Quick Question Concerning "Puller"-type Rockets

[Caiaphas]

So I'm attempting to design my first >20 km/s dV orbitally constructed nuke rocket to intercept an E-class asteroid, and after browsing through some people's designs I decided to go with a puller-type rocket with the propulsion bus up top and the propellant down at the bottom.  What concerns me is that my propulsion bus doesn't angle the rockets, and when it's fully assembled some of the propellant tanks and the asteroid might be in the way of the exhaust.

At what range does the exhaust start to affect other objects?  At present there's a separation of about 10 meters from the nozzle ends to the top of the first propellant tank, and I'm wondering if I should add in another spine to increase that separation.

[bewing]

As I understand it, the standard answer is 20 meters. But this is something that you can test for yourself on the launchpad, or using the SetOrbit command in the cheat menu.

[Caiaphas]

...cannot believe I didn't think of that.  Thanks!

[Human Person]

What advantage do you expect from such a design?

I only see the disadvantages you mentioned yourself plus the reduced efficiency of angled engines.

 

Look Here: 

 

Edited March 15, 2017 by Physics Student

[Bornholio]

Pullers allow the grabber to angle naturally along the line of action and center of asteroid mass.

Much easier to fly.

[Caiaphas]

3 hours ago, Physics Student said:

What advantage do you expect from such a design?

I only see the disadvantages you mentioned yourself plus the reduced efficiency of angled engines.

 

Look Here: 

 

You'll note that his main point is that GRAVITATIONAL force is acting equivalently across the rocket.  This isn't the case in interplanetary space.

EDIT: okay, that's not as precise or helpful as it could be.  Suffice it to say that out at Kerbin's orbit and outside its SOI, gravity is negligible compared to everything else acting on the rocket.

Also, Mr. Manley doesn't really get into the root cause of why pendulum rockets are unstable, which is outlined here (https://web.archive.org/web/20091018232320/http://geocities.com/jim_bowery/pendrock.html) but which basically boils down to the assumption that the rocket motor is going to be fixed relative to the mass that it's dragging, which isn't necessarily the case, since you can free the gimbal on the Klaw.

Edited March 15, 2017 by Caiaphas

[Ten Key]

4 hours ago, Physics Student said:

What advantage do you expect from such a design?

I only see the disadvantages you mentioned yourself plus the reduced efficiency of angled engines.

This video is misleading.

Scott Manley is correct in that a perfectly rigid rocket will not benefit from a puller configuration, since the thrust vector is always locked relative to the rest of the rocket. However, rockets in KSP are rarely rigid, and if you watch that video all the way to the end you can see that introducing even a small amount of flexibility does result in a pendulum effect. 

For launch vehicles this isn't necessarily desirable. . .we want the rocket to be able to tip in a controlled manned. But for a tug operating in space, it does help. In a puller configuration, any sort of wobble or flex in the system will naturally damp out when the vehicle is under thrust. In a pusher configuration, it's apt to get worse. There's a reason tow trucks are favored over push trucks.

[Human Person]

1 hour ago, Ten Key said:

any sort of wobble or flex in the system will naturally damp out when the vehicle is under thrust.

 

That's the most kerbal statement I read so far. The rocket bends and flexes, and that's good, we'll use that to our advantage. Amazing.

I wouldn't do it nontheless. My pusher-tugs work perfectly fine even with Class-E Asteroids.

You don't need huge amounts of dv either, I once aerocaptured a Class-E asteroid using less than 50 m/s for all of my maneuvering.

Edited March 15, 2017 by Physics Student

[Ten Key]

58 minutes ago, Physics Student said:

That's the most kerbal statement I read so far. The rocket bends and flexes, and that's good, we'll use that to our advantage. Amazing.

Yep. Sometimes, it is just that simple.

 

 

 

Edited March 15, 2017 by Ten Key

[Caiaphas]

1 hour ago, Physics Student said:

That's the most kerbal statement I read so far. The rocket bends and flexes, and that's good, we'll use that to our advantage. Amazing.

I wouldn't do it nontheless. My pusher-tugs work perfectly fine even with Class-E Asteroids.

You don't need huge amounts of dv either, I once aerocaptured a Class-E asteroid using less than 50 m/s for all of my maneuvering.

You'd be surprised at how often you want structures to bend and flex in mechanical engineering (provided you're working with materials that don't suffer from excessive fatigue cracking); for one thing a mildly flexible structure will often easily absorb forces that would crack or shatter a much more rigid structure.

Back on topic.  Could you please tell me at what altitude you aerobraked the asteroid?

[Human Person]

11 hours ago, Caiaphas said:

Back on topic.  Could you please tell me at what altitude you aerobraked the asteroid?

28 km. Make sure your craft is behind the asteroid, it's your heat shield 

Edited March 16, 2017 by Physics Student

[bewing]

They no longer work as heat shields.

[Kryxal]

15 hours ago, Caiaphas said:

You'd be surprised at how often you want structures to bend and flex in mechanical engineering (provided you're working with materials that don't suffer from excessive fatigue cracking); for one thing a mildly flexible structure will often easily absorb forces that would crack or shatter a much more rigid structure.

And then you make a mistake, and you get the Tacoma Narrows bridge ... but it's probably 100:1 the other way, isn't it?

 

1 hour ago, bewing said:

They no longer work as heat shields.

If that's so, how are they on drag?  If they tend to act as an anchor, that's easy to deal with, but if you end up as the draggy part, it could be trouble...

[bewing]

Unless you've completely hollowed them out, they are always much heavier than the rest of your ship, so they always end up in front, no matter how draggy. But they are pretty draggy.

[Caiaphas]

6 hours ago, Physics Student said:

28 km. Make sure your craft is behind the asteroid, it's your heat shield 

Cool, thanks.