Heavy SSTO help

[LordFjord]

Heya,

I need some help to get this turkey chicken bird into orbit.

And this time efficiently, without the brute force approaches via MOOOAAAAR BOOSTERS like I did before.

I have followed some advice by O-Doc from here, but I am still stuggling.

The goal:

Get 40 tons into orbit and return to KSC in order to refuel and attach a new module - all in the spirit of the affordable space program challenge.

The cargo is attached with a docking port sr. to the hull and a docking port to the top. It's a bit wobbly but I don't want to strut it (due to the challenge) - but this isn't the issue.

The plane itself flies well and stable, can keep a direction and pitch, can take off maybe at halfway down the runway (thanks O-Doc for the plain jets hint, these really help a lot down there). It can survive a landing with the cargo (did this after a failed attempt).

The problems:

Gaining speed at high altitude, the use of nukes on heavy SSTOs and transition to space.

Some stats:

Total mass on runway: 138 tons

4 jets, 10 turbojets, 6 nukes

total ~40 intakes

part count 400something

Here are some notes I did during my latest test flight:

Altitude m      Pitch °       Speed m/s
17000           26            495
18000           28            605
19000           29            677
20000           30            736
21000           31            800
22000           32            930  <-- turned off the 4 jets here
24000           33           1000  <-- throttling down slowly
29000           35           1200  <-- turned on the 6 nukes, throttle is at 1/3rd
33800           38           1300  <-- flameout on jets, KER shows ~2.5km/s dV left

The problem is, that the nukes cannot carry it to space, it lost speed, then lost altitude.

I guess I could slap 2 orange tanks on the wings with a skipper and 4 turbojets each and bruteforce my way to space, but I have seen some other heavy SSTO planes that could do it on nukes alone. This is what I'd like to learn - and this is the opener for heavy SSTO interstellar flight as well.

Some pics of the craft during various attempts.

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Here is my latest version of the craft, it requires the station science mod (payload) and Kerbal Engineer Redux:

https://www.dropbox.com/sh/uoy3uv1qw5e0zrk/Qrn78V50Eq/SSTO%20Cargo%2040%20MK%204.craft

I'll make a mod-free version of the craft later with an orange tank, which is similar in size and weight, sorry that I havent thought about this earlier.

Any help and advise is appretiated. I have tried to build something that also looks more or less appealing (at least to me).

Fjord

edit: thanks folks, I got the bird up there.

Edited April 17, 2014 by LordFjord

[O-Doc]

I'll wait for your stock version as I don't run mods yet.

One tip would be to run your turbos as high and fast as possible without throttling down. Then turn your nukes on and get as high and fast as possible before you have to throttle those down together.

If you are still not breaking through then try the dipper maneuver once your at your highest speed on full throttle.

[Mesklin]

Fjord, I will test your craft tomorrow and then try to advise something . I hope

P.S. Can you give me a link to the station mod what you are using in payload?

Edited April 17, 2014 by Mesklin

[KerikBalm]

Well, all my designs use struts, not just two docking ports (how do you get them both to attach? I've wondered, I've seen other people do it).

But .. your payload bay, formed by wing connectors... do not the downward sloping wings simply create drag and decrease net lift? Could you not get to a higher speed without them?

If you're loosing altitude after firing your rockets, your rockets don't have enough thrust, its that simple.

In my designs I don't use nukes because RP reasons, and 2, low thrust to weight ratio, I just lift more fuel in the lifter. I use LvT 30s, 45s, and aerospikes (the aerospikes are simply for higher ISP with more TWR than lv 909s - at the altitude they fire, its basically vacuum ISP), because when the rockets fire, they need to take over all the thrust of the Jets, and add to it. Nukes are too low TWR for me (at least if I'm carrying a heavy payload).

I'm now making designs with the liquid fuel boosters (better weight distribution, better TWR, still decent ISP)

My first SSTO lifter - 40 tons or so... had problems on reentry if it didn't have enough fuel left over, and the intakes were opened too soon (too much drag up front, not enough mass)

https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-ash3/t31.0-8/p600x600/1957798_10102506321827793_636161842_o.jpg

https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-frc3/t31.0-8/p600x600/1891452_10102506322172103_1122623981_o.jpg

My heavier, more re-entry stable lifter (double the payload capacity)

https://scontent-b.xx.fbcdn.net/hphotos-frc3/t31.0-8/p600x600/1781709_10102522555220943_38363739_o.jpg

https://scontent-a.xx.fbcdn.net/hphotos-prn1/t31.0-8/p600x600/1277967_10102522555340703_937978531_o.jpg

https://scontent-b.xx.fbcdn.net/hphotos-ash3/t31.0-8/p600x600/1961831_10102522555101183_621013543_o.jpg

Also note, no part clipping, not really intake spamming, and stock (I modded the intakes to also produce intake atmosphere for use with mod electric fans, but those weren't used, and the intake atmosphere modules were closed)

[LordFjord]

Fjord, I will test your craft tomorrow and then try to advise something . I hope

P.S. Can you give me a link to the station mod what you are using in payload?

here you go.

Thanks for the help folks.

I guess 6 nukes just don't cut it and that I'll need more thrust. But a different engine configuration will most likely need a lot more fuel, then I'll need more turbojets and more lift as well -> I'm spinning in circles here and might end up redesigning the whole thing.

Alternatively I give it a try with lighter payloads.

Fellow314 posted a nice heavy SSTO here. It seems to have a very good payload fraction, will take a closer look at it later.

[O-Doc]

Based on your engine setup I'd say you're carrying way too much fuel. My rule of thumb for lifters is for each turbo jet(basics are ignored) carry 150-200U(1T) LF as your baseline for a standard 10min to complete burnout flight. With nukes, 32T of rocket fuel is far too much. Start with 2-3T of rocket fuel per nuke and adjust accordingly. So, you can lose about 25T or fuel straight up.

You'll find Fellow314's designs are air-hoggers. He likes to clip intakes so there are far more than it appears.

[KerikBalm]

Well, I did some calculations... compare the LV-T30 vs the LV-N

Thrust: Mass

215:1.25 vs 60:2.25

For 1 LV-T30, you need 3.5833... LV-Ns to equal the same thrust. Those LV-Ns will weigh 8.0625 tons, while the LV-T30 weighs 1.25 tons -> 6.8125 tons lighter

For every 3 LV-Ns, you can replace them with 1 LV-T30 and 5.5 tons of fuel, and have nearly 20% more thrust.

Your 6 LV-Ns weigh 13.5 tons, and have a thrust of 360 kN

A single pair of LV-T30s weighs 2.5 tons and gives you 430 kN,

and you can throw on 11 tons of fuel - ie nearly 2.5 FL-T800 tanks -> which for sufficiently small delta-V amounts means that your dV/payload is actually higher because you carry more than enough fuel to compensate for lower ISP.

So, switching for a pair of LV-T30s gets you more thrust, and you can carry more fuel.

You should only need about 1000 m/s of dV when you turn on your rockets.

Or you could do pairs of LV-T30s, and a pair of LV-Ns.

Use the LV-T30s to raise your apopasis into a relatively steep climb, then once climb rate is established, shut them off and use LV-Ns to circularize.

Ie, the LV-T30s are your boosters to fire when jet thrust is too low to continue climbing.

Also, I don't run jets and rockets at the same time at reduced throttle.

if I fire my rockets, I want them going full throttle, so if I have many jet engines, I may shut off most of them and go full throttle on the remaining ones, so that when my rockets do fire, they fire at full thrust to boost me up out of the atmosphere (or I don't bother to try and make use of residual intake air, and just shut engines down, close intakes, and then go full throttle).

Of course, my larger designs now use the liquid fuel boosters.... the TWR ratio on those is great (assuming you subtract the equivalent mass of fuel tanks), and the ISP of 360 isn't bad either.

Note that my 40 ton payload to orbit design had 4x aerospikes, and 4x rapiers, so 8 x 175 kN of thrust = 1400 kN of thrust -> 700 kN at 360 ISP, 700 kN at 390 ISP

You'd need over 23 nukes (23.33) to get that much thrust, and you'd take those nukes to orbit.

23*2.25 = 51 tons of nuclear rocket engine to orbit compared to my 7 tons of engines to orbit...

Getting the same mass to orbit, I'd have over 44 tons more of that mass as payload.... granted, I'd use more fuel, but I still find nukes to simply be too heavy for my SSTO lifter designs.

Maybe with more intake spamming and part clipping to get a design that flies on jetsat 40km and closer to 2,000 m/s, nukes are fine, but for my non spamming designs which normally top out at under 30km and under 1,400 m/s, nukes just don't cut it

Edited April 17, 2014 by KerikBalm

[LordFjord]

I tried a different engine setup, here is the result:

I used 2 jets, 12 turbojets, 4 LV-T30 and 2 Nukes. Same fuel setup (all tanks full).

I had some excess jetfuel left, but ran outta oxidizer, so I had to push the last bit with RCS into orbit. A big step up from last attempt.

I think KerikBalm summed it up nicely, so you have either:

- excessive intake spam in order to reach very high speeds in high atmosphere, so the nukes are sufficient to carry it to orbit

- some rocket engines with more thrust (and less weight) to boost the plane from high athmosphere, but only "high" speed into orbit.

Here is a stock craft file, an orange tank as payload. It will some more refinement but I think I have the basics covered.

https://www.dropbox.com/s/w6tv07q1aqdag7d/SSTO%20Cargo%2040%20MK%205%20%28stock%29.craft

Note: it aint finished and barely reaches orbit. Launch has to be done carefully, steering causes the payload to wobble, which results in certain crashes. I messed up some fuel line so I had to pump fuel around. But thats nothing that cant be fixed.

I will fiddle around some more with engines, intakes, fuel. Also need some work on the landing gear in order to stabilize it and get rid of my central row of intakes behind the cockpit. After loading the file the Kraken disapproved too narrow intake spam and bursted it from inside.

I will throw up the finished and tested plane on the spacecraft exchange when I am happy with the result. 29% payload fraction aint a bad start

Btw, I have tried Fellow314's plane. He uses 4 jets, 10 turbojets, 4 nukes and an unholy number of intakes. I just couldnt get it off the runway, 5 crashes in row, always the gear crumbling under the weight and then some part colliding into the runway.

I dont doubt that it can get into orbit and its payload fraction is very high, it just might not be a very reliable design

All in all:

thanks a lot folks, this helped me a lot.

Fjord

Edited April 17, 2014 by LordFjord

[gchristopher]

Nice, LordFjord! I'd been trying to figure out the same thing, and run into roughly the same analysis before I ran out of attention span and wandered off.

I glanced at Fellow314's craft. 104 ram intakes and 6 circular intakes. My understanding of that approach is that you get to a high enough velocity, then creep upward in altitude until apoapsis gets up out of the atmosphere, then fire nukes to raise PE. Seems like once you get to high altitude, thrust/drag is the dominant factor, and intakes are all that matter for thrust?

Have you tried a claw for payload holding instead of the large docking port?

[LordFjord]

Nice, LordFjord! I'd been trying to figure out the same thing, and run into roughly the same analysis before I ran out of attention span and wandered off.

I glanced at Fellow314's craft. 104 ram intakes and 6 circular intakes. My understanding of that approach is that you get to a high enough velocity, then creep upward in altitude until apoapsis gets up out of the atmosphere, then fire nukes to raise PE. Seems like once you get to high altitude, thrust/drag is the dominant factor, and intakes are all that matter for thrust?

Have you tried a claw for payload holding instead of the large docking port?

Hmm, havent thought of the claw - good idea

As a small relief from heavy SSTOs, I just threw together a small one (25 tons). 3 turbojets (2 and 1 each in an actiongroup) and a nuke. 12 intakes. 3k dV left once in orbit and fully science equipped.

Wanted to practice the nuke-ascent. Instead of really massive intake spam I staged the engines. Just before the flameout, I turn off the 2 turbos at the sides, the central turbo with the clipped in nuke can really nicely push it to orbit.

Sometimes I just need a little distraction

[Redshift OTF]

I find there is a magic barrier around 36km up where, although the air makes lift difficult, drag really starts to fall off. If you can punch through that you are usually on to a winner. Trying to thrust through that barrier with only jets requires a lot of intakes and nukes on their own tend not to have the thrust to overcome the drag or you end up catching up with your apoapsis and thus falling. Getting that punch may only take a few more small supplemental engines to your nukes that you can hotkey off when you no longer need them and wont weigh you down in space when they are dead weight. Sounds like you have cracked the hard part. There is a lot to learn with space plans but you've broken a barrier.

[O-Doc]

I just had a chance to put your stock plane into orbit. It nearly made it just on two nukes. I pulled the basics, the T30s and the cans they were attached to and setup on 4 nukes, the final result in the resources tab is 4000LF + 5000OX. So, there's plenty of room to cut fuel out of the craft. I'd also pull out 4 turbos as they are simply not needed considering the payload weight and the number of intakes you have.

Overall, I'm guessing you could pull out 20T of excess weight before you look at optimising the structural elements. For example; too many I beams. You should mount your under carriage on girders(6-8) that are strutted together. Not only will you save weight but, the landings will have more give.

Monoprop? I carry monoprop as permanent cargo on some of my heavy lifters and if I need to bring more it goes into the cargo bay. I noticed you have RCS thrusters. I don't actually agree that docking a heavy lifter is useful or fun. I prefer putting a tug in the cargo hold or better yet, keep a tug on my station ready to assemble new parts or interplanetary ships.

Re: wings. There are a couple of places where you should stitch them together. It's an advanced construction technique where you get inside the wing and strut one edge of the wing to another.

Also, the plane looks a lot better with the canards replaced with winglets tucked into the nose. That's just an aesthetic opinion.

Bottom line, you have enough air intakes, I would work on your ascent with nukes. Progressively switch off and throttle down turbo engines so that your turbos are using all of the available air to assist your nuke 'stage'. But, by your last post, it looks like you are starting to travel in that direction.

[Mesklin]

LordFjord, I tested your first craft and it can reach orbit with right ascent profile (dV on LKO is ~1400 m/s):

1. Delete all basic jets, you are really do not need this engines, TWR with only turbojets is enough

2. Try next profile:

Trubojets only (full throttle)

H, km Pith Speed

0-19 60 0 - ~900

19-27 50 ~900 - ~1300

27-28 15-30 ~1300 - ~1700 (vertical speed should be low, do not fly over 28 km)

On atomic engines, pith 20-25 and climb with slowly decreasing throttle till throttle will be ~1/3 (orbital speed should be >2000 m/s at this time, height ~ 45 km). Off jets, full throttle, pith 45-40 and climb till 50 km and speed 2100-2200, then slowly set pith to 0 and go to desired apoapsis.

Edited April 19, 2014 by Mesklin

[Mischief]

Some basic advice that I've had luck with -

The juggle to get turbojets to be useful is almost never worth it. Use Rapiers instead, drop about one 'step' (depending on tank size) of oxidizer, it doesn't take a lot of fuel for Rapiers to get your playload to orbit.

Two to four nukes, depending on how far interplanetary you need to go, at most. You get a little over 2x the fuel efficiency for Nukes but you've got to lug 5-10 tons of useless weight up that first 40km. I only bother with nukes if I'm going to Duna or Minmus or wanting a SSTO one-shot trip to the Mun without refueling.

Best way to airhog doesn't take going nuts with clipping.

http://i.imgur.com/kVhrGP3.png

It's easier to balance weight forward and keep it balanced even with an empty tank by slipping pairs of smaller tanks up near the neck. This gives you room for two more of the better air intakes, one forward one back. This lets you air hog just fine.

At this point I've taken to using FAR; it just lets me make better, more challenging and interesting SSTOs but without it the whole thing is even easier. This build here is leaving the atmosphere at just over 2k m/s; with the right launch timing I can literally go straight from runway to the Mun and not even circularize. This ship can get to the Mun without nukes and have about 1800 dV when I finish circularizing Munar orbit.

http://i.imgur.com/bjoIpue.png

Landing is the fun part with FAR; hardest thing I've ever done in this game is land at KSC on re-entry in an SSTO with FAR installed. Tiny miscalculation in angle of descent and you'll get into an unrecoverable stall or rip yourself apart in a flat spin. COMEDY GOLD.

[LordFjord]

A little update and again, many thanks to all.

@Mesklin: I tried to follow your ascent path -> SUCCESS!

Added some pics to the gallery, pics 7-9. Redesigned the engine nacelles, in total it has now 10 turbojets and 4 nukes. I emptied the rear 2 tanks.

The trick was really to follow a SLOW ascent, I levelled the plane out several times to gain speed, then ascend a bit higher, then levelled it out again. I turned off more and more turbos, while the nukes were driving me slowly, but steadily out into orbit.

It really felt like creeping to space, but it worked.

In the end, I got punished for some intake spamming (behind the cockpit). The Kraken ate the plane as I undocked the cargo.

The docking port senior is attached to the same I-Beam as the intake line behind the cockpit.

-> Best avoid that in the future

Pics:

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[O-Doc]

Good to see you are getting the hang of the ascent. I would turn off SAS for undocking as a quick fix.