1.0.2 - Any hope left for SSTOs?

[GoSlash27]

I looked at the .cfg files, I don't see why that would be the case. To me the Rapiers just seem plain better. Sure, they are slightly heavier, and their Isp is 6400 instead of 8,000... but when you start getting high payload fractions, that doesn't really matter... its like if the liquid fuel is 5% or 7% of your total mass... who cares? The mass lost to jet fuel is recovered in the gain from less expended rocket fuel.

I'm not sure why either, but my empirical testing so far shows that the turbojet is much more powerful around Mach 1 at 10-12km altitude, which allows it to drive a much larger mass into the hypersonic regime.

The problem is it peters out at a lower altitude and velocity, which hurts you when you transition into rockets. The 3-400 m/sec deficit and increased drag costs a lot more mass in fuel than I would expect to be saved by going that route, but as I said... I haven't tested it yet.

Best,

-Slashy

[Wanderfound]

As i mentioned, i assume (without any testing in 1.0) that the combination of RAPIERs and LV-909 might work. The Rapiers take the plane as fast and high as possible in jet mode and they have enough thrust for the time critical final kick in rocket mode. As soon as the time critical phase is over, the LV-909 can do the rest of the trip.

I still assume that the poodle and 909 are rather good and fuel saving choices in vacuum as they were in 0.90....i haven't compared the exact changes yet.

The difficulty with relying on just a pair of RAPIERs during the ascent is that you're likely to have trouble punching through the sound barrier unless it's a very lightweight ship; the new RAPIERs have a rather significant flat spot in their power curve, right in the transsonic range.

OTOH, a Turbojet/LV-909 ship can be made to work. My version was a small Mk1 with a single Turbo and two 909's, though.

[Rune]

*whistles innocently*

http://i.imgur.com/Vjpcw5U.jpg

(I actually have no idea whether the bi-adapters get shielded by the cargo bay or not, I never thought about it until now... Looks boss though so I'm keeping it Trying to work out if I can stick a third adapter on the back of the first, then a fourth on the resulting doubled node and have a jet underneath as well for three vertically aligned engines...)

Whether it does or doesn't, don't worry: the rule of cool gets a pass to that plane, every time.

I'm not sure why either, but my empirical testing so far shows that the turbojet is much more powerful around Mach 1 at 10-12km altitude, which allows it to drive a much larger mass into the hypersonic regime.

The problem is it peters out at a lower altitude and velocity, which hurts you when you transition into rockets. The 3-400 m/sec deficit and increased drag costs a lot more mass in fuel than I would expect to be saved by going that route, but as I said... I haven't tested it yet.

Best,

-Slashy

Not to mention on a Turbojet SSTO, you can use the extra rocket system for a brief push over Mach 1 to start accelerating if you are really tight on TWR. I'm thinking that could save some marginal design cases.

And, with the extra TWR jets give (if only because they are lighter), you can push over 1km/s lower, so even if they give out earlier (I still have them usually lit with decent thrust until 20kms), you end up at pretty much the same speed when the rockets take over. Yeah, I'm a bit sad about it (I love the RAPIER and its new fuel logic), but turbojets may very well outperform RAPIER designs.

- - - Updated - - -

I'd like to do a Jet+LV-N only SSTO, so that it only uses liquid fuel, but I don't think that is workable with the even lower LV-N TWR.

I've actually done that already and yeah, the TWR on the nukes makes it very challenging and the payload fraction ends up being ridiculous, even if your fuel fraction is low... most of the GLOW ends up being engines. You need such a kick in the arse on airbreathers that you can get a decent time to apoapsis (and the apoapsis itself over the draggier parts of the atmosphere) before the jets give out. That means about 1,100-1,200m/s before 20kms, and an ascent so shallow you won't see the temperature overlays over the heat effects themselves. This puppy has TWR like 2.5 on turbojets... And still you stay burning all the way to apoapsis, maybe even dropping a bit before you reach orbital velocities and time to apoapsis climbs again.

Rune. 33% payload fraction to orbit. Wow. At least now I know I have a lot of margin to improve...

Edited May 6, 2015 by Rune

[Sattorin]

As i mentioned, i assume (without any testing in 1.0) that the combination of RAPIERs and LV-909 might work.

It should. I've gotten a SSTO to LKO with 1,200m/s to spare using 2 RAPIERs and a LV-T30. I believe it could be better by switching the T30 with a 909 and relying more on RAPIER closed cycle.

Rapier-Nuclear hybrids are good, because you can go very heavy on the liquid fuel, without worrying about carrying a lot of useless mass in orbit.

I have yet to see a nuclear SSTO design that surpasses conventional rockets either for dV in orbit or in payload capacity. You could probably do a really nice "Two stage to Laythe and return" mission though, since I expect RAPIERs to be much more effective there.

I looked at the .cfg files, I don't see why that would be the case. To me the Rapiers just seem plain better.

The RAPIERs actually lose thrust as they go from Mach 0 to Mach 1. Then thrust skyrockets after you pass Mach 2. If your plane has too much mass, RAPIERs won't be able to push through Mach 1 to reach their ideal speed. I can imaging some very large designs for which it would be more efficient to haul along a few turbojets rather than burning LFO to break that barrier.

Edited May 6, 2015 by Sattorin

[Acus]

No, they don't... Jets already get you to over half orbital velocity. IRL, a Scramjet won't get you anywhere close to there. THe Turboramjets and Rapiers already have performance better than a SCRAMjet, proportionately speaking.

I don't think you're right there - just look at angine efficiency http://en.wikipedia.org/wiki/Scramjet#/media/File:Specific-impulse-kk-20090105.png

I didnt check rapiers after patch, you gave me a hope:)

And quite interesting fact is that space plane can be fueled by liguid fuel only(turbos+909) - greatly reducing mass of plane.

[Jovus]

The RAPIERs actually lose thrust as they go from Mach 0 to Mach 1. Then thrust skyrockets after you pass Mach 2. If your plane has too much mass, RAPIERs won't be able to push through Mach 1 to reach their ideal speed. I can imaging some very large designs for which it would be more efficient to haul along a few turbojets rather than burning LFO to break that barrier.

I found this to be the case with the Mk 3 proof of concept SSTO I just built. It has six engines; if all of those are RAPIERs, it can't break the sound barrier. Replace a pair with a pair of turbojets and getting to orbit is a breeze. (You still have to dive a bit, but that's as it should be.)

[KerikBalm]

Well, my first 1.0 SSTO design used a mix of rapiers and turboramjets.

That may be the way to do it.... climb through mach 1 better... then in the final stages of flight, intake air seems to be the limiting factor anyway.... and the rapiers aren't running at full anyway (while you still accelerate), so switching a few to turbojets may not harm the final airbreathing speed.

So lets see what the relevant parts of the .cfg file say:

Turboramjet:

mass = 1.8

maxThrust = 180

velCurve

{

key = 0 1 0 0

key = 0.2 0.98 0 0

key = 0.72 1.716 2.433527 2.433527

key = 1.36 3.2 1.986082 1.986082

key = 2.15 4.9 1.452677 1.452677

key = 3 5.8 0.0005786046 0.0005786046

key = 4.5 3 -4.279616 -4.279616

key = 5.5 0 -0.02420209 0

}

I assume this means:

- it produces 0 thrust at mach 5.5...

- a 5.8x multiplier at mach 3 (mach 3 is is maximum performance, 1044 kN)

- a 3.2x multiplier at mach 1.36

- a 1.7x multiplier at mach 0.72

I don't know how to use the other numbers to define the curve and find its thrust at mach 1... I assume it is somewhere between 1.7 an 3.2

Lets just split the difference and assume: 2.45x multiplier -> 441 kN of thrust

The rapier:

mass = 2.0

maxThrust = 140

velCurve

{

key = 0 1 0 0.08333334

key = 0.2 0.98 0.42074 0.42074

key = 0.7 1.8 2.290406 2.290406

key = 1.4 4.00 3.887193 3.887193

key = 3.75 8.5 0 0

key = 4.5 7.3 -2.831749 -2.831749

key = 5.5 3 -5.260566 -5.260566

key = 6 0 -0.02420209 0

}

Zero thrust at mach 6.... but still significant thrust at 5.5 (3x multiplier)

8.5x multiplier at 4.5 -> 1190 kN

4.0 and mach 1.4, 1.8 at mach 0.7...

splitting the difference again: 2.9 * 140 = 406 kN.... ok, this does seem signficantly less than the turbojet... nearly 10% worse... and I think when accurate numbers for mach 1, it may be better

I still haven't looked at the atmosphere curve, that may favor the turbo at the altitude that you typicaly go through mach 1

- - - Updated - - -

I don't think you're right there - just look at angine efficiency http://en.wikipedia.org/wiki/Scramjet#/media/File:Specific-impulse-kk-20090105.png

I didnt check rapiers after patch, you gave me a hope:)

And quite interesting fact is that space plane can be fueled by liguid fuel only(turbos+909) - greatly reducing mass of plane.

Well,

http://en.wikipedia.org/wiki/Scramjet

it depends on what the maximum is... when etimates vary betwwen mach 12, and mach 24.... that's quite a difference.

Even on the absolute high end, the Isp is much much much worse than the rapiers, the TWR much much much lower, and it still comes up more than 1,000 m/s less than what is needed to acieve orbit.

And no, if can't be LF only with turbos+ 909... it has to be turbos + LV-N, because the 909 needs oxidizer. I doubt you'll get turbos/rapiers and an LV-N to work (assuming no oxidizer for the rapiers)

I have yet to see a nuclear SSTO design that surpasses conventional rockets either for dV in orbit or in payload capacity. You could probably do a really nice "Two stage to Laythe and return" mission though, since I expect RAPIERs to be much more effective there.

Did you not see mine?

There is a big difference between dV in orbit, vs payload capacity.

No SSTO does better than my rapier-LV-N ssto in the payload fraction challenge. Although, admittedly, if I drop the LV-Ns and carry more fuel, I get about the same results.

The difference comes when I don't use it to drop off a cargo... but rather when 37% of the starting weight is still there as fuel in orbit.

That leads to very high dV numbers with LV-Ns. Over double that of a conventional rocket.

For a payload to LKO... the LV-Ns are so weak, they save about the same amount of fuel mas, as their own mass... and are sort of neutral.

[Sattorin]

I still haven't looked at the atmosphere curve, that may favor the turbo at the altitude that you typicaly go through mach 1

Have you been using Kerbal Engineer to compare thrust at speed/altitude? It has a feature which is absolutely perfect for planning out SSTOs. Not able to load the game now to screenshot it, but I encourage you to check it out if you haven't.

[Korizan]

First I haven't built a SSTO yet.

Will I, most likely at some point once I work my way through the science tree.

I hate saying this, but considering the game makers are trying to emulate at least in part RL, that means SSTO should not be easy.

In fact making an SSTO should be one of the hardest things to make in the game.

So if the squad decides to make the game even more realistic then it is now expect the SSTO's to be even harder to make.

[KerikBalm]

The RAPIERs actually lose thrust as they go from Mach 0 to Mach 1. Then thrust skyrockets after you pass Mach 2. If your plane has too much mass, RAPIERs won't be able to push through Mach 1 to reach their ideal speed. I can imaging some very large designs for which it would be more efficient to haul along a few turbojets rather than burning LFO to break that barrier.

Actually, look at the curve, they lose thrust only slightly as you accelerate to mach 0.2... then it continuously increases until mach 3.75

[Sattorin]

Did you not see mine?

There is a big difference between dV in orbit, vs payload capacity.

No SSTO does better than my rapier-LV-N ssto in the payload fraction challenge.

I said a nuclear only design that didn't use conventional rockets (such as RAPIER in closed-cycle mode). Your closed-cycle design is easily one of the best, but I don't think a liquid-only SSTO is going anywhere.

Actually, look at the curve, they lose thrust only slightly as you accelerate to mach 0.2... then it continuously increases until mach 3.75

A lot of my designs were stalling out around Mach 1, which is why I mentioned that number (don't have access to the actual numbers atm). In any case, their TWR is considerably lower than turbojets at low speed, which may make turbojets a good choice in some designs.

[FlipNascar]

But surely the fact that the turbo gives out a) at lower speeds and at lower altitudes, how can it make sense to favour one of those over a Rapier? If you're hitting max TWR with a turbo at around 13km, and max TWR with a Rapier at around 16-17km, surely the fact that you've got more power, at a higher altitude will always be more favourable than petering out much sooner? Even if it requires a reduction in vertical speed, or a short period of diving to push through that drag boundary.

To me, at least, it's all about getting as much thrust as high as possible?

Unless of course, you've got some gargantuan monster hauling up a bucketful of drag and/or payload. Maybe I'm just too darn stubborn, but I've yet to find a reason to replace a Rapier with a turbo. And if I'm struggling to make orbit on account of need moar boaster, surely I'm going to need moar boaster at both ends - airhog and rocketry phases - and therefore I'm again inclined to add a Rapier...

However, adding a Rapier requires more mass in terms of ancillary bits like er fuel. But, at least this is useful mass for the entire ascent. A turbojet becomes dead weight beyond around 15km (yes it will still give power but you're way beyond it's peak and while it's helping, surely its contribution isn't that spectacular?)

Of course, I should probably explore options using real rocket engines too...

[Wanderfound]

I'm about to go to bed, so I can't do it myself right now, but does anyone feel like doing a comparison between a RAPIER/Turbo/RAPIER ship and an identical craft with the central turbo replaced by a third RAPIER?

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Craft file at https://www.dropbox.com/s/ga8orb9xrpxgk21/Kerbodyne%20Triske.craft?dl=0

[eddiew]

Actually I find turbos are useful up to about 19km and rapiers can deliver usable thrust to 24, if you've got some speed going already. Where turbos excel is supporting rapiers to get off the line, since they have much higher low speed power delivery. One turbo can push a 2-4 rapier plane past the mach barrier with ease (often below 10km in a steep ascent), allowing the rapiers to deliver at their full power for longer.

One of the problems people are having with 1.0.2 and SSTOs is that rapiers have a poor power curve at low speeds; twin engine planes can't always deliver enough to get through the mach barrier on their own, but once you can push them to m1.5 or above they become serious monsters. Add a turbo and all of a sudden you've got a dragster.

doesn't work without the turbojet on the aft end. With it, it'll hit a 45km apoapsis on air, so you can save your rockets for much nearer vacuum conditions for better efficiency

[xtoro]

I'm enjoying the new aero, it's more of a challenge. SSTO planes getting to orbit and then visiting several planets were pretty unrealistic, overpowered and ridiculous anyways... Still, I've managed to get almost 6t to minmus, land with ssto plane, deliver and return to land on kerbin. I've now lost interest in ssto planes

I think anyone upset about the change is upset because they mastered an incomplete aero concept, or they're mad because all their hard work on their designs is now garbage and unusable, so now they need to do rockets to do what they did before. Welcome to reality

[eddiew]

I'm about to go to bed, so I can't do it myself right now, but does anyone feel like doing a comparison between a RAPIER/Turbo/RAPIER ship and an identical craft with the central turbo replaced by a third RAPIER?

Got some napkin maths for you - at mach 1 and what I suspect is about 10-12km altitude, two turbojets are worth three rapiers. Rapiers get better as you get higher and faster, but this is why turbojet assisted craft get up to speed better

Made a thread in the support forum a while back to try to draw attention to this weakness: http://forum.kerbalspaceprogram.com/threads/118686-Rapiers-seem-too-weak

Edited May 6, 2015 by eddiew

[Rune]

I'm sorry to throw water at that excellent analysis you guys are making, but intake air is also a factor in engine thrust, and intake air is in itself proportional to altitude and speed (so, in a way, KSP already uses dynamic pressure). The analysis would be multi-dimensional, and I suspect, without a lot of graphs and computer aid, you won't really get a meaningful answer unless one of the factors clearly dominates the others (like intakeArea used to dominate in the souposphere).

And of course, you also have to throw into the analysis the rocket and extra weight that comes installed in a RAPIER, and what that will do to your TWR and mass ratio as a rocket. A handful!

Rune. Of course that shouldn't stop us from trying to figure out trends. Enough experimentation can compensate for a total lack of theory! To the SPH!

Edited May 6, 2015 by Rune

[Sattorin]

I've managed to get almost 6t to minmus, land with ssto plane, deliver and return to land on kerbin.

If you can do that, you can probably ISRU Minmus and do an interplanetary return with what is technically a SSTO!

[Wanderfound]

If you can do that, you can probably ISRU Minmus and do an interplanetary return with what is technically a SSTO!

If you can get 1,500m/s ÃŽâ€V plus a mining rig into LKO, you can go anywhere. And even the big ISRU parts will fit inside a Mk3 cargo bay...

[Kuu Lightwing]

If you can get 1,500m/s ÃŽâ€V plus a mining rig into LKO, you can go anywhere.

Even to Moho?

[Wanderfound]

Even to Moho?

I'm assuming that the 1,500m/s is what's left in your tanks after the climb to LKO; refuel at Minmus and you should have several thousand to play with, plus you can stop for further top-ups at any planet you pass along the way.

[KerikBalm]

I said a nuclear only design that didn't use conventional rockets (such as RAPIER in closed-cycle mode). Your closed-cycle design is easily one of the best, but I don't think a liquid-only SSTO is going anywhere.

Actually, you quoted me saying:

Rapier-Nuclear hybrids are good, because you can go very heavy on the liquid fuel, without worrying about carrying a lot of useless mass in orbit.

ANd you said:

I have yet to see a nuclear SSTO design that surpasses...

There was no mention of "only" when calling it a nuclear design. In fact, I initially interpreted your statements as comparing SSTOs to conventional rockets... Ie the disintegrating totempoles.

That you were quoting me mentioning a nuclear-rapier design which implicitly used rapiers in closed cycle ("very heavy on the liquid fuel" is not "only liquid fuel", and implies that some oxidizer would be carried for the rapiers).

So I thought you were saying that LV-Ns were more weight than they are worth... which for payload purposes, I sort of agree with... because my design gets basically the same fraction to LKO with or without them... Obviously once in orbit though, its very nice to have LV-Ns and mk3 liquid fuel fusalages.

Rune: Yes, intake air also determines it... but if altitude is held constant, as you accelerate from a standstil to mach 1, the rapier's thrust generally stays steady at first, and then increases.

So it is incorrect to say that thrust decreases as you approach mach 1.

Thrust decreases as intake air runs out.

Thrus decreases as you move away from mach 3.75

Thrust decreases as the atmospheric pressure drops.

Thrust does not decrease when approching mach 1 from below mach 1.

[Sattorin]

Actually, you quoted me saying:

Yeah, I misinterpreted you there, sorry!

[Ion Man]

It's actually not thrust, it's heat and altitude/ angle you have to face and work into your concept.

I have a spaceplane SSTO (red tank lifting) with a 29% payload ratio, that stalls at 9k and 280 m/s, if I mess up.

On the other hand I'm able to load the 10 Rapiers to 450 kN of thrust each, making me burn up in the atmo.

When I do everything right, I get up to 350 kN thrust, 1275 m/s at 20k alt max and a nice 80 km orbit, with a full red tank in the cargo bay and enough fuel to land at KSC.

It's more technical and physical now, than it was in 0.9.

[Rune]

Even to Moho?

You can top up at Gilly, that should leave you with 1,500m/s, plus about 1,500 more that you used to get to LKO, leaving you 3km/s to play with at a significantly lower orbit than Kerbin's. Then again, landing on Moho alone from escape velocity will eat around a km/s, so... maaaybe (if you are a god with transfers and nail Moho just in a node)? A flyby for sure, that should be peanuts.

Rune. Then again, with multiple gravity assists (which I can't plan for the life of me) everything is possible, you could use Moho to circularize at Moho's orbit, and yes I know that sounds wrong. Think what Messenger did.

Edited May 6, 2015 by Rune