Dnepr third stage

[Shadow Addict]

I've recently become interested in the Dnepr launch vehicle. I'm considering possibly attempting it as an addon project when I get better at developing. However, I'm a little confused about its third stage; from what I can tell, it jettisons after second stage burnout with its engines facing forward and ignites them before turning 180 degrees so they are burning prograde. The engines are weak and burn for a long time, so I guess the retrograde thrust it delivers isn't too much, but why have any at all? What's the point of igniting before rotating?

 

[SiberianTiger]

This upper stage implements a pulling scheme to deliver payloads to target orbits. At the same time, the engine and tanks are positioned below payloads during ascent. Once after 2nd stage's cut-off, it extends the combustion chambers with nozzles to the sides, ignites them (they can only be ignited once) and performs turning to 180 degrees to acquire the right attitude. These nozzles are main and vernier thrusters at the same time and are the only means of controlling the attitude.

 

[Urwumpe]

One reason behind it is packaging - the Dnepr is an ICBM, that has to fit into silos, also all later Russian ICBMs use such a post-boost vehicle design.The reason behind the pulling-scheme is also not too complex: By pulling the payloads, you can release multiple warheads without having to cut the engines.

 

[Shadow Addict]

Ohh, I see. So how exactly does that work? Do two engines ignite to start the rotation and then the other two to stop it, or do they all ignite and once and two of them are throttled? Also, the third stage continues to accelerate after payload separation, right?

 

[Urwumpe]

Ohh, I see. So how exactly does that work? Do two engines ignite to start the rotation and then the other two to stop it, or do they all ignite and once and two of them are throttled? Also, the third stage continues to accelerate after payload separation, right?

AFAIR, all four nozzles use the same propellant utilization system and are thus practically ignited all at once.

And yes, it moves away for collision avoidance.

 

[Shadow Addict]

Ohh, so the thrusters can rotate to provide attitude control?

 

[Urwumpe]

Ohh, so the thrusters can rotate to provide attitude control?

Yes. Each thruster is 2DOF.

 

[Shadow Addict]

Ah, finally makes sense! One last question for now:

I don't understand some of this picture. I know that the plug is being blown out of the way at the bottom, but what are the rings that are detaching from the body of the rocket?

 

[Urwumpe]

I don't understand some of this picture. I know that the plug is being blown out of the way at the bottom, but what are the rings that are detaching from the body of the rocket?

Guides and pressure seals for the cold gas ejection. Basically, the rings ensure that the rocket is ejected straight up and does not suffer too much blow-back.

Look here how the cloud of the cold gas ejection only leaves the silo AFTER the rocket is ejected.

[ame="http://www.youtube.com/watch?v=fB-vtc-d0Cg"]YouTube- Dnepr rocket launch[/ame]

 

[Shadow Addict]

Ohhh, I'd been wondering what the blast before the plug getting blown to the side was. Thanks so much for all the help!