnes and just put 755 fuel and 300 oxidiser in to a 115km orbit. I think I'm getting there. Now to go up a size...

Edited November 11, 2014 by RizzoTheRat