Flight Question RCS in linear mode, but my ship still pitches and/or yaws

[Pithecanthropu]

From reading Orbiter.doc, I gathered that the purpose of Linear Mode is to allow you to apply small amounts of thrust from the RCS engines linearly, which you would want to do in close range maneuvers such as docking. Clearly the main engines are far too powerful for this purpose. Also, presumably, Linear Mode is supposed to be useful for small orbital adjustments such as raising or lowering the apoapsis by a few kilometers, or zeroing out a few percent of eccentricity. IIRC the RCS system in the Gemini spacecraft was actually called the "Orbital Maneuvering System".

But when I try to use the RCS in Linear Mode, I notice that my spacecraft still yaws or pitches. For example, when trying to align my orbit with that of a fueling station, I was at the ascending node of the intersection, so I positioned my ship to antinormal. I set to Linear Mode and fired the forward thrusters (Keypad 6). The relative inclination of the orbits did diminish appreciably. However, for some reason I must have swung around to retrograde, because my PeA dropped 30k, from about 232 to 190. This is using a Saturn I Single Stage To Orbit add on (SASSTO).

Is there something I need to do, when using Linear Mode, to maintain my current attitude?

 

[RisingFury]

Normal and Antinormal will always rotate your ship. That didn't come from RCS. Try putting your ship in normal or antinormal and don't touch the keyboard, then look at the stars and you'll notice that your ship rotates.

Oh and there are two reasons why you'd lose or gain PeA and ApA:
One, the normal/antinormal autopilot might not keep up with the changes of orbit.
Or two, in order to prevent the ship from spinning, the RCS jets are actually pointing towards the center of gravity... but if that means that they can also push you in undesirable direction as well.

 

[Zatnikitelman]

In Orbiter at least, burning normal and anti-normal will add or subtract some velocity along the velocity vector. How big was the plane change burn?

 

[Hielor]

What vessel?

 

[ar81]

If space station is drifting, gravity gradient could make it to rotate. So the problem is the space station, if you want to dock.

Or it could be that you have uneven engine trimming or hovers turned on.

Or it could be an autopilot that remains active.

 

[Pithecanthropu]

In Orbiter at least, burning normal and anti-normal will add or subtract some velocity along the velocity vector. How big was the plane change burn?

The planar adjustment was four degrees or so.

---------- Post added at 08:59 PM ---------- Previous post was at 08:41 PM ----------

The vessel is a Saturn SSTO, essentially a Gemini spacecraft bolted to the third stage of a Saturn V. Here's the link:

[ame="http://www.orbithangar.com/searchid.php?ID=1882"]SASSTO v1.0[/ame]

Zatnikitelman, I'm not sure I understand. I know that there's apparent rotation over the course of an orbit if you do nothing at all, but I thought that was basically inertia--i.e. the vessel holds its position relative to an observer outside the earth-vessel system, but appears to slowly rotate relative to the earth. So that type of rotation I would expect. But shouldn't the normal/antinormal buttons hold the ship in the same attitude, in the same way as Level Horizon, Prograde, and Retrograde? I'm not a physicist or engineer by any stretch, but can you explain in layman's terms why thrusting along the anti/normal would affect the speed along the existing orbit? Does it sum up to force along a vector midway between the normal and prograde? If so, I can see how it would result in a higher orbit, but less so how it could result in a lower one.

 

[the.punk]

The planar adjustment was four degrees or so.

---------- Post added at 08:59 PM ---------- Previous post was at 08:41 PM ----------

The vessel is a Saturn SSTO, essentially a Gemini spacecraft bolted to the third stage of a Saturn V. Here's the link:

SASSTO v1.0

Zatnikitelman, I'm not sure I understand. I know that there's apparent rotation over the course of an orbit if you do nothing at all, but I thought that was basically inertia--i.e. the vessel holds its position relative to an observer outside the earth-vessel system, but appears to slowly rotate relative to the earth. So that type of rotation I would expect. But shouldn't the normal/antinormal buttons hold the ship in the same attitude, in the same way as Level Horizon, Prograde, and Retrograde? I'm not a physicist or engineer by any stretch, but can you explain in layman's terms why thrusting along the anti/normal would affect the speed along the existing orbit? Does it sum up to force along a vector midway between the normal and prograde? If so, I can see how it would result in a higher orbit, but less so how it could result in a lower one.

Look what RisingFury said.
In perfect case burning to antinormal and normal don't effects the orbit eccenciary. But as you are burnig you change the plane of your orbit. And then the autopilot must follow the corrections, because the antinormal or normal point is changing. And because the autopilot in orbiter is not perfect it could that you are burning a little bit retrograde or prograde depending if you use normal or antinormal.

 

[Quick_Nick]

From reading Orbiter.doc, I gathered that the purpose of Linear Mode is to allow you to apply small amounts of thrust from the RCS engines linearly, which you would want to do in close range maneuvers such as docking. Clearly the main engines are far too powerful for this purpose. Also, presumably, Linear Mode is supposed to be useful for small orbital adjustments such as raising or lowering the apoapsis by a few kilometers, or zeroing out a few percent of eccentricity. IIRC the RCS system in the Gemini spacecraft was actually called the "Orbital Maneuvering System".

But when I try to use the RCS in Linear Mode, I notice that my spacecraft still yaws or pitches. For example, when trying to align my orbit with that of a fueling station, I was at the ascending node of the intersection, so I positioned my ship to antinormal. I set to Linear Mode and fired the forward thrusters (Keypad 6). The relative inclination of the orbits did diminish appreciably. However, for some reason I must have swung around to retrograde, because my PeA dropped 30k, from about 232 to 190. This is using a Saturn I Single Stage To Orbit add on (SASSTO).

Is there something I need to do, when using Linear Mode, to maintain my current attitude?

I don't believe your orbit will change altitude/eccentricity if you burn precisely at a node. However, you are only at a node for an instant. This means, you have to start burning before the node and finish burning about equally long after reaching the node to not affect altitude. I wouldn't be too precise with this though. I would just burn a while before the node (preferably using Align Planes MFD) and correct altitude later.

 

[Zatnikitelman]

Technically, everywhere on an Orbit is a node. The equator is nothing special, it's just an arbitrary line (ok, not really, but work with me here). If you burn AN/ON at any point in your orbit, you're changing your plane in relation to your "last" orbit last meaning your orbit instantaneously before the current moment (Orbiter: last timestep; RL: Last unit of Planck Time). Yes it's hard to visualize because we're used to looking at a sinusoidal orbit drawn on a flat map, but if you can picture a perfect sphere (I know Earth isn't one) then where you draw your equator doesn't matter.

 

[Pithecanthropu]

OK, I think I get it. The lateral burn changes the orbit plane as intended, but this causes the direction of thrust to drift from true lateral. Consequently a small component of the effective vector of thrust is now prograde or retrograde.

At least altitude is fairly easy to correct.