Beast won't turn - Boosters (Answered. Kind of)

[Tokay Gris]

With the science update I learned to build my rockets small.

With the contracts update I am learning to build cheap.

Which leads to boosters. Lots of boosters. (Jebediah style, so to speak)

Sometimes rockets and/or payload aren't small.

Currently I am building a fuel shuttle to be used on Mün and Minimus. Two of the biggest tanks, a 12-ring of nuclear engines, landing abilities.

Of course, it lifts off empty besides the 12 tanks for the radial engines. It is a tanker to be refueled on Mün. No sense lifting up full tanks.

But still, this beast has more than 100 tons on the launch pad.

The first stage consists of a big bunch of the large boosters. Gets that thingie basically to space.

But I can only get it up straight. That thing won't TURN. I can't for the life of me get it to do the gravity turn.

I tried putting a bunch of SAS modules on it. No use. I tried RCS in different configurations and in different places. Tried the "new" RCS-outlets that run on liquid fuel. Last try I had 16 of them.

Still, that beast does not turn. It gets to about 10 degrees off the straight line and no amount of RCS or SAS usage gets it to turn further. I do have Wings on that vessel. But still, no use.

So I get it to 75 km. More or less straight up. Boosters burn out, get discarded, vessel turns just fine (a bit sluggish, of course). But the circulation burn is about 2000, which takes forever with the nuclears and doesn't work at all, because I get into the atmosphere again.

TWR of the payload is not so bad considering that the vessel is almost empty. 0.6.

So, anyone any idea how I can get this beast to actually turn east?

Hm... maybe put some liquid engine in the middle of my booster pack? The boosters accelerate the rocket well beyond terminal velocity. (Not too much. Just barely fireworks...

What else can I do? Put lateral engines on it and disable them once the turn is complete and the solid fuels are discarded?

Deliberately put the vessel off balance? Hm... that might be an idea....

Edited September 25, 2014 by Tokay Gris

[Pecan]

This is a 100t SSTO launch-vehicle, fully recoverable with drogue & powered landing.

(That's 4 KR-2Ls)

Without pictures or even a description of your vehicle it's rather impossible to say why it won't turn. My only real advice is to not lift what you can't lift; there's no real reason for such a large payload except "Hey, I can".

ETA: Oh - and on short trips to Mun/Minmus the LV-Ns probably aren't worth their mass. 2 (x 0.1t) 48-7Ss will give the same thrust, if not the same economy.

EETA: (Oops) it has exactly 1 large SAS unit and handles nicely.

Edited September 24, 2014 by Pecan

[xtoro]

Picture will help yes. So, are the wings just wings? Or are they control surfaces? Static wings won't help you, you need control surfaces that will actually move and steer it. Also, they should be as close to the bottom of the rocket as possible.

[Tokay Gris]

Without pictures or even a description of your vehicle it's rather impossible to say why it won't turn. My only real advice is to not lift what you can't lift; there's no real reason for such a large payload except "Hey, I can".

I thought I gave a description. Payload basically two of the big tanks (biggest there are) plus "stuff". 12 LV-N laterally attached.

Lifts off empty on the big tanks, weights 100 tons.

And this is supposed to be a refueling tanker. Land on Mün, refuel at kethane rig, lift off again, refuel interplanetary mission in Kerbin orbit (or Mun orbit. Will have to see.)

ETA: Oh - and on short trips to Mun/Minmus the LV-Ns probably aren't worth their mass. 2 (x 0.1t) 48-7Ss will give the same thrust, if not the same economy.

Like I said, this is a big beast. Not meant to set down on Kerbin ever again. Tests with different engines gave the highest delta-V for the LV-Ns.

This is a 100t SSTO launch-vehicle, fully recoverable with drogue & powered landing.

100tons the whole thing?

My payload is 100 tons. The whole rocket sits at slightly above 1000tons on the launchpad.

[Tokay Gris]

Picture will help yes. So, are the wings just wings?

Both. The structural wings are just to get a bigger footprint. The actual control surfaces are way down on the bottom of the rocket.

This is a second try:

Top half is the payload. Bottom half is the "lift" stage.

[Sirine]

Put some Advance SAS. About 8~12 for your ship size and some electricity enought to power them.

Edited September 24, 2014 by Sirine

[Tokay Gris]

Put some Advance SAS. About 8~12 for your ship size and some electricity enought to power them.

Just tried that. 12 advanced SAS, 6 "normal" ones.

No change.

This was the "boosters only" first stage:

It seems to want to get straight up. And nothing else.

[Tokay Gris]

Hm... will try tomorrow with a somewhat funny ascent profile.

The LVN stage has delta-V enough. Over 4000 m/s. Even with the big tanks empty.

So I will try to lift up straight (or turn as far east as it will go) to about 150 and then burn sideways in the hope that I can achieve orbit before I hit atmosphere.

Sounds silly... Might just work.

[Skorpychan]

This is exactly what the verniers are for. Mount some up at the top of the rocket so you can get control authority that way.

[Laie]

Is that big boosters all the way to the core? Those beasts are heavy, it takes a lot of torque to turn them around. I don't know when you want to turn, but chances are that you're too high and too slow for the rudders to have much impact.

My advice: spend the money and put a proper gimballing Liquid Fuel Engine on the thing. You've already got the tanks, which are by far more expensive than the fuel they can hold -- just fill them up (well, one of them -- both would be a bit much). Use boosters to get to 10-20km; from there, one KR-2L plus one large tank worth of fuel should get you on such an trajectory that the Nervas can handle the rest. With some creativity, you may even mount several rockomax engines (might be cheaper). Keep in mind that you can additionally run the Nervas from as early as 2-3km (provided nothing is underneath them), and the final 600-1000m/s may come from Nervas alone.

I've launched similar-but-larger contraptions without any trouble; but then again, I never tried a booster-only launch.

Edit to add: this is my heavy tanker / refuelling station. 173 tons empty, it went up on four mainsails that were discarded during ascent. It took me several attempts to get the ascent just right, though. Launch cost (fuel, engines, decouplers &c) was ~51k funds.

Side note: since 0.24, the green SAS has more than twice the torque of the brown SAS; and it weighs slightly less, too.

Side note2: refueling the radial tanks will be a hassle. If you use the 100kg structural fuselage to mount the Nervas, they will tap the central tank for propulsion, no fuel lines required (150 funds apiece!) Or just use empty tanks with crossfeed disabled, like this. If you need the additional fuel capacity, add Rockomax or Kerbodyne tanks to the core -- much easier to refill.

Edited September 25, 2014 by Laie

[xtoro]

What Laie said. You can't steer with boosters. SRB's just thrust in a staight line, and it's lots of thrust, so trying to fight that force pushing you straight up with wings will be a losing battle. Use more liquid fuel rockets with engines that gimble. You can still use SRB's, but they're mostly only used in the beginning of the launch to get you moving through the lower altitudes.

[Progressm]

In real life, solid rocket full stages are used in satellite launchers, I think... wonder how they steer, then.

[Sirine]

Just tried that. 12 advanced SAS, 6 "normal" ones.

No change.

This was the "boosters only" first stage:

img...

It seems to want to get straight up. And nothing else.

Did you press the "T" button? You must 'ON' the SAS. With those wings and SAS. Everything is turn-able.

[xtoro]

In real life, solid rocket full stages are used in satellite launchers, I think... wonder how they steer, then.

Well this game isn't real life is it... I'm sure in real life there are SRB's that gimble

[Pecan]

I thought I gave a description. Payload basically two of the big tanks (biggest there are) plus "stuff". 12 LV-N laterally attached.

Lifts off empty on the big tanks, weights 100 tons.

Yes - that was a description of your payload, not of the launch vehicle that wouldn't turn! All you said about that was 'boosters', which could be anything but the pictures you've since posted make it all clear.

And this is supposed to be a refueling tanker. Land on Mün, refuel at kethane rig, lift off again, refuel interplanetary mission in Kerbin orbit ...Not meant to set down on Kerbin ever again. Tests with different engines gave the highest delta-V for the LV-Ns.

So why the 4km/s deltaV you've since mentioned? LV-Ns will always give the best figures 'eventually' because that's just down to fuel vs Isp. It's unlikely you'll ever be burning them long enough just within Kerbin's system to get through all that fuel though, that's why I suggested lighter engines such as 48-7Ss. Let's get you working at all first though ;-0

100tons the whole thing?

My payload is 100 tons. The whole rocket sits at slightly above 1000tons on the launchpad.

If you click on the picture of the launch-vehicle I posted you can see it in more detail, with a bit of description, on imgur. It is 741t itself but only 52 parts and launches an additional payload* of 100t - so should work for you. Although it costs ~321,000 it is a fully-recoverable, vertical landing, SSTO so each launch you only have to pay for the fuel it uses plus whatever percentage you lose from not landing exactly on the launch-pad or runway at KSC.

(*The yellow and white pointy thing on top of the rocket is an NRAP test-weight, put the real payload there instead)

SRBs are high-thrust and cheap but they are only really useful for a bit of extra thrust at launch on big builds, if you need it. It's easier to use a conventional design with less staging and, if you can recover parts, more cost-effective too. (In my experience, anyway. I've only found SRBs cost-effective for much lighter payloads, since you generally have to throw them away each launch).

[Laie]

SRBs are high-thrust and cheap

Also: heavy. Which doesn't matter if you use them the ordinary way, but if you throttle them, or put them on top of each other, they quickly lose their lightning.

[Tokay Gris]

This is exactly what the verniers are for. Mount some up at the top of the rocket so you can get control authority that way.

Tried that. Had 16 on top of the rocket. Saw them fire at the beginning of the gravity turn. Slightly more turning, but still no more than 10 degrees.

[Tokay Gris]

Is that big boosters all the way to the core? Those beasts are heavy, it takes a lot of torque to turn them around. I don't know when you want to turn, but chances are that you're too high and too slow for the rudders to have much impact.

That I noticed. Rudders didn't do squat after about 8000 m. I think that is where the 10 degrees come from.

My advice: spend the money and put a proper gimballing Liquid Fuel Engine on the thing. You've already got the tanks, which are by far more expensive than the fuel they can hold -- just fill them up (well, one of them -- both would be a bit much).

Hm... Might have to do some redesign then. If I only put one tank below the vessel, I might not have the space to put the big boosters on. Will try, though.

Side note: since 0.24, the green SAS has more than twice the torque of the brown SAS; and it weighs slightly less, too.

Really? What use are the brown/grey ones then?

Side note2: refueling the radial tanks will be a hassle. If you use the 100kg structural fuselage to mount the Nervas, they will tap the central tank for propulsion, no fuel lines required (150 funds apiece!)

Maybe I do have tomatoes on my eyes. (If that sentence does not make sense, then this german proverb does not translate to english)

But I can't find a fuselage/mounting point for 150 and 0.1 tons. What am I overlooking?

But you gave me some ideas. Will try that next.

[Tokay Gris]

So why the 4km/s deltaV you've since mentioned? LV-Ns will always give the best figures 'eventually' because that's just down to fuel vs Isp. It's unlikely you'll ever be burning them long enough just within Kerbin's system to get through all that fuel though, that's why I suggested lighter engines such as 48-7Ss. Let's get you working at all first though ;-0

Sorry, was late yesterday.

Well, the idea is to refuel that monster from a kethane rig stationed on Mün (and/or Minimus) and get as much of that fuel to orbit again, probably Kerbin orbit.

In the "old" times (when funding was irrelevant), I just launched a bunch of heavy tankers and refueled immense interplanetary rockets that took of from the surface empty. (Even had a monster that burnt fuel sitting on the ground. That was before you could empty tanks in the VAB.)

(After the first dozen, I "cheated". I decoupled the tanker, used Hyperedit to fill it up again, docked again, transferred the fuel... Reason was that Mechjeb went totally of course if I used hyper edit to refuel the vessel directly. And besides, there were kethane tanks I did not WANT to be full.)

So, with the Nuclears, I use less fuel to get to orbit again and therefor have more fuel to fill up interplanetary craft.

Thats the idea, at least. (And a pretty similar design was able to lift off again fully fueled from Moho surface. Quite a tedious liftoff with TWR only slightly above 1.0 relative to Moho)

SRBs are high-thrust and cheap but they are only really useful for a bit of extra thrust at launch on big builds, if you need it. It's easier to use a conventional design with less staging and, if you can recover parts, more cost-effective too. (In my experience, anyway. I've only found SRBs cost-effective for much lighter payloads, since you generally have to throw them away each launch).

Some of my vessels (like this one) are not supposed to be recovered. Ever. If I get this monster to orbit, there is no chance in hell it will ever get down to Kerbin surface again in one piece.

I mean, I can build a good old asparagus and get this thingie in orbit no hassle. But that is everything, but not cost effective.

So I was trying to get that thing to orbit as cheap as possible.

And again, sorry if I was a bit less than polite yesterday. I was pissed, I was frustrated and nothing worked.

[Tokay Gris]

A little update:

I did get the thing to orbit, using a slight modification of the one in the first picture.

A bit above 400.000 creds and a rather funny ascent profile.

I have used the "boosters only" first stage before and managed to get some big vessels to orbit. (Circum-Mün and Circum-Minimus stations did get up that way)

With this design though, it doesn't really work. And even with the KR-2L in the middle, it doesn't burn on the way up (MechJebs "limit to terminal velocity") and just kicks in after the boosters are discarded.

Hm... maybe the solution here is actually LESS boosters....

[Sirine]

So, now I know why your craft won't turn.

[ratchet freak]

to those that suggest putting vernier at the top, you can also put them at the bottom to provide the kick sideways into the grav turn that a gimbaled engine would otherwise provide

[KerikBalm]

Are you using NEAR/FAR?

I think its aerodynamic forces that prevent you from turning...

I think you need a lot more controllable fins, and bigger ones (try the canards), and set their tweakables to have the maximum deflection

That thing is truly massive... you really want to take 4 OTEs worth of fuel up at a time?

[kurja]

How about those control surface-less wings on above the rocket's center line, won't they effectively stabilize the craft and prevent turning while going through atmo at speed??

[SRV Ron]

Well this game isn't real life is it... I'm sure in real life there are SRB's that gimble

SRBs in real life cannot be gimbled. They can be steered some using graphite veins mounted in the exhaust blast.