Smart NASA...

[Spaceisbeautifulul]

So I was trying to make an accurate spaceshutte replica in KSP. I failed, mainly because of the boosters overpowering the smaller engines on the shuttle.

I tried looking up pictures, and it showed that NASA actually pointed their engines on the shuttle towards the large orange tank. This centered the center of lift! I tried it out and it worked like a charm.

[architeuthis]

So I was trying to make an accurate spaceshutte replica in KSP. I failed, mainly because of the boosters overpowering the smaller engines on the shuttle.

I tried looking up pictures, and it showed that NASA actually pointed their engines on the shuttle towards the large orange tank. This centered the center of lift! I tried it out and it worked like a charm.

Aerodynamic drag, and the weight of the orbiter create a positive torque which wants to rotate the entire rocket. If the SSME were pointed straight down they would produce a negative torque which would counter act the rotation, but the force applied from the rocket is much greater than the other forces so it would tend to cause the Shuttle to rotate in the other direction. Instead they point the engines at roughly the center of mass of the entire stack, as a vector whose line of action passes through the center of mass wont create any rotational moment. This is why the Space Shuttle would roll to an 'upside down' position during the boost phase: you can work with the torque created by gravity instead of fighting it when you make the gravity turn.

[DerekL1963]

Aerodynamic drag, and the weight of the orbiter create a positive torque which wants to rotate the entire rocket. If the SSME were pointed straight down they would produce a negative torque which would counter act the rotation, but the force applied from the rocket is much greater than the other forces so it would tend to cause the Shuttle to rotate in the other direction. Instead they point the engines at roughly the center of mass of the entire stack, as a vector whose line of action passes through the center of mass wont create any rotational moment. This is why the Space Shuttle would roll to an 'upside down' position during the boost phase: you can work with the torque created by gravity instead of fighting it when you make the gravity turn.

Um... no. (If I read your word salad correctly.) The Shuttle rolls "heads down" to control the effects of lift (from both the wings and the ET) on the stack and trajectory. (Also to point the antenna, which are located on the Orbiter's upper surface, towards the ground stations.)

[Thaniel]

That is prettu much what he writes though. Just very differently. The placement of the antenna, I am pretty sure came after the aerodynamics and propulsive forces where pretty much nailed down.

[architeuthis]

Um... no. (If I read your word salad correctly.) The Shuttle rolls "heads down" to control the effects of lift (from both the wings and the ET) on the stack and trajectory. (Also to point the antenna, which are located on the Orbiter's upper surface, towards the ground stations.)

Thanks DerekL1963, those are other good reasons for the the maneuver as you stated, another advantage is that an RTLS abort mode is easier accomplished. But considering the context of the original post do you think this is pertinent to the discussion at hand (granted KSP's lack of meaningful communications or dynamic pressure)?

Ahh, delicious word salad. Should I take you to mean that you don't understand the words that I used in the previous post? Are you familiar with the concept of torque?

Edited June 12, 2013 by architeuthis

[DerekL1963]

Thanks DerekL1963, those are other good reasons for the the maneuver as you stated

No, they're practically the only reason the maneuver is accomplished. That's why, by the mid-late 90's they were turning heads up sooner and sooner (even while still performing the gravity turn) - the downrange stations were being decommissioned for budgetary reasons and they needed to turn heads up to communicate via TDRSS.

But considering the context of the original post do you think this is pertinent to the discussion at hand (granted KSP's lack of meaningful communications or dynamic pressure)?

Considering the context of the original post, which was NASA's reasoning behind certain details of the Shuttle's design, yes.

Ahh, delicious word salad. Should I take you to mean that you don't understand the words that I used in the previous post? Are you familiar with the concept of torque?

I'm quite familiar with the concept of torque... What creates the word salad is when you use somewhat meaningless terms like "positive torque" when you mean "pitch up" or "positive pitch" or some other proper technical or Shuttle term. In the same way, it took a bit to work out that by "the rocket", you seem to mean the SRB's rather than the SSME's (both of which are rockets), etc... etc...

[architeuthis]

No, they're practically the only reason the maneuver is accomplished. That's why, by the mid-late 90's they were turning heads up sooner and sooner (even while still performing the gravity turn) - the downrange stations were being decommissioned for budgetary reasons and they needed to turn heads up to communicate via TDRSS.

I agree with you Derek.

I'm quite familiar with the concept of torque... What creates the word salad is when you use somewhat meaningless terms like "positive torque" when you mean "pitch up" or "positive pitch" or some other proper technical or Shuttle term. In the same way, it took a bit to work out that by "the rocket", you seem to mean the SRB's rather than the SSME's (both of which are rockets), etc... etc...

I think you misunderstand me. Concerning my earlier post I was less interested in the actual maneuvers of the Space Shuttle per se than I am in the simple application of kinematics to a recreated shuttle in KSP, specifically the cant of the SSMEs, which is what the original poster was talking about. As I mentioned earlier, there is no "max q", nor tracking stations in KSP, so here the main practical reasons for the Shuttle roll program don't really apply. Pitching is an angular displacement, torque is a moment of force, I didn't confuse these terms, but if you read it that I way I apologize. Reading back I agree, the post was rather jumbled. The negative, positive torque distinction only refers to the directions of the vectors, although I notice now that by the normal convention the directions I gave earlier were backwards. IRC the SSME's produce much less thrust than the SRB's so the gimballing of the SRB's are what do most of the steering until separation. Also I realize that the torque caused by gravity is small for the shuttle, still I think it is an interesting consideration. Thanks!

[rdfox]

Ironically, in KSP, I always make sure to rotate my manned spacecraft in the VAB so that they'll launch in a "proper" heads-down orientation. (For whatever reason, NASA has *always* launched with the astronaut heads-down during the gravity turn--I've never been able to find out why it was for the capsules.)

If MechJeb included the ability to have a roll program, I'd skip that and do it the way NASA does, with a roll program starting just after tower-clear into the heads-down attitude, but for now...

[architeuthis]

Ironically, in KSP, I always make sure to rotate my manned spacecraft in the VAB so that they'll launch in a "proper" heads-down orientation. (For whatever reason, NASA has *always* launched with the astronaut heads-down during the gravity turn--I've never been able to find out why it was for the capsules.)

I think this let's the astronauts keep the horizon in view as a point of reference, as opposed to the empty sky.

If MechJeb included the ability to have a roll program, I'd skip that and do it the way NASA does, with a roll program starting just after tower-clear into the heads-down attitude, but for now...

I don't use mechjeb very often but I understand that it has a custom scripting capability, autom8 I think it's called? I bet you could write a roll program script with out much difficulty:D

[MDBenson]

Perhaps launching 'heads down' also lessens the chance of blacking out at high g-forces because the head is the 'lowest' point in relation to earth's gravity?

Edited June 13, 2013 by MDBenson