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

[michaelhester07]

Sounds like a 2 part solution

1. Put a liquid rocket in the center with a little bit of fuel (it should be fairly powerful, like a mainsail or higher).

2. Do your launch without mechjeb.

If MJ is turning off your liquid rocket you'll have no steering, so turn MJ off and keep your throttle to 20% or lower. This way you can use the center rocket to steer for your gravity turn. The wings higher up will actually interfere with your steering too. If you can replace them with girders that would help.

[Tokay Gris]

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

I still don't. And it still does not turn. It just runs out of boosters and THEN turns.

[Tokay Gris]

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

Nah. Those tanks are empty. This is supposed to be a fuel shuttle from Mün or Minimus.

- - - Updated - - -

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??

Hm... Probably. But I need them to get the footprint big enough and not look crappy.

[Tokay Gris]

Ok, I'll mark as "answered".

Still does not turn if I use only Boosters in the first stage. Probably the Surfaces on the payload. But likely this can't be done the way I had in mind.

I can get it to orbit, I have a kethane rig waiting to be launched and landed and currently am sending a rover to Mün to find a nice and flat place, preferably on the equator, to set up shop. Window for Duna opens up in 40 days...

[John FX]

I generally put 4 active winglets on a single SRB or two on each outer one if there is a cluster.

On a cluster as large as yours I would put a total of 24-36 active winglets to get it to turn fast enough. You currently have 12.

Also replace the delta wings with active winglets. AFAIK delta wings have no control surfaces so will `straight line` your craft slowing its turning ability although I may be wrong about this.

[Pecan]

Sorry, was late yesterday...

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.

...

There's no need to apologise for anything but you weren't being clear so we couldn't help. I'm glad you launched the thing but you really did do it the hard and expensive way. I understand that you didn't want the tanker to land again, the point of the design I showed you was a) it works, it turns, c) it is very low part-count, d) you can recover the LAUNCH VEHICLE (not the payload) so get nearly all the launch-cost back (>=98% if you can land at KSC, which is what it's designed for).

It's that last that makes it more cost-effective than throwing-away all those boosters. While it's possible to get them to work for fairly large payloads they stop paying for themselves very quickly beyond mid-teens tonnes unless they are just used for a very short thrust-boost at launch. Trying to use them beyond a few kilometers, at most, just isn't worth it for a 100t payload.

[Laie]

ide 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?

They cost half as much and come earlier in the tech tree.

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

Misunderstanding. The 100kg fuselage saves you 150 funds for the fuel line (not to mention part count).

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

That's what I was trying to get at.

This is a simplified version of your vessel. With an initial load of 1.7 large tanks of fuel, this one can easily get into 80km orbit under Mechjeb control. Jettison the Mainsail when your total fuel drops below 1000 units. Notice the complete absence of boosters.

As to your original question, "why does the beast not turn": it might also be a center-of-drag issue. With the empty tanks up front and the heavy boosters behind, the thing might fly like an arrow (that is, always point into the wind). An actual arrow will slowly turn back towards the ground, though -- but that's because it's going ballistic, whereas your vessel has a constantly increasing TWR driving it upward.

[Tokay Gris]

On a cluster as large as yours I would put a total of 24-36 active winglets to get it to turn fast enough. You currently have 12.

Tried with 60 to no avail.

Also replace the delta wings with active winglets. AFAIK delta wings have no control surfaces so will `straight line` your craft slowing its turning ability although I may be wrong about this.

You might be right, but those delta wings are for the footprint.

[Tokay Gris]

This is a simplified version of your vessel. With an initial load of 1.7 large tanks of fuel, this one can easily get into 80km orbit under Mechjeb control. Jettison the Mainsail when your total fuel drops below 1000 units. Notice the complete absence of boosters.

Tried to replicate your version. Does turn, of course, but does not reach orbit...

What am I doing wrong here? TWR on the launchpad is above 1.0, but not much. Fuel gets down to about 1000, but that leaves me with about 700 m/s left, which is not enough for circularization. And the TWR is abysmal and not enough to keep it up.

Best case - haven't achieved that yet - is that I reach orbit, but on empty tanks. And getting out and pushing that thing to Mün is not really possible... ;-)

And thats with about 200.000 creds less than my "version". Ok, so 200.000 is a bit expensive for 4000 m/s, it might be possible to achieve the goal (getting that thing to land on Mün) with a few boosters, but then the price difference would shrink further.

That said, your fuselage version is great! Never occurred to me to do it that way.

As to your original question, "why does the beast not turn": it might also be a center-of-drag issue. With the empty tanks up front and the heavy boosters behind, the thing might fly like an arrow (that is, always point into the wind). An actual arrow will slowly turn back towards the ground, though -- but that's because it's going ballistic, whereas your vessel has a constantly increasing TWR driving it upward.

Very possible.

[wasmic]

The reason that you can't steer with so many SRBs is simple: the stock "aero"dynamics are a piece of ...., so stuff won't fly as you expect it to fly. The thing is, in stock "aero"dynamics, each part that is not a wing or winglet has a drag coefficient assigned. The SRBs have a higher drag coefficient than other parts, which means that your ship is simply too aerodynamically stable. I have some suggestions that could fix this:

1: Mount wing parts at the top of your launcher. Be careful, though, as it's a fine balance you're trying to achieve.

2: Try to place the boosters around the payload instead of under it.

3: Install NEAR or FAR. Then the aerodynamics will behave as you'd expect them to, and situations like this would not arise.

(choose one of these options)

[sonicsst]

The reason that you can't steer with so many SRBs is simple: the stock "aero"dynamics are a piece of ...., so stuff won't fly as you expect it to fly. The thing is, in stock "aero"dynamics, each part that is not a wing or winglet has a drag coefficient assigned. The SRBs have a higher drag coefficient than other parts, which means that your ship is simply too aerodynamically stable.

I don't think drag works that way. As I understand it, the drag isn't applied to each part but instead to the ship as a whole, so if the center of drag is near the center of mass it shouldn't have an effect on handling. Any parts with lift would behave quite differently, but I don't think drag alone is his problem.

[Laie]

Best case - haven't achieved that yet - is that I reach orbit, but on empty tanks. And getting out and pushing that thing to Mün is not really possible... ;-)

Oh, and I thought getting into orbit and refueling there was an option. If you want to go further, you need more thrust to start with. The mainsail solution burns insane amounts of fuel early on just because it takes so long to climb. This would be a prime example for using boosters, but I'm too lazy to explore that route (it would take a lot of stuff just to attach the boosters, and I'm not *that* interested in this vessel after all).

However, replacing the Mainsail with a KR-2L also works. Keep the rockomax decoupler, that makes it easier to set up the fuel line. Apply a few struts if necessary. Fill her up, completely (2.5 large Kerbodynes).

Mechjeb Options:

turn start 7km
turn end 60km
final angle: 0°
shape: 54%
--
[X]Limit to terminal velocity
[x]Limit to 25m/s²
[ ]corrective steering
--
Jettison the KR-2L at 35300m 

Possibly the most refined ascent I've ever seen, that's why I'm quoting detailed mechjeb parameters. Leaves you with ~2000m/s in an 80km orbit.

That said, your fuselage version is great! Never occurred to me to do it that way.

On the shoulders of giants... rep goes to Kasuha.

Please keep in mind that my vessel has less fuel capacity than your original one; that alone makes it more lightweight, not to mention the lack of lander legs & stuff.

the drag isn't applied to each part but instead to the ship as a whole, so if the center of drag is near the center of mass

And as pretty much all parts have the same drag coefficient, the center of mass and center of drag are nearly identical. You're right. But Boosters have a higher-than-usual drag... so they *do* work just like tailfins in this case. So while my reasoning was all wrong, the effect is there.

Edited September 25, 2014 by Laie

[Kryxal]

Might I also suggest doing the math and kicking in the nukes just as soon as their ISP is above the "booster" engine?

[Hannu]

That should be possible to get on orbit. I have used more than 40 largest boosters. That was quite hard to turn and second stage had to correct wrong course, but did not took insane extra dv. They need enough SAS-devices. I put three large SASes and 10 boosters around that. It needs considerable amount of electricity, few extra batteries are good idea, if payload have not large capacity. Wings are quite useless in this kind of rockets. They have limited force, you do not need much torque below 10 km and wings lose gradually their effect after that.

Edited September 26, 2014 by Hannu

[Hannu]

Here is the example.

http://imgur.com/a/36YsN

[Hannu]

3: Install NEAR or FAR. Then the aerodynamics will behave as you'd expect them to, and situations like this would not arise.

... and forget thousand ton monsters with nasty shaped payloads. That's why I uninstalled it.

[Tokay Gris]

Oh, and I thought getting into orbit and refueling there was an option.

Not really. Well, that IS an option.

But not a practicable one, since I thought of this in the first place because getting surplus fuel up from Kerbin is a hassle.

It would make the mission even more complicated.

So far mission profile is this:

Land on Mün (done, no problem there), find a flat spot with kethane deposits (that is the currently active phase. Haven't found one yet) with a rover (looked nice, but tends to overturn on Mün), get a kethane rig there (already build, works in testing but needs a flat spot), get the tanker (the vessel from this thread) there and fill it up, use said tanker to fill up an interplanetary vessel in Kerbin orbit (or Mün orbit, if I can make that work).

It would be kind of counterproductive to fill up the tanker in Kerbin orbit just to get to Mün... then, I could fill the interplanetary vessel directly.

Possible, yes. Practicable, no.

And a refueling mission for the tanker kind of annihilates the cost effectiveness completely.

On the shoulders of giants... rep goes to Kasuha.

Why am I not surprised? ;-)

Does he have a real life as well? His knowledge is immense.

[Tokay Gris]

1: Mount wing parts at the top of your launcher. Be careful, though, as it's a fine balance you're trying to achieve.

Tried that. Had 24 Winglets right on top.

Didn't change anything.

2: Try to place the boosters around the payload instead of under it.

Will have to try that, but so far no idea where to place them without changing the design completely.

3: Install NEAR or FAR. Then the aerodynamics will behave as you'd expect them to, and situations like this would not arise.

Not at this point. What I heard of those mods doesn't sound right for me.

And besides, if I would start with such "game changing" mods (as opposed to the "doodad"-kind of mods I am using, like MechJeb or KAS) I could do what a friend of mine (in RL) does: Last I heard, he is rebuilding an imperial class star destroyer (hm... 16 km long?) and to do that, he is moding heavily on his own. Make it into one part, as far as I understood him. Haven't seen the results yet....)

[Sirine]

Perhaps.

Mun Maps?

http://www.kerbalmaps.com/

[Tokay Gris]

Perhaps.

Mun Maps?

You are referring to my search for a flat spot?

First of all, I need a REALLY flat spot. Preferably 1 degree deviation in any direction max. My kethane rig is rather high. And does not have a big footprint. Yet.

Kerbalmaps doesn't tell me that. At least not as exact as I'd need it.

On Minimus this would be easy, since the "oceans" (as I call them. The flats, you know...) are flat. And at least ONE kethane deposit should be there somewhere. Maybe I should rather aim for Minimus for this purpose.

Secondly, as far as I know, kethane is not always the same. Different for each user.

[Laie]

Maybe I should rather aim for Minimus for this purpose.

Minmus would indeed be better, because of it's lower gravity. You need less fuel to lift off from your refinery, and can deliver more to LKO. Your tanker might also get away with fewer LV-Ns; never mind the weight, if you have only eight of them wrapped around the tank, you can place decouplers below and needn't worry about the fairings getting stuck. An obvious mountpoint for boosters.

[Tokay Gris]

Minmus would indeed be better, because of it's lower gravity.

Yes, I know. I do have a kind of "generalized" fuel tanker in mind.

Back in 0.23 I had a Cruiser based on a similar design that could actually take off from Moho fully fueled. Granted, very slowly and inefficient, but it worked.

[tsotha]

Did you put the Vernor engines on the very top? Putting them at CoM won't do anything for you.

I wouldn't expect aerodynamic control surfaces to do much - above 10k there isn't much air.

Have you considered mounting, like, a T-30 horizontally? I think there was a "hands off gravity turn" challenge a few weeks back where people employed that tactic pretty successfully.

[Dkmdlb]

TL;DR the thread.

You should try tilting the rocket 5 degrees to the east on the launchpad. Secure it with launch clamps. Report back with results.

[Tokay Gris]

Did you put the Vernor engines on the very top?

Of course. I even put a special later to be discarded small tank on top, just for those.

I wouldn't expect aerodynamic control surfaces to do much - above 10k there isn't much air.

I left a few on, just to get more control from start to around 10 k.

Have you considered mounting, like, a T-30 horizontally?

Actually, yes. I have. I just couldn't figure out how to control it.

That is one shortcoming of the game: You couldn't build a quadrocopter-like vessel.

Well, you can, but you can't control it like a quadrocopter, meaning, you can't control throttle differently on different engines.

And I would need an engine on top that works on a different throttle. It might be possible to use the thrust limiter, but that would be pretty crude.