Help me tune my SSTO

[KatzOhki]

Hey guys, so I've got a SSTO design and it almost works. Actually I did get it into orbit once, but the problem is that I can just barely reach a maybe 75x75 orbit. I want to be able to reach a higher orbit so I can dock at some of my stations. I have a lower one at 100x100, but I have some that are as high as 300ish orbit.

As it is the design has four intakes (a couple are clipped over eachother). I didn't want to do it that way, but it's all I could do to get it to work. I'm not sure which last design worked, but it's probably somewhere around E, F or G. Here are some pics of the variations:

And here are the craft files:

https://sites.google.com/site/katzcospace/downloads

Any help is appreciated

[doggie015]

Replace the rocket engines with nuclear engines. On something that small it should not make that much difference. Also: Try to fly a shallower ascent profile. Take it up to 15-20 KM and build up horizontal velocity. Once it reaches 1500 m/s or so pitch up and get your apoapsis to 100-150 km or so. Try to use your jet for as much of the ascent as you can, the atmosphere is your friend here!

Edited October 14, 2013 by doggie015

[Galacticruler]

move the front RCS firther forward, adds more control.

[Captain Sierra]

Single LV-T30 oughta do the job. Unlike Doggie, I do not advertise irradiating the atmosphere, as Squad may later add penalties for that.

Go with two jet engines. YOu can get higher faster and crank out more speed. I suggest an intake-engine ratio of 3-1. THat usually works.

You have way too much monopropellant and a little too much rocket fuel. Replace the center rocket fuel tank and 1 monoprop tank with a jet fuel fuselage (one is more than enough for two jet engines).

[Octagon]

Single LV-T30 oughta do the job. Unlike Doggie, I do not advertise irradiating the atmosphere, as Squad may later add penalties for that.

Go with two jet engines. YOu can get higher faster and crank out more speed. I suggest an intake-engine ratio of 3-1. THat usually works.

You have way too much monopropellant and a little too much rocket fuel. Replace the center rocket fuel tank and 1 monoprop tank with a jet fuel fuselage (one is more than enough for two jet engines).

I was doing some experiments while you typed this, I guess we thought almost the same:

Based on Vikingshelm Rev G

Edited October 14, 2013 by Octagon

[KatzOhki]

Hey Octagon, that looks pretty good. Can I ask what the SAS and ASAS modules are for? Also, what orbit did you get it to?

[Dispatcher]

As you can see, everyone has their own "system" or preferences. Another thing you might want to do is replace rocket engine(s) (not sure if you will keep two or go with one); use an aerospike instead of the 909. It drinks fuel a little faster; but its thrust is between 3 & 4 times that of the 909, and it puts out almost another 100 iSP in atmosphere. That combined with turbos is a fair combination.

@ Octagon: I can understand the use of a battery. Is the SAS for use as a "spacer" or a counterweight? If so, that's why I use them.

Edited October 14, 2013 by Dispatcher
left out something

[arq]

Do not use LV-Ns or aerospikes, as some are suggesting for ascent. At 20km you have 98% of vacuum ISP, so the aerospike is wasted (and even above 10km they lose most of their usefulness - really they're only useful for first stages or Eve launchers). The LV-T30 gets much higher thrust for less weight, for only a small ISP penalty. THe LV-N is *very* heavy and as such has *abysmal* TWR. It is only viable for ascent with extensive airhogging (you need to be around 40km for drag to be low enough for it to work well), but for a craft this small even then it is heavier than it is worth. Only use LV-Ns if you plan to go to Mun or further.

Lose some RCS, you only need a roundified tank or two here, at most. Also, lose the parachutes. Add slightly bigger wings, if necessary. Parachutes are deceptively heavy and one of the biggest advantages of planes is the ability to do horizontal landings.

[KatzOhki]

Seems like the overall suggestion is to go with two jet engines and to do some weight reduction. Thanks guys ill work on that.

Also, saw my thread got moved. Sorry about that

[RoboRay]

Do not use LV-Ns or aerospikes, as some are suggesting for ascent. At 20km you have 98% of vacuum ISP, so the aerospike is wasted (and even above 10km they lose most of their usefulness - really they're only useful for first stages or Eve launchers). The LV-T30 gets much higher thrust for less weight, for only a small ISP penalty. THe LV-N is *very* heavy and as such has *abysmal* TWR. It is only viable for ascent with extensive airhogging (you need to be around 40km for drag to be low enough for it to work well), but for a craft this small even then it is heavier than it is worth. Only use LV-Ns if you plan to go to Mun or further.

Lose some RCS, you only need a roundified tank or two here, at most. Also, lose the parachutes. Add slightly bigger wings, if necessary. Parachutes are deceptively heavy and one of the biggest advantages of planes is the ability to do horizontal landings.

OP, listen to this guy. He knows what he's talking about.

The key to improving a spacecraft's performance is reducing mass, not adding more. This is even more true for SSTO designs which, by definition, require harsh compromises on performance and capability. Replacing very lightweight but capable engines with far heavier ones for even double the fuel efficiency simply isn't a good solution. You may not be firing the rocket low enough that the aerospike would be of any benefit, and the LV-N is so massive that you'll expend about as much fuel lifting it as it will save you on efficiency. And the extra weight cuts into the effectiveness of your wing lift.

I still think you'd benefit from more wing-area, though. That, and getting rid of the parachutes should help.

Edited October 14, 2013 by RoboRay

[arq]

The number of jets to use can be a tricky number. Essentially, you just need enough to reach 20km relatively quickly. Once you hit somewhere between 22 and 25km, you need to start throttling back and max thrust is no longer the limit. Here, you enter the 'air-limited' region. Thrust is no longer limited by engines, but rather air intake. At this point, extra engines actually slow you down (extra mass and drag) and add no thrust.

[Dispatcher]

OP, listen to this guy. He knows what he's talking about.

(RoboRay is referring to a post by arq, "Do not use ... aerospikes, as some are suggesting for ascent."

I suppose it depends upon your own designs and experience. From simple testing with my plane design, my altitude with the same amount of fuel is between 1 & 2 times (not 2 & 3 times as I posted prior to editing) the altitude gain using the LVT 30 when I use the aerospike. Yes, there are short term trade offs between engine and fuel tank mass (which is persistent), and fuel & oxidizer mass, which translates into propulsion. As to its short length and better runway clearance, that was just a bonus. Other people have succeeded using the spike. Here is an example:

Your mileages may vary.

Edited October 14, 2013 by Dispatcher
Correcting 2&3 to 1&2 times altitude.

[lammatt]

for planes, the lighter the better

- you have way too much RCS fuel

- and parachute is kind of meaningless for a plane this small

- 4 intakes for 1 turbojet are too many

[Octagon]

Originally Posted by KatzOhki

Hey Octagon, that looks pretty good. Can I ask what the SAS and ASAS modules are for? Also, what orbit did you get it to?

Originally Posted by Dispatcher

@ Octagon: I can understand the use of a battery. Is the SAS for use as a "spacer" or a counterweight? If so, that's why I use them.

In fact I combined "business with pleasure", my intention was to gain some control, make the plane longer and to counter balance at same time. In battery's case, I used it as backup in case o the plane need to maneuver, note that there are no solar panels. Of course this is just a experiment, you can see an example of a serious plane of mine here: http://forum.kerbalspaceprogram.com/threads/52561-Vulcano-Mk5

Vulcano Mk5:

[RoboRay]

I suppose it depends upon your own designs and experience. Other people have succeeded using the spike.

Oh, certainly. I didn't mean to imply that the aerospike is a wrong choice... it's simply not often the best choice. I've built good SSTOs using the aerospike myself. However, the spike's one big benefit, the entire point about which it was designed, is that it offers consistent performance regardless of atmospheric pressure (or, the total lack of it). The aerospike's reason to exist is as core engines that will be used continuously from the surface into space. They're still good engines if used in a narrower niche, but you're not taking complete advantage of their capabilities over the full operating-time of your craft while enduring their disadvantage (relatively high mass) for the entire time.

If you're designing a craft that must light the rockets at lower altitude (limited wing area, few air intakes, or for whatever reason), the aerospike becomes a better choice than if you're building a craft that can get extremely high and fast just on jets.

Edited October 14, 2013 by RoboRay

[Dispatcher]

RoboRay, we can agree that each tool has its use. Yes, my current design requires the spike to burn in low atmosphere. Its also interesting how the tandem use of various kinds of rockets (as well as with jets) can change the flight dynamics. We can agree also that there is no "one size fits all" engine. If that were the case, this thread wouldn't even exist.

Now for the OP: you'll find that if you use an LVT engine, the 30 (mentioned by others above) has some preferred characteristics over the 45. Its lack of vectoring means its less massive and has a bit more thrust (and thus a better TWR). You can do a lot with your RCS which would often be done with a craft possessing a vectoring engine. Good luck on whatever engine combination you decide upon!

Edit: That's a nice looking plane, Octagon.

Edited October 14, 2013 by Dispatcher
Vulcan MK5

[KatzOhki]

I have to admit, the LVT-30 was not an engine I had considered

[RoboRay]

Honestly, the little orange Rockomax engines are my favorites. They don't have a great specific impulse, but their mass is so low and their thrust so high that they are extremely well-suited to SSTO craft.

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Two seats, more than 600m/sec excess ÃŽâ€v for maneuvering around LKO, and docking capability, in just 13 tons total mass on the runway. All stock.

Edited October 14, 2013 by RoboRay

[Starglider]

Parachutes are nice for SSTOs imo, but only if you're going to decouple the landing gear after takeoff. Seriously - each landing gear weighs 0.5 tons, for a total weight (assuming 3 wheels) of 1.5 tons, while the parachutes you'd need to float the plane to the ground (and the decouplers required to drop the wheels) weigh much less. I have a personal distaste for doing this (I like to do proper landings) but on my early spaceplanes I couldn't argue with the weight savings.

As for rockets, I like the Rockomax radials. They're very light, and their low thrust values are compensated for by no drag, so the thrust adds up over time (and once you're in space, you have time).

Typically, one mk1 fuselage tank is sufficient for each jet engine, but for multiple engines, you may wish to look at the mk2 and mk3 fuselages, which pack more fuel for their mass.

Also, RCS thruster blocks and RCS fuel really do add up. Your design is obviously meant for docking, so I can't argue with their inclusion, but you should minimize the RCS fuel, and optimize your thruster block placement. Each thruster block adds weight, and you don't need many given the low net weight you're going for. If you can, put them straight on the center of mass (well, where you expect the center of mass to be, once you reach orbit); you lose some rotational ability this way but you have SAS for that and you retain translational ability while cutting down weight. Remember you're having to lift all that stuff into orbit; it doesn't contribute anything at all to getting you out of atmo. Take a close look at the weight of each of these parts, and ask "how much do I really need?"

The constant consideration here, can be summed up as "weight is not your friend". More weight means you need more lift. More lift means more wings, but that also means more drag (lift being created from drag). There is a balance between too much wing (which impacts your top speed) and too little wing (which impacts your top altitude). Too much weight disrupts that balance. Adding engines often just makes the problem worse (another engine needs another tank and set of intakes; the heavier craft needs more wings; more wings means it flies slower); the only gains are when the mission your spaceplane is designed for requires it (I've seen a spaceplane deploy full orange tanks to orbit; it had lots of jet engines, and deserved every one).

Another consideration is jet engine flameout. Even numbers of jet engines can be problematic because when an engine flames out, all of the sudden you have unbalanced thrust. This can be a problem too with odd numbers of jet engines, although if you add the center engine last it seems to flameout first, which gives you warning. Single jet engine spaceplanes seem to be the easiest to start out with, as you avoid this issue entirely.

[Guest]

While it seems like it in the VAB, landing gear does not have weight or drag. You can test this yourself by placing them in odd places on a balanced craft and flying it. The same goes with struts. Just remember always balance your plane BEFORE putting landing gear on it as they show a faulty COM

[Dispatcher]

Honestly, the little orange Rockomax engines are my favorites. They don't have a great specific impulse, but their mass is so low and their thrust so high that they are extremely well-suited to SSTO craft.

The Radial 24s are not the most impressive things by look and sound, but you are right. Three of them alone can dead lift nearly 6 tons (falling between an LV-N and a Turbojet engine) of fuel, and convert that to thrust sufficient to to reach an altitude of over 1.2 million meters.

[KatzOhki]

I tried the newer tiny orange engines, but that didn't work for me. I did only have two though.

[Octagon]

Edit: That's a nice looking plane, Octagon.

Thank you Dispatcher.

I did a new experiment:

Wings are outdated ! This "airplane"(SSTO) is at a 160X160Km orbit with half of it's fuel to expend. I managed to make the jet engine work at +60Km with all that air-intakes. I guess it is on of smallest things(SSTO) that have ever gone to orbit.

Edited October 15, 2013 by Octagon

[KatzOhki]

I'm happy to report that the 8th iteration of the Vikingshelm spaceplanes made it to space last night! So that's the good news. The bad news is that there was a strong left handed torque when firing the rocket thruster in space. This effect was strong enough to overcome automatic SAS correction and caused the waste of a lot of fuel. Could this be caused by the ladder that is on one side?

The changes were to use two jets and the lvt-30 and change some fuel tanks.

[numerobis]

However, the spike's one big benefit, the entire point about which it was designed, is that it offers consistent performance regardless of atmospheric pressure (or, the total lack of it).

The other benefit of the aerospike is its low drag. Lower drag means more speed off jets, which means more air intake at a given altitude, which means a higher service ceiling off jets. Nevertheless, the 48-7S and 24-77 have so much higher TWR that even on drag terms it's a wash between them and the aerospike. And they're more versatile, since you probably don't need much rocket thrust.

The number of jets you use really only affects your low-altitude performance. As you get up high, all that matters is the number of intakes. 1 jet with 6 intakes has a higher top speed than 2 jets with 6 intakes, because the latter is heavier. You just have to make sure you can get to high altitude.