SSTO Fuel Issues

[kodack]

That said, I've never had luck with long-range SSTO unless it involved orbital refueling. In my experience getting anything with that much fuel up high enough requires more than 1 or 2 jet engines, or it lacks the TWR and lift to make it up off the runway. More jet engines means more problems with fuel consumption, air consumption, and flame-outs, not to mention more weight to lug around once you get into space. You can give it more wing, but that also increases dead weight in space. It's a very difficult balance to strike. Does anyone have any tips for building really big spaceplanes in an efficient manner?

The devil is in the details. For instance when you need more lift you jump to giving it more wing. Why not replace the delta with a higher lift wing so you get more lift without more weight?

When I'm not building large, beastly, over the top, vehicles my motto is less is more. Try to make every part serve more than one purpose. Instead of having a fuel tank, and a fuselage, combine them. Instead of adding more control surfaces to handle a shifting CG as tanks empty, reposition the tanks so the CG works for you. Instead of adding more jet engines to get it off the runway, consider changing your center of lift to help get the nose up. In extreme cases, consider a vertical takeoff. It's still SSTO and a spaceplane if it launches vertically so long as it can land horizontally.

[m1xte]

How did you spam the intakes like that?

[HoY]

turn on part clipping with alt+f12, then in 4 way symmetry grab an intake from the menu and I used shift+q/e to rotate them to the desired position. I was going to go for a completely circular intake but I got bored after pressing shift q 300 times for the first 27 intakes.... but as has been mentioned a couple times already in this thread, theres a good chance this method will get nerfed (technically its balanced) in a coming update.

Edited May 14, 2013 by HoY

[HoY]

The only problem with this ship is that it's 500 parts.. And it continues to be 500 arts throughout its entire mission lol. A 500 part on the pad usually stages down to 100-150 on mission. This thing just keeps onnnnnnnnn lagging. 30 mins in game time to get to a 90km orbit, but with delta set to lowest it's actually been probably double that.

[Scialytic]

That said, I've never had luck with long-range SSTO unless it involved orbital refueling. In my experience getting anything with that much fuel up high enough requires more than 1 or 2 jet engines, or it lacks the TWR and lift to make it up off the runway. More jet engines means more problems with fuel consumption, air consumption, and flame-outs, not to mention more weight to lug around once you get into space. You can give it more wing, but that also increases dead weight in space. It's a very difficult balance to strike. Does anyone have any tips for building really big spaceplanes in an efficient manner?

Spaceplanes are particularly tough to give guidelines for, even more so than rockets, because people's subjective preferences about part count, handling, and aesthetics play a bigger role. One person's ideal is another person's rubbish, and big SSTOs will force you to make hard choices. That said, here are some tips based on my experience building interplanetary SSTOs:

1. Iteration is very important. Don't try to get everything right all at once. Test and refine. Test and refine.

2. You want just barely enough turbojet engines + wings to takeoff and ascend using only jet power. In my experience, this typically means 4 turbojets for a Mk1 or Mk2 cockpit-based design and 5 or 6 turbojets for a Mk3 cockpit-based design. So basically, once you have settled on a number of turbojets, add wings and canards as needed until it just barely flies. It's hard to describe, but the handling should start to feel good at around 10 to 15 km altitude.

3. I usually end up with designs that takeoff by running off the end of the runway. This is mainly a symptom of the center of lift being quite a bit behind the center of mass. My reasoning is that takeoff is much easier than landing, so I design such that the handling will improve as fuel burns and the CoM moves rearward. When you think you've got it right, test landing from orbit on empty tanks.

4. Use only ram air intakes. Start with about 5 ram air intakes per turbojet, and add more as needed. I usually end up closer to 10 per engine, but it varies. This is generally the most grueling part of testing for me. You want just enough intakes to get you above 2100 m/s using only turbojets. Given your low TWR, this requires you to be quite high (typically well over 30 km, otherwise your acceleration is too slow and you waste a lot of fuel). Finding the right ascent path can get quite time-consuming, but it can yield really impressive results.

5. In almost all cases, a pair of LV-N engines should serve as your main rocket engines. I like to supplement them with a pair of aerospikes, primarily for takeoff on Duna. Plus it's nice to have the option of some extra kick when you want it. During ascent, I usually run the aerospikes for a few seconds when transitioning from jet to rocket power. Some people really seem to like the Rockomax 24-77s, but for heavy SSTOs I don't (they are great on small vehicles though). Keeping the part count down and maximizing Isp are your top priorities.

6. I've seen people really overdo the landing gear. 3 is all you need on most designs; maybe 4 or 5 if it's really heavy.

7. Make sure you have the right ratio of liquid fuel to oxidizer. You'll be surprised how much fuel 4 turbojets burn though, especially if you're visiting Laythe. Also, a little excess fuel makes landing on the runway at KSC easier when you return; excess oxidizer is completely useless.

I hope that helps. Good luck.

[bsalis]

You will need to make the most of your jet assisted boost -snip-

The problem isn't enough fuel to get Orbit. The problem is getting to the Mun, landing, and to orbit again, via refueling in LKO.

The single LV-N should also be able to land you on the Mun if you don't carry too much else. If your craft is too heavy, you can top off the nuke with some 24-77s for the landing and takeoff.

Totally agree. I suggested the exact same thing in my reply.

Do people actually ready posts!!!

Holy hell that's goddamn impressive. .craft file? I'd really love to get a chance to pick at that ship.

Thanks. I have been Spaceplane-ing for a while. I put it on the exchange for you - see my sig for link.

I usually end up with designs that takeoff by running off the end of the runway. This is mainly a symptom of the center of lift being quite a bit behind the center of mass.

More control surfaces should fix that. Or having them further away from the CoM. Or just having your wet and dry CoM closer together. Admittedly running out of runway is no biggie in my books.

[Queue]

Do people actually ready posts!!!

I always like and enjoy reading your space plane tips. Being a plane nut myself i learn alot from fellow space planers. Btw i love your avatar!

[Colonel_Panic]

Thanks. I have been Spaceplane-ing for a while. I put it on the exchange for you - see my sig for link.

Very nice, thanks!