Does anyone know how to launch Atlas V 551 configuration in KSP???

[dennislee]

I am trying to launch Atlas V in 551 configuration to replicate JUNO and New Horizons launch. but because of asymmetrical srb, the rocket keeps getting out of control. Also, even if I get it right, I have to keep rocket straight up, which causes a lot of delta v loss. I want to know how you guys launch Atlas V...

plz help

[CaptainTrebor]

Scott Manley did a video on the Atlas V 411, however that also has asymmetrical thrust so in theory it should be the same.

https://www.youtube.com/watch?v=f-o6BLBzsUA

[LameLefty]

Like a real Atlas 551, you have to have sufficient control authority to handle asymmetric thrust. Give increased control authority to the core engine, offset and lock the nozzle from your SRB, have a strong SAS at the top of your rocket ... whatever it takes. Fins would make it easy but of course a real Atlas doesn't use them.

[Gaarst]

Use Vectors: the real RD-180 has sick gimbal range that helps making the rocket go straight up even in asymmetric configurations.

The real Atlas V uses boosters that have fixed but oriented nozzles: instead of being directed straight down like in KSP, the SRB's thrust is headed slightly away from the center; that way the thrust vector of the booster is roughly aligned with the CoM of the rocket, making it going a bit sideways but reducing torque to a minimum. Unfortunately there is no way to replicate this in KSP, apart from clipping a thrust-limited Flea inside your booster but that usually ends up looking terrible, so you won't get as good behaviour as the actual rocket.

[XB-70A]

I got my own solution for it with the stock parts. I actually use one of the game problem  (its pretty bad aerodynamic) to compensate the thrust one : by placing some wings and their control surface inside the main core.

Here are some screenshot taken while testing my X-37 shuttles family after having installed the "wings" (sorry it's during the night) : 

As you can see there is no visible lifting surface, but the visible aerodynamics marks clearly show that the one inside are compensating the strong lifting surface of the top vehicle.

It's too bad I dont have any screenshot of it now as my actual model got 4 Thumper for the light shuttle version and 5 for the heavy one, anyway the lifting surfaces are still perfectly capable of compensate the imbalanced CoT. And as the boosters are lighted at the same time and got a pretty short time of burning the launcher is still in the atmosphere when we have to jettison them.

 

--- EDIT ---

 

I got some time this morning, so just launch the game to make a quick flight with the actual version :

I use the ascent guidance to get my screenshot's hand free while the other is controlling the X-axis.

 

Here the core control surface actions are clearly visible.

 

Once freed of the SRB nearly nothing remains.

 

And you enjoy the flight.

Edited January 22, 2017 by XB-70A

[GoSlash27]

dennislee,

 the real life SRB used with the Atlas V has the exhaust nozzle canted to drive through the CoG, so it doesn't really matter how they're arranged.

 If I were to replicate it in KSP, I'd reduce the propellant and throttle on the 3 booster side to 70% so they make the same thrust as the other side but still burn out at the same time.

Best,
-Slashy

 

[septemberWaves]

Asymmetrical SRBs are kind of a pain to get used to but they're pretty simple once you know what you're doing. The trick is to have quite a large rocket compared to the SRB, attach the SRB on a TT-70 radial decoupler, and use the rotate tool to change the SRB angle. When doing this, you'll want the Center of Mass and Center of Thrust indicators to be visible, and you'll want to change all engine thrusts to zero except for the SRB and the first stage engines. The trick is to then rotate the SRB so that the combined thrust vector points through the center of mass. You also need to use high-gimbal engines on the first stage. Moa engines from SpaceY work quite well, or if you're using stock then your best option is the Vector engine.

When flying an asymmetrical rocket, the best thing to do is to rotate it in such a way that the thrust vector points slightly towards your intended direction of travel. For a single-booster rocket, this means the booster should be on the side of the launch pad facing the VAB (assuming you're launching into a prograde orbit). This way, the rocket will gain some horizontal velocity as well as vertical velocity at liftoff even when it's pointing directly up.

If you're very patient and you tune the thrust in the right way, it's possible to set up an asymmetrical launch vehicle so that it performs a gravity turn with no input to the controls other than pressing space to stage when necessary. This is extremely difficult to achieve though, and it's probably easier to get used to flying asymmetrical rockets manually.

Edited January 22, 2017 by eloquentJane

[ILikeIke]

@dennislee-if you want methods for controlling stock asymmetrical LVs, I'm afraid I can't help you.

If you're ok with using mods/changing the hardware, I would direct you to @CobaltWolf's Bluedog Design Bureau, which has Atlas-V-alike parts, and whose RD-180 analogue has plenty of gimbal. The Daleth-CGR-60 "Sapphire", which is also part of the pack, is capable of acting as an analogue for the SRMs and does not produce too much thrust for the RD-180 analogue to handle.

 

[CobaltWolf]

1 hour ago, ILikeIke said:

@dennislee-if you want methods for controlling stock asymmetrical LVs, I'm afraid I can't help you.

If you're ok with using mods/changing the hardware, I would direct you to @CobaltWolf's Bluedog Design Bureau, which has Atlas-V-alike parts, and whose RD-180 analogue has plenty of gimbal. The Daleth-CGR-60 "Sapphire", which is also part of the pack, is capable of acting as an analogue for the SRMs and does not produce too much thrust for the RD-180 analogue to handle.

Missing from the pack is a proper length fuel tank for the centaur, a proper interstage+fairing base combo for said centaur (if we're talking 5xx with the 5m ((3.125m in KSP)) fairing) and the traditional AJ-60 SRBs. With that said, the CGR-60s are analogues for the GEM-60 SRBs from Delta-IV, which I've heard will be replacing the AJ-60s some time in the next couple years for the remaining Atlas V flights before the planned transition to Vulcan, which will also use them.

The Atlas V parts are also some of the oldest parts in the mod. I personally hate looking at them but I think I hate everything I've done after a week or so

Edited January 22, 2017 by CobaltWolf

[dennislee]

12 hours ago, CobaltWolf said:

Missing from the pack is a proper length fuel tank for the centaur, a proper interstage+fairing base combo for said centaur (if we're talking 5xx with the 5m ((3.125m in KSP)) fairing) and the traditional AJ-60 SRBs. With that said, the CGR-60s are analogues for the GEM-60 SRBs from Delta-IV, which I've heard will be replacing the AJ-60s some time in the next couple years for the remaining Atlas V flights before the planned transition to Vulcan, which will also use them.

The Atlas V parts are also some of the oldest parts in the mod. I personally hate looking at them but I think I hate everything I've done after a week or so

I am using launchers pack atlas V currently

[WuphonsReach]

20 hours ago, LameLefty said:

Like a real Atlas 551, you have to have sufficient control authority to handle asymmetric thrust. Give increased control authority to the core engine, offset and lock the nozzle from your SRB, have a strong SAS at the top of your rocket ... whatever it takes. Fins would make it easy but of course a real Atlas doesn't use them.

Reaction wheels (SAS) should always be at the center of mass point (adjusted towards the nose of the rocket if you need better control after the prior stage is 1/2 to 2/3 empty of fuel).  

Place a broomstick on the floor.  It's far easier to rotate it if you grasp it in the middle, then if you try and rotate it by grabbing near one of the ends.

[Kerbart]

55 minutes ago, WuphonsReach said:

Reaction wheels (SAS) should always be at the center of mass point (adjusted towards the nose of the rocket if you need better control after the prior stage is 1/2 to 2/3 empty of fuel).  

Place a broomstick on the floor.  It's far easier to rotate it if you grasp it in the middle, then if you try and rotate it by grabbing near one of the ends.

That's not how torque works, it can be applied from any point. If you're generating torque by applying a force placement matters because you need a lever, but pure torque as the SAS unit generates can be applied from anywhere. The only concern is flexing your craft by inducing torque on it, and that is a consideration.

The broomstick is not an optimal example as you are accelerating the center of mass when grasping it by the end. Force is required for that and that's why you have to apply more torque when grasping it at the end. Grasp it at the center and the COM of the broomstick doesn't move. No force required, less torque need to be applied.

Edited January 23, 2017 by Kerbart

[KAL 9000]

This is why we need 5x symmetry!

[LameLefty]

12 hours ago, WuphonsReach said:

Reaction wheels (SAS) should always be at the center of mass point (adjusted towards the nose of the rocket if you need better control after the prior stage is 1/2 to 2/3 empty of fuel).  

Place a broomstick on the floor.  It's far easier to rotate it if you grasp it in the middle, then if you try and rotate it by grabbing near one of the ends.

I've been playing KSP for almost 4 years and I've got an undergraduate degree in aerospace and mechanical engineering. No need to try to teach me anything about how rockets work.

[jhousen]

yes, adjust the thrust limiters and fuel loads.

what you are after:

center of thrust at center of mass

and the srb's to burn out at the same time

I use kerbal engineer, I'm guessing this will be....difficult without it

[Snark]

Moving to Gameplay Questions.

[eagle92lightning]

I have built a fully stock Atlas V 551 and the best way to do it is to start your roll shortly after liftoff and really as long as you have seratrons on the boosts and sas is on the game handles it fine. 

 

Note: when I did my 551 it had a 400 series uper stage not a centaur