SSTOs - What am I doing wrong?

[yorshee]

I've been trying to make an SSTO but I just can't seem to do it. I've tried making small, fast ones to large, fuel-heavy ones, but no matter what I do, the same thing happens.

I tend to use Turbojets and Nervas and I have seen people make SSTOs with just these two types of engines, but whenever I try it, I get to around 20km (apoapsis is around 35km) and then the turbojets run out of air so I switch to the nuclear engines, but the TWR is so bad that I get to my apoapsis of around 35km and start to fall again.

I have tried using RAPIERS but I don't like how you have to carry oxidizer to fuel them - it weighs you down a lot and I find that it chews through oxidizer like no tomorrow. I never carry enough oxidizer to get to 70km, and if I do decide to make my entire spaceplane out of rocket fuselages then it barely has enough fuel to do anything - I now have to halve the amount of liquid fuel I had to make room for the oxidizer.

I have only managed to make one successful SSTO, and even then, it barely even had the fuel to deorbit itself and land - there's no way I could even flyby the Mun with that.

Does anyone have any tips? D:

[Slugy]

You need to get both your speed and ascent path right with any ssto - even more so with a LF only one. Check the thrust on the engines as you fly - if they never get up to 400+ you probably need to try again. Climbing to14-17km and then diving back to 10km to build speed and thrust may be worth a try.

Remember nukes are heavy - maybe try 2 jets to one nuke? Personally I think the Rapiers are great: they fill the gap between jets not working and the nuke not having enough thrust to keep you going to orbit.

[KerikBalm]

So far, I haven't had any pure-liquid fuel SSTOs... I accept that I'll be gulping oxidizer as I get the last 1,000 m/s needed for orbit.

If I recal correctly, people couldn't get anywhere near the payload fraction of oxidizer using SSTOs... but they were built so that one could refuel them after they were in orbit, and have no wasted space for oxidizer... just a lot of liquif fuel, and good range on the nukes.

They're probably much more viable on laythe due to the lower gravity and velocity needed for orbit

If you're not getting over 1,200m/s at 20km before switching over, my advice is to change your ascent profile, or use more turbos/rapiers.

It sounds like you've got the right conditions (apo of about 35, at 20km).

I haven't had a problem carrying enough oxidizer.

Sandbox heavy lifter: 33% payload capacity demonstrated, based on leftover fuel >37% possible

Note the 3 nukes at the end of the payload (can be redocked to the front of the cargo space after payload separation)

sandbox mk3 lifter, no nukes at all:

My current(ish) career medium lifter:

That is docking with a Mun station, to deliver the cargo (a lander + RCS supply + battery packs and solar arrays because the lab didn't have enough power).

No, it did not refuel from the depot, but actually made a deposit of LF, and a lot of oxidizer.

IIRC, it had 8 turbojets, 3x LV-T45s, and 2 nukes.

So far, my problem has been too much oxidizer. I modified that design to incorporate 2 more long mk2 liquid fuel fusalages (removed 2x mk1s)

Edited May 12, 2015 by KerikBalm

[AaronLS]

If you're not getting over 1,200m/s at 20km before switching over, my advice is to change your ascent profile, or use more turbos/rapiers.

It sounds like you've got the right conditions (apo of about 35, at 20km).

Yep, and make sure the lift and/or thrust from offset turbo jets keeps your prograde lined up with your angle of attack. If you are eskew of prograde, you won't accelerate as much. I have my stabilizer locked to prograde after 10km because that's when acceleration is important. But if your plane's weight and lift isn't balanced well, then a prograde lock will either tip your too far forward or back.

For me, my final angle at around 18km is usually around 20 degrees. Too low of an angle and you'll be fighting the atmosphere for too long after switching to rockets.

I usually hold a steep angle until around 10km altitude, then angle down to 25 degrees and set stabilizer to prograde. The way my plane is weighted the prograde will drop very slowly and I'll be around 20 degrees at 18km.

Between about 10km altitude and 18km is the crucial time that you need to accelerate quickly.

Tips:

- Close air intakes to reduce drag once your jets shutdown.

- Inline airtakes won't blow up as easily as external or front facing intakes. You only need one or two nacels per turbo jet.

- Nose cones make a huge difference to reducing drag. A very sharp protective nose cone is good balance between drag and resistance to heat. I personally use a reversed tail as a nose cone because it has less drag but they aren't as heat resistance, so if my plane is going to exceed 1200 m/s, I use a sharp protective nose cone shell which can survive 1400 m/s for a short period. If you are reaching top speeds easily though, then a lightler nose cone like an Advanced Nose Cone is probably a better choice.

- Reaching very high speeds too soon will overheat parts prematurely, making them already hot and less able to stand the heat when you finally reach top speed.

- If no matter what you do, you aren't reaching 1200 m/s, then you've got a fever, and the only cure is more ram jets. Either that or you need to reduce drag, your flight profile is too far off prograde because of your lift/mass balance, or angle is too high and your passing threw the critical acceleration altitudes(10km-18km) too quickly before reaching top speed.

- Mine tend to fly more like rockets than planes. Even though they have landing gears, I launch them at an angle from a mount. You don't need much wing surface, because at a very high speed you'll get plenty of lift. I *think* wings like delta's and swept wings generate less drag at high speed as well.

- Less wings means less weight to push when you switch to rockets.

- I use probodobodyne controllers instead of cockpits because they are so much lighter. I mount them inside my cargoe bay on the opposite side of the payload. Sometimes they absorb heat from the external parts and overheat easily, so I put a girder between them.

I've put a payload of a lab and cupola, which is 5.3t payload I think, with 5 ram jets and a poodle. But it took alot of tweaking and practice launching though. The plane pictured below is alot easier to get into orbit.

You want to practice your ascent so that your angle limits your early acceleration from reaching a point where your parts start over heating soon. If you're heating your parts up at 8km in thicker atmosphere, then they aren't going to be able to absorb enough heat to stand the top speeds in the 12km+ range. Keep them cool until you get above 10km and start pushing for your top speed.

IMO, having a ton of air intakes isn't worth the drag. After 18km the atmosphere starts thinning out extremely fast and you'd need to exponentially increase the number of air intakes. 20 or 30 ram air intakes won't help much. My SSTOs usually have one or two nacels per ram jet.

This is one I use for small payloads. I only added the wings as an afterthought to allow landing. It gets into orbit fine without them.

Edited May 13, 2015 by AaronLS

[Wanderfound]

A nuke spaceplane is a specialised thing, and considerably more difficult to get right than an LFO boosted one. Walk before you run.