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Can't get this SSTO in orbit even if it reaches 2200 m/s at 35000 m!


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As the title says, I managed to reach 2200 m/s at 35000, but I can't really push it in orbit. I managed to reach (roughly) 40000m with the nuclear engine but at that point I start falling back on earth, with speed decreasing.

Please notice that I'm not using FAR and I have some experience (I played 300 hours, so I'm not noob but far from expert considering the ~2000 hours people played this game).

The issue is obviously that LV-N doesn't have enough thrust to push me outside og Kerbal gravity. I would love to hear a few suggestions.

Also, I would like not to disintegrate the design, it's the first time I managed to get a nicely-designed SSTO with a lot of delta V (6200), I'm aiming for a trip to laythe, land, come back to kerbin.

Any good suggestion? I need to push this guy in LKO!

2014-12-27_01-12-40.jpg2014-12-27_01-17-17.jpg

Delta V with turbojets:

2014-12-27_01-19-57.jpg

Delta V with LV-N:

2014-12-27_01-21-22.jpg

Edited by Fire-Dragon-DoL
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That's a nice-looking spaceplane right there :D I think getting to Laythe and back with it might be a tiny bit overambitious though - at the very least it'd be very very close.

If you can get up to 2200m/s at 35km, I think you should be able to get to space using only the jet engines - just keep (slowly!) going higher and throttling back as you get higher. Once you're in space the LV-N's low thrust obviously isn't a serious issue :D. I know mechjeb has a thing that will handle throttle automatically for you based on intake air; you could try that to see if it helps, but it's quite prone to letting engines flame-out asymmetrically in my experience. Kerbal Engineer Redux has an intake air usage/consumption readout which I find to be more useful for managing intake air manually :)

Edited by armagheddonsgw
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I'm using that already (I was doing it manually previously, argh!), but 2200 was really the limit, I had the throttle at roughly 10%, maybe less. I have no idea how can I push it out with jet only o.o

Laythe too much? Mhhh... How much delta v do I need from lko for a trip and back? I usually have 7k on my big spaceships, but they don't need to land and go back to orbit :P

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Fire Dragon,

It looks to me like you've got too many wings there and the induced drag (even at 35km altitude) is too much for the LV-N to overcome.

You really only need 1.0 Cl per ton of aircraft to get you to the jump-off point to orbit.

You also wouldn't really need 4 turbojets for this job and are you remembering to close your intakes when transitioning to the LV-N?

http://forum.kerbalspaceprogram.com/threads/102182-So-you-want-to-build-a-space-plane

Best,

-Slashy

Edited by GoSlash27
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You might just need a few more intakes (lol), but it's certainly possible to do with turbojets (easier for small planes though). Ideally you should be able to keep going past 40km, but probably not for long.

As for Laythe, I'm not sure exactly how much deltaV you need (it's been way too long since I've went there lol) but going by this you need at least 2k for the transfer from Kerbin to jool, plus maybe 10-40m/s to adjust your trajectory to be a Laythe encounter. For the return trip, you need ~2.8k for escape velocity from Laythe (a little less just to get back to Jool's SoI) and then roughly another 1.7k for the transfer from there to Kerbin. So as a rough estimate 6.5km/s?

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As for Laythe, I'm not sure exactly how much deltaV you need (it's been way too long since I've went there lol) but going by this you need at least 2k for the transfer from Kerbin to jool, plus maybe 10-40m/s to adjust your trajectory to be a Laythe encounter. For the return trip, you need ~2.8k for escape velocity from Laythe (a little less just to get back to Jool's SoI) and then roughly another 1.7k for the transfer from there to Kerbin. So as a rough estimate 6.5km/s?

It takes only about 1200 m/s to return from Laythe, maybe less. For Alexmoon's calculator, plug in Laythe's orbital characteristics to figure out when you should leave Low Laythe Orbit (i.e. where should Laythe be when you get set to leave). That tells you you need about a 1500 m/s burn, but it's not taking into account that you're going 1800 m/s already, and that you're in a gravity well. At the appointed time, put down a maneuver node that has you leave Laythe's SoI directly prograde, and keep pushing until it also escapes Jool's SoI (directly retrograde) and eventually intersects Kerbin's orbit. It'll be about 1000-1200 m/s.

So the total trip is under 3.5 km/s, and that includes some reasonably large corrective burns.

If you're talking about leaving Laythe's surface, my experience is that it's about a 3-4 km/s burn, but it's on jets. The best thing there is to align your orbit such that you end up with periapsis in the right place for your later interplanetary burn, and apoapsis at about 1,000 km altitude. Then you can save a couple hundred m/s.

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For Alexmoon's calculator, plug in Laythe's orbital characteristics to figure out when you should leave Low Laythe Orbit (i.e. where should Laythe be when you get set to leave.

That's exactly what I did... the produced figure was 1.7k 1.6k (actually more like 1.8k 1.7k if you don't want to wait on laythe for 2 years) 1.6k for getting from Laythe's orbit to Kerbin (also considering the earliest departure is approx 640 days year 3 day 12). As for the rest, sure but a direct return's a little difficult to get the timing/nodes right for. I have done that before though (but it's probably been more than a year)

EDIT: I may have slightly forgot the flight length in estimating earliest departure >_>

Edited by armagheddonsgw
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First off, the 2200m/s is surface speed, not orbital speed. Right?

If you switch off the jets then, don't do that -- milk them for every last bit of thrust. Run the Nerva in parallel to the jets; two jets and one Nerva at 1/3rd throttle probably still provide more thrust than the Nerva alone at 100%. If you can run more than 30% throttle, even better.

If that still doesn't help: what Slashy said.

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I agree the drag from all those intakes is probably not helping. However, a single LVN for a 41t aircraft is marginal also. Once you reach orbit, TWR becomes less important, long burns are acceptable. But when you are giving the final nudge out of the atmosphere, and circularization burn, you need to complete that in a limited time, before you start dropping back into the atmosphere.

If removing drag (intakes and maybe some wing) doesn't work, you might try adding a second LVN. Or, if the weight of the extra LVN isn't agreeable, you could add some 48-7S engines, just to push you to orbit. You can then disable them in space where the extra thrust isn't needed. They weigh next to nothing.

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That's exactly what I did... the produced figure was 1.7k 1.6k (actually more like 1.8k 1.7k if you don't want to wait on laythe for 2 years) 1.6k for getting from Laythe's orbit to Kerbin (also considering the earliest departure is approx 640 days year 3 day 12). As for the rest, sure but a direct return's a little difficult to get the timing/nodes right for. I have done that before though (but it's probably been more than a year)

The optimal for the first two years is to exit Laythe's SoI at 1828 m/s. Assuming you're in circular orbit around Laythe at 55 km, you've already got a lot of that speed, so you only need a burn of about 1179 m/s. If you set it up right (which requires some additional care) you can have a couple hundred m/s of that done with jets.

Kosmo-not had a great tutorial for the math required here:

http://forum.kerbalspaceprogram.com/threads/16511-Tutorial-Interplanetary-How-To-Guide

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The optimal for the first two years is to exit Laythe's SoI at 1828 m/s.

That's fine, except it takes 'til just after (the start of) year 3 to get there in the first place if you're trying to do the initial transfer from Kerbin somewhat efficiently - and that's even being a little wasteful to keep the travel time reasonably low.

Edited by armagheddonsgw
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I agree the drag from all those intakes is probably not helping.

At high altitude, drag from intakes is precisely offset by thrust from engines, in a 1:1 ratio. Adding intakes just lets you get up to higher altitude.

As for wings, the lift:drag ratio is greater than 1 until you get stupidly fast -- as in, until you get to solar escape velocity. It's not so much their drag that's a problem, as it is their mass.

I've just added code to calculate all the forces that a wing suffers to my KSP-scripts. Previously it ignored gravity.

Say you're going 2200 m/s surface speed at 35km with 100 m/s vertical, maintaining a 15-degree angle of attack. What forces does a delta wing on this plane produce? It's got a flight path angle of 2.625 degrees, 15 degrees angle of attack, so we write:

lift.deltaWing.forceVector(2.625, 15, 2200, altitude = 35000)
(-0.3690115475061154, 0.7273193796026425)

Well that looks good: we have to fight 0.37 kN in drag (aerodynamic drag and parasitic drag), but we gain 0.73 kN in lift. However, the scene changes when you take apparent gravity into account:


lift.deltaWing.forceVector(2.625, 15, 2200, altitude = 35000, includeGravity=True)
(-0.3690115475061154, -0.04646433649130555)

You get drag, but the lift doesn't outpace the mass.

That said, delta wings are the least efficient. Switch to the wing strake, and you're still getting a net benefit under those circumstances.

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First off, the 2200m/s is surface speed, not orbital speed. Right?

If you switch off the jets then, don't do that -- milk them for every last bit of thrust. Run the Nerva in parallel to the jets; two jets and one Nerva at 1/3rd throttle probably still provide more thrust than the Nerva alone at 100%. If you can run more than 30% throttle, even better.

If that still doesn't help: what Slashy said.

No, I meant orbital speed, forgot to specify. And when I say "I switch jet off", I already milked everything, speed is going down (very, very slowly, like 0.1 every 2 minutes), and I'm about to descend, so really it was the best I could do.

I'll try with two LV-N today and see if it goes better.

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That's fine, except it takes 'til just after (the start of) year 3 to get there in the first place if you're trying to do the initial transfer from Kerbin somewhat efficiently - and that's even being a little wasteful to keep the travel time reasonably low.

Well, I'm trying to make it a little bit cool, otherwise I have a big SQUARE SPACESHIP (that's the name I gave it) which brings a full gray tank in space without using it.

But you know, it's terrible, lol

I agree the drag from all those intakes is probably not helping. However, a single LVN for a 41t aircraft is marginal also. Once you reach orbit, TWR becomes less important, long burns are acceptable. But when you are giving the final nudge out of the atmosphere, and circularization burn, you need to complete that in a limited time, before you start dropping back into the atmosphere.

If removing drag (intakes and maybe some wing) doesn't work, you might try adding a second LVN. Or, if the weight of the extra LVN isn't agreeable, you could add some 48-7S engines, just to push you to orbit. You can then disable them in space where the extra thrust isn't needed. They weigh next to nothing.

Your suggestion is very good, I'm going to try a few times in this way before breaking the SSTO apart and rebuild it.

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Maybe try and replace 2 turbojets with 2 rapiers?

The rapiers in rocket mode should give you enough thrust to enter orbit without having to rebuild the entire plane to add a second nuke.

The plane was well built, I managed to remove the single nuke and add a coupler for 2 nukes, but it wasn't enough in any case, so I went back to one nuke and added two rapiers.

Unfortunately, rapiers consume tons of fuel, so I end up with 3800 delta v in LKO, sigh, looks like I'll need an EVEN LARGER SSTO to go to laythe

2014-12-27_13-17-14.jpg

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It takes only about 1200 m/s to return from Laythe, maybe less. For Alexmoon's calculator, plug in Laythe's orbital characteristics to figure out when you should leave Low Laythe Orbit (i.e. where should Laythe be when you get set to leave). That tells you you need about a 1500 m/s burn, but it's not taking into account that you're going 1800 m/s already, and that you're in a gravity well. At the appointed time, put down a maneuver node that has you leave Laythe's SoI directly prograde, and keep pushing until it also escapes Jool's SoI (directly retrograde) and eventually intersects Kerbin's orbit. It'll be about 1000-1200 m/s.

So the total trip is under 3.5 km/s, and that includes some reasonably large corrective burns.

If you're talking about leaving Laythe's surface, my experience is that it's about a 3-4 km/s burn, but it's on jets. The best thing there is to align your orbit such that you end up with periapsis in the right place for your later interplanetary burn, and apoapsis at about 1,000 km altitude. Then you can save a couple hundred m/s.

Man, thanks a lot, having an idea of how much delta v I need while in orbit really helps me.

So IN THEORY my SSTO now can reach Laythe and come back, but I feel like I'll have troubles when trying to come back after landing there (expecially I don't have a clever idea of how much I'll consume while on Laythe).

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Fire-Dragon,

You don't need a bigger spaceplane to get to Laythe, you need a *smaller* one.

For the amount of mass you're lifting, you've got way too many turbojets and way too many wings. You need just enough wings to get you above 32km altitude and just enough turbojets to overcome drag through the 25-32km "wall".

For the top-end speed portion of the flight, more turbojets don't help you to go faster, they're just dead weight.

Same for wings; they help get you over the 25-32km "wall", and after that they're dead weight.

You really only need 1 turbojet for each 15 tonnes of aircraft, 4 radial intakes per engine, and enough wings to generate 1.0 lift coefficient per ton of aircraft.

If you build to those specs, your spaceplane will easily achieve orbit and be light enough for your LV-R to do it's thing.

SATFix7_zps8aad1571.jpg

This plane is designed to shuttle Kerbals to a station in LKO and back to KSC, but you can see how much payload that is. That 12t plus the fuel to feed the LV-909 buys you a lot of DV with an LV-N.

For wings, the delta wings are the least efficient choice (somebody said that upstream; good advice). The ideal wings are the strakes.

For engines, I don't recommend RAPIERs for any reason other than cool factor. They stop producing thrust 200m/sec earlier than turbojets and it's less thrust.

tl;dr

Don't think "moar boosters", think "moar efficient".

Best,

-Slashy

Edited by GoSlash27
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The key to getting such a design into orbit is having enough vertical inertia when you end up with flameout going over 2,200 m/s. And the higher the altitude you are when that happens, the better the chances of coasting to 100k or more altitude where you can then make the circulation burn.

7j7T4zU.jpg

Needless to say, the insertion window is extremely critical for achieving orbit using turbojets. You have the speed, you must now find the climb angle needed to achieve the desired orbital apogee without having to use the LV-N

Edited by SRV Ron
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I agree with those who say you've probably got too much wing area, but if you're getting above 2000m/s surface speed at 35km, you should be able to make orbit anyway even with just that little nuke. Rather than trying to fly right out of the atmosphere at a steady rate of climb, I think what you need to do is get to your max speed at a somewhat lower altitude, like 27-28km, then pitch up more for your the end of your jet burn, trading forward speed for rate of climb. With that little LV-N engine, if you don't have a high enough rate of ballistic climb at jet flameout, even at 35km you'll dissipate too much of your kinetic energy with drag for it to overcome. If you can get to where you're climbing at or over 100m/s when you cross 35km, even if you're going significantly slower than 2000m/s you'll be out of the soup long enough for the LV-N to put you in orbit.

Another solution might be to replace one pair of turbojets with RAPIER engines. I know you want to avoid burning fuel at such a low ISP, but if you're just trying to get that last little push into space you'll only need to burn them as rockets for a few seconds. If you have any destinations other than Laythe in mind, they'll also give you the power you'll need to make efficient landings on airless bodies.

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Fire-Dragon,

You don't need a bigger spaceplane to get to Laythe, you need a *smaller* one.

For the amount of mass you're lifting, you've got way too many turbojets and way too many wings. You need just enough wings to get you above 32km altitude and just enough turbojets to overcome drag through the 25-32km "wall".

For the top-end speed portion of the flight, more turbojets don't help you to go faster, they're just dead weight.

Same for wings; they help get you over the 25-32km "wall", and after that they're dead weight.

You really only need 1 turbojet for each 15 tonnes of aircraft, 4 radial intakes per engine, and enough wings to generate 1.0 lift coefficient per ton of aircraft.

If you build to those specs, your spaceplane will easily achieve orbit and be light enough for your LV-R to do it's thing.

http://i52.photobucket.com/albums/g13/GoSlash27/KSP/SATFix7_zps8aad1571.jpg

This plane is designed to shuttle Kerbals to a station in LKO and back to KSC, but you can see how much payload that is. That 12t plus the fuel to feed the LV-909 buys you a lot of DV with an LV-N.

For wings, the delta wings are the least efficient choice (somebody said that upstream; good advice). The ideal wings are the strakes.

For engines, I don't recommend RAPIERs for any reason other than cool factor. They stop producing thrust 200m/sec earlier than turbojets and it's less thrust.

tl;dr

Don't think "moar boosters", think "moar efficient".

Best,

-Slashy

Thanks slashy, that's a very good suggestion. I'll be trying a new SSTO this evening.

One note, remember that some wings have been chosen for design purposes.

Are there any table with most efficient wings?

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Unfortunately, rapiers consume tons of fuel, so I end up with 3800 delta v in LKO, sigh, looks like I'll need an EVEN LARGER SSTO to go to laythe

It's so easy to get caught up in that mentality, but this tiny little craft can make it to Laythe and back easily:

napMQ9Z.jpg

It uses 4 ion engines instead of a nuke, and with some small modifications you can put an actual cockpit on it. As I said above, the key trick to getting to orbit with something like this is to have enough climb rate at flameout so that even the puny ion engines can circularize it.

Edited by herbal space program
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It's so easy to get caught up in that mentality, but this tiny little craft can make it to Laythe and back easily:

http://imgur.com/a/napMQ9Z

It uses 4 ion engines instead of a nuke, and with some small modifications you can put an actual cockpit on it. As I said above, the key trick to getting to orbit with something like this is to have enough climb rate at flameout so that even the puny ion engines can circularize it.

Ahem, that image man, lol

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