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endl

SSTO what to do to concerve fuel?

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so im trying to get my SSTO space plane to reach my station 150km in orbit but im having problems conversing fuel across all my design attempts.

most of my planes only have enough air intake until 21km regardless of intake spam. ive tried different engine combinations for 21-100km stage and nothing seems to work. i know turbo jets are best air breathers and 909 are best for orbital translations unless were talking nukes for longer distances. but whats a good "work horse" for the gap in between. right now im using rapiers because they save on weight but they eat fuel like crazy so that leads to my plane getting heavier since i just cant seem to strike a balance i feel like ive hit a brick wall.

Edited by endl

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@Kasuha: Because you made this statement.

Intakes per mass are IMO much better measure - at least in stock aero.

Which is not true. It's intakes to TWR as the measure in stock aero. Show me a 100T craft with 400 RAM intakes and one turbo jet engine that can achieve orbital velocity.

This is not the topic of the thread. We are talking about efficiency to orbit. I have 6-7 ton craft with 2 RAM intakes that can achieve a periapsis at their own altitude in atmo but, they don't go to orbit as efficiently as different ascent profiles or, with craft on that type of ascent with a higher TWR.

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You should have around 4 RAM intakes per turbojet
Intakes per mass are IMO much better measure

I don't see anything wrong on my statement. TWR of course plays role, too. I was not arguing with that.

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Impressive instructions but let me add my own experience. I am assuming stock physics and aerodynamics here so I'm not sure if it applies to OP.

A/ build your plane right. Rules for intake air distribution have already been linked so I don't have to do that. Use enough engines for the plane mass, the more engines the less time and fuel will you need. Optimal engine numbers for planes, minimizing problems with asymmetric flameout are 1 and 4. 1 engine is obvious, 4 engines need to be interleaved with intakes properly and placed in order outer-inner-outer-inner. For instance: (1)-(2)-(4)-(3). Why? Because when first stage flameout occurs, both your outer engines will flameout at once and inner engines will keep running. No asymmetry, and giving you enough time to lower the throttle. One RAM intake per engine is enough to reach orbital speeds, but two or three make it more comfortable. Don't make the plane too heavy, have 2 intakes per 10 tons at least (some will call that airhogging)

B/ flying

1/ reach 10 km as fast as possible. Use maximum pitch that allows you to keep your speed. 70 m/s is fine as long as you're getting out of the soup fast.

2/ above 10 km concentrate on building your speed. Switch to map view and watch your apoapsis. Switch navball to orbital mode. Raise your apoapsis to 20-25 km, not higher until you reach at least 1000 m/s. Regulate the apoapsis with pitch, don't be afraid to burn below prograde if your apoapsis is too high. Then raise it to 30 km and build your speed to about 1800 m/s. Then raise the apoapsis to 36 km and burn strictly prograde. If you have enough power, your orbit will grow out of atmosphere at this point. If you don't have enough power and reach the apoapsis, pitch up and keep the plane at the apoapsis (or rather the apoapsis below the plane) by changing pitch slightly up and down, continue burning until the apoapsis jumps to the other side of planet. After that, burn prograde.

3/ build orbital speed by burning prograde. As you lose air, your engines will start decreasing their thrust. When you get a flameout, decrease thrust to 2/3, then 1/3, and then go to just one tick above zero thrust. You can use physics time warp at this phase to wait till your plane gets out of atmosphere.

this was very helpful but i dont think my design is working, if you want to have a go at it maybe you can offer some more advice on how to set it up. atm i think it needs more thrust. im trying to dock with my station in 150km orbit.

download (requires spaceplane +)

3ytUa5ll.jpg

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Not too many details in that picture, but you seem to have 4 rapiers for a vessel of about 40 tons. That should be enough thrust. Barely enough, IMO, but others are sure to disagree.

BUT: How many intakes do you have? I'm not familiar with SP+ parts, so I may fail to spot some... to me it looks as if you'd be suffering not from a shortage of engines, but from a shortage of air for your engines.

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Not too many details in that picture, but you seem to have 4 rapiers for a vessel of about 40 tons. That should be enough thrust. Barely enough, IMO, but others are sure to disagree.

BUT: How many intakes do you have? I'm not familiar with SP+ parts, so I may fail to spot some... to me it looks as if you'd be suffering not from a shortage of engines, but from a shortage of air for your engines.

i provided a download link the picture is just if your too lazy i didnt want to spam the forums with a full reso image. i have 2 shock cones and 12 radial scoops (most of which are on the bottom). im not saying you should install a mod but the next version makes sp+ official so you might as well.

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More testing using NovaPunch and KW Rocketry came up with this highly efficient SSTO with a payload of the second stage. Launch weight 56.43 tons. Orbit 15.22 tons. No parts dropped off other then launch clamps.

crlztEV.jpg

SJGulvD.jpg

Staging set to allow main engine to power up before launching.

Edited by SRV Ron

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most of my planes only have enough air intake until 21km regardless of intake spam.
i have 2 shock cones and 12 radial scoops (most of which are on the bottom).

In stock, the Ram Air Intake is known to work much better than any other intake at high altitudes. SP+ parts may have similar attributes. Could well be that radial scoops aren't made for a thin atmosphere and effectively stop working, leaving you with the two shock cones as your only source of air.

Aaaaand.... it's been brought up already, but in case you missed it: Two identical-looking planes can differ wildy in performance, depending on the order in which the parts have been put together in the SPH. If you're not aware of that, you should really look into it (links have been posted a few posts back). Building your plane the right way is the single most important factor; give and take 20% TWR are insignificant in comparison.

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In stock, the Ram Air Intake is known to work much better than any other intake at high altitudes. SP+ parts may have similar attributes. Could well be that radial scoops aren't made for a thin atmosphere and effectively stop working, leaving you with the two shock cones as your only source of air.

Is that actually true? None of my testing seemed to reveal any special properties of the ram intake, aside from it's significantly larger area.

Well that, and it has the same base mass as all of all the other lightweight intakes with the larger area, making it less draggy per intake area thanks to #lolstockairphysics.

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Not too many details in that picture, but you seem to have 4 rapiers for a vessel of about 40 tons. That should be enough thrust. Barely enough, IMO, but others are sure to disagree.

BUT: How many intakes do you have? I'm not familiar with SP+ parts, so I may fail to spot some... to me it looks as if you'd be suffering not from a shortage of engines, but from a shortage of air for your engines.

Basically speaking, SP+ shock cone = stock ramscoop (actually, the SP+ cone is a smidgeon better than a scoop, particularly at extreme altitude) and SP+ structural intake (the long things with the square cross section) = stock radial intake. There's definitely enough engine, intake and fuel there to get to orbit in either stock or FAR/NEAR (although airhogging it up would make it easier in stock). The problem is in the piloting.

Essentially all of my designs are for FAR, but I do have one that is proven to be able to easily make it to orbit in stock air. See https://www.dropbox.com/s/lbnz9s8k9h7gwgb/Kerbodyne%20Benchmark%20StockAir.craft?dl=0 for the download.

screenshot409_zps875352d5.jpg

That's running three engines from two intakes. Intake spam is not necessary; it's just a way of covering for dodgy piloting. Crank it up as fast as you can before you climb out of the rich air, then shut down the RAPIERs and continue climbing on the turbojet alone. Once the turbo chokes, turn the RAPIERs back on (using closed cycle oxidising mode) and let the rocket-driven ram air revive the turbo. Do it right and you'll have the apoapsis over 70km before the turbo chokes again, with plenty of fuel remaining for circularising once you're in space.

Edited by Wanderfound

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Okay: just took the OP's plane for a spin in stock aero.

Yeah, you've got problems, but they're nothing to do with air intakes or thrust. You're massively lacking in pitch authority and lift; the plane requires a huge angle of attack just to maintain level flight (not enough lift) and you have to stress the controls to the max in order to keep that angle (not enough pitch authority).

Lose some weight, get some more wing surface, and add some more pitch-relevant control authority (i.e. placed as far away from CoM longitudinally as possible; canards up front or elevators out back). The control surfaces on your wings aren't doing much for pitch; they're too close to CoM. The pair of canards you've hung out the back just aren't cutting it.

While you're at it, increasing the height of the front gear with a hardpoint, pylon or strut would ease takeoff considerably. And you'd be better off rearranging your action groups so that you can toggle and mode-switch the engines in pairs.

Alternately, ditch the stock aero and give NEAR a try. The plane would probably work (after a bit of aero tuning) if it didn't have to fly in soup.

Edited by Wanderfound

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Okay: just took the OP's plane for a spin in stock aero.

Yeah, you've got problems, but they're nothing to do with air intakes or thrust. You're massively lacking in pitch authority and lift; the plane requires a huge angle of attack just to maintain level flight (not enough lift) and you have to stress the controls to the max in order to keep that angle (not enough pitch authority).

Lose some weight, get some more wing surface, and add some more pitch-relevant control authority (i.e. placed as far away from CoM longitudinally as possible; canards up front or elevators out back). The control surfaces on your wings aren't doing much for pitch; they're too close to CoM. The pair of canards you've hung out the back just aren't cutting it.

While you're at it, increasing the height of the front gear with a hardpoint, pylon or strut would ease takeoff considerably. And you'd be better off rearranging your action groups so that you can toggle and mode-switch the engines in pairs.

I agree with this. Lift is one of the things that people seem to forget about when discussing space planes. Below is a pic of my workhorse fleet in career. The smallest is by far my most efficient design (not a spaceplane expert). If you compare wing area between that one and the largest one in the pic you'll see that as the total weight increases the wing area increases even more, but although the thrust does increase, it doesn't increase as much as the wing area. OTOH, it is a balancing act, too much wing just increases drag.

Thrust is another key point when considering what setup works best for your design. I almost always use turbo jet and small rocket motors on my smaller spaceplanes. My small spaceplane in the pic can easily achieve orbit with a single Rapier replacing the jet and smal rocket engines, but at the cost of quite a bit more fuel. OTOH, bigger designs will require bigger rocket engines (most of the time), making the design more complicated, and usually heavier. Most of my medium and larger spaceplanes use Rapiers for this reason.

4Z2iWzS.png

Edited by Beachernaut

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Okay: just took the OP's plane for a spin in stock aero.

While you're at it, increasing the height of the front gear with a hardpoint, pylon or strut would ease takeoff considerably. And you'd be better off rearranging your action groups so that you can toggle and mode-switch the engines in pairs.

@Wanderfound:

While i agree with your other points, i have to say that your statement with the gears maybe cause some new troubles. Because mostly every plane will "dance" on the RW if the front gear is higher than the rear gear.

@endl:

I haven't tryed out your plane jet, but will do it afterwards. One of your problems may be the engines. RAPIER's tend to loose mutch faster thrust and ISP than ordenary jets and have - even with enough air-intake - very early an flameout. Because of that you have to switch the mode very early witch results in a low fuel capacity when you achieve orbit.

The second thing:

Call it a "bug" or "game feature". If you achieve an airspeed over 1800 m/s with jet's - preferably below 30k altitude - it doesn't matter if there is zero airintake. The engines will work up to 55k altitude (with ever less thrust, of course).

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Call it a "bug" or "game feature". If you achieve an airspeed over 1800 m/s with jet's - preferably below 30k altitude - it doesn't matter if there is zero airintake. The engines will work up to 55k altitude (with ever less thrust, of course).

What's right: jets will function as long as they have air to work with. Up to 69km or wherever the atmosphere is cut off. You will need to throttle down, of course: if you demand too much of them, they will flame out and you get nothing. In the above picture (no.2), the throttle is almost, but not quite, at zero. The thrust you can tickle out of a jet at extreme altitudes may seem miniscule, but it can make quite a difference, like bringing periapsis fom -60 to +30km.

What's wrong: this has nothing to do with airspeed. When and where you reach 1800m/s, and whether you do it under jet power, plays no role.

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@Wanderfound:

While i agree with your other points, i have to say that your statement with the gears maybe cause some new troubles. Because mostly every plane will "dance" on the RW if the front gear is higher than the rear gear.

Only if there's something else seriously wrong with the design. Check the Omnibus thread in my sig; there are about twenty planes there, almost all of which run nose-high, none of which dance on the runway. The craft files are all available in my thread; take 'em for a test spin if you don't believe me. They're all FAR planes, but stock aero won't make any real difference on the runway.

screenshot298_zps39f53b8c.jpg

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screenshot318_zps65f4ffc2.jpg

screenshot419_zps22633733.jpg

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In my experience, gear-tracking problems in KSP are almost always caused by off-vertical gear or gear placed on mounting points that flex. Wing-mounted gear is especially troublesome unless the wing is very well braced.

Edited by Wanderfound

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Main problem with uneven gear height is in my experience that the game will drop the plane on runway from greater height and parts may start falling off.

My best experience is with putting two wheels slightly behind the plane's CoM (so the plane does not turn under its own weight but it needs only a little force to do so), one wheel at the same height below the nose, and one elevated wheel at the back to protect the engine from touching the ground on liftoff/landing.

The model below has also two stabilization wheels at ends of its wings but these are not mandatory.

n89xvKm.jpg

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i made some changes based on the feed back im getting. i got to 1700ms maybe its my piloting :(

Nice plane! People are reporting issues with the radial intakes on SP+. Try replacing them with shock cone intakes to see if that solves the problem. It's a shame because the radial intakes look awesome and are easier to attach to a plane!

I haven't got this mod but since it's going to be part of the main game anyway I will attempt to install it and give your plane a fly to see if my theory is correct, (when I have finished my current mission).

Edited by Redshift OTF

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Nice plane! People are reporting issues with the radial intakes on SP+. Try replacing them with shock cone intakes to see if that solves the problem. It's a shame because the radial intakes look awesome and are easier to attach to a plane!

I haven't got this mod but since it's going to be part of the main game anyway I will attempt to install it and give your plane a fly to see if my theory is correct, (when I have finished my current mission).

ty, im trying to keep it simple, its supposed to be a crew transport so im trying to keep it small, i also wanted to try to retain a jet fighter profile, the original was modeled after the f-22 this configuration forced me to change the wing shape. which reminds me i forgot to add wing brakes ><

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What's wrong: this has nothing to do with airspeed. When and where you reach 1800m/s, and whether you do it under jet power, plays no role.

Actually, intakes are speed sensitive. Their flow rate is proportional to their area AND the air speed.

Nice plane! People are reporting issues with the radial intakes on SP+. Try replacing them with shock cone intakes to see if that solves the problem. It's a shame because the radial intakes look awesome and are easier to attach to a plane!

The SP+ structural intakes have a very small intake area in their config - it's something like half of a stock scoop, if I recall correctly. I'm hoping that they get bumped up a bit for 0.25 personally, because they do look quite neat, until you have to attach like five on each side of the plane to equal a ram scoop..

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i made some changes based on the feed back im getting. i got to 1700ms maybe its my piloting :(

endl, I like your design. However, for a plane that size, 2 Rapiers should be plenty to get to orbit. The extra tanks and engines on the side could be dropped to save quite a bit of weight. OTOH that leaves you with just the radial intakes. Although I use them, they are never my primary source for air. Using a couple radial intakes to compliment a ram air for a single engine seems to only give marginally more air. Edit: Renegrade hit it right on the head with the intakes.

Now that I look closer I'm guessing that's a cargo bay right behind the cockpit rather than a fuel tank? When I get home I'll do a bit of experimenting with your design.

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Yeah I think the side intakes, (whatever they are called), only pull in a quarter of the air of shock intakes so 12 side intakes equal 3 shock intakes, (actually shock intakes pull in a bit more than ram intakes so it's a little less than 3). That only gives you around 5 intakes for 4 engines so it's going to struggle to get into orbit. I usually use 4 ram intakes for each engine! You could always use part clipping in the debug menu and add more shock intakes on top of the shock intakes you already have if you want to retain aesthetics.

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endl, I like your design. However, for a plane that size, 2 Rapiers should be plenty to get to orbit. The extra tanks and engines on the side could be dropped to save quite a bit of weight. OTOH that leaves you with just the radial intakes. Although I use them, they are never my primary source for air. Using a couple radial intakes to compliment a ram air for a single engine seems to only give marginally more air. Edit: Renegrade hit it right on the head with the intakes.

Now that I look closer I'm guessing that's a cargo bay right behind the cockpit rather than a fuel tank? When I get home I'll do a bit of experimenting with your design.

Personally I use at least 1 jet engine per 10 tons of weight. Minimum. But that's my preference. He has the same number of engines I would use.

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yea its a bay, i store extra mono propellent in there and a bit of extra fuel, its basically a service bay.

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OK, I've flown the plane you posted in post #41 and it has nowhere near enough air coming into it to provide enough thrust. I clipped extra shock intakes onto the current ones so there were 8 each side, (4 per engine, probably overkill for this plane), and it gets into space easily. Of course if you don't like part clipping then you'll have to find another way of mounting 16 shock intakes :D

BTW, you can use Mechjeb for flying space planes. Settings I used where 100km orbit and under "edit ascent path" I used 15 for turn start altitude, 35 for turn end altitude, 0 for final flight path angle and 20 for turn shape. Fly it manually up to 15 km and engage. As soon as your speed gets near 1900-2000 m/s switch to closed cycle and Mechjeb will do the rest.

Your plane is actually lighter than I thought so you probably don't need as many engines as I thought but it is really fun to fly with that much thrust. Also maybe move the rear wheels forward a bit so it is easier to get off the runway and some more tail fins as it can get a bit twitchy with the yaw. I don't think your RCS thrusters will give you side to side movement if you are planning to dock with anything.

Edited by Redshift OTF

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OK, I've flown the plane you posted in post #41 and it has nowhere near enough air coming into it to provide enough thrust. I clipped extra shock intakes onto the current ones so there were 8 each side, (4 per engine, probably overkill for this plane), and it gets into space easily. Of course if you don't like part clipping then you'll have to find another way of mounting 16 shock intakes :D

BTW, you can use Mechjeb for flying space planes. Settings I used where 100km orbit and under "edit ascent path" I used 15 for turn start altitude, 35 for turn end altitude, 0 for final flight path angle and 20 for turn shape. Fly it manually up to 15 km and engage. As soon as your speed gets near 1900-2000 m/s switch to closed cycle and Mechjeb will do the rest.

Your plane is actually lighter than I thought so you probably don't need as many engines as I thought but it is really fun to fly with that much thrust. Also maybe move the rear wheels forward a bit so it is easier to get off the runway and some more tail fins as it can get a bit twitchy with the yaw. I don't think your RCS thrusters will give you side to side movement if you are planning to dock with anything.

welp i did it, thanks so much for all the help. ill be sure to +rep. im pretty sure i could have done it with even more fuel left over but my flying skills still suck.

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if anyone is interested in a 25t, 6 kerbal crew ship here is the craft file im not sure if it has enough fuel left over for a return flight but you can refuel it since it has a docking port. (it was mostly full at 130k orbit but after some translations to get it to my station at 150k i somehow managed to burn most of it)

all thats left is to give it an official name instead of mission designation, i think im going to go with Valkyrie. i named my science station odin ;p

Edited by endl

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