ISS orbit boost.

[Genius]

How do they boost the ISS into an higher orbit. I learned that there is some factors that cause the ISS to loose altitude with time.
I know it is with the shuttle, but it is getting more mass all the time. And will the delicate Solar panels not snap off as it getting it's boost? And what about the center of mass. Is the docking ports in the center?

 

[Suzy]

They usually use whatever Progress ship is docked to the ISS, or the Space Shuttle if it's visiting. The reboost is done very slowly so it doesn't put stress on the panels.

 

[ryan]

I think they wouldnt use the shuttle, the orbiter is low on fuel as it is. Though the ATV Jules Verne boosted its orbit before its undocking.

 

[SiberianTiger]

How do they boost the ISS into an higher orbit. I learned that there is some factors that cause the ISS to loose altitude with time.
I know it is with the shuttle, but it is getting more mass all the time. And will the delicate Solar panels not snap off as it getting it's boost? And what about the center of mass. Is the docking ports in the center?

There are several ways to do an orbit reboost for the ISS. The most frequently used is having the Progress docked to the Zvezda (SM) aft port to burn its main engines for a length of several minutes. Its attitude control jets take responsibility for keeping the station in the correct attitude in pitch and yaw channels. Meanwhile, the vehicle docked to the Pirs (SO-1) nadir port is kept in active state with its jets ready to keep the station in the right roll attitude.

The orbit boosting burn can also be performed by the main Zvezda engines, by the ATV engines if one docked to the same port, or by the Shuttle's OMS engines - but this is a bit different procedure, as it takes the Zvezda to keep the right station attitude with its RCS.

 

[Urwumpe]

The orbit boosting burn can also be performed by the main Zvezda engines, by the ATV engines if one docked to the same port, or by the Shuttle's OMS engines - but this is a bit different procedure, as it takes the Zvezda to keep the right station attitude with its RCS.

Not the OMS, the RCS. The OMS would make the ISS spin as it is orthogonal to the center of mass.

 

[SiberianTiger]

Not the OMS, the RCS. The OMS would make the ISS spin as it is orthogonal to the center of mass.

Oh, well. 6 kN for the double Zvezda engines vs 27 kN for the OMS... I believe you are right - unless they can throttle the OMS down a bit.

 

[Urwumpe]

Oh, well. 6 kN for the double Zvezda engines vs 27 kN for the OMS... I believe you are right - unless they can throttle the OMS down a bit.

Has nothing at all to do with the thrust magnitude, but rather with the thrust vector.

Torque = F x r

Which means: The torque is maximal, when the angle between the vectors F and r is 90°.

It would simply overload the attitude control system, if the OMS gets fired while docked. And the power of a single OMS engine is already enough for making the ISS spin out of control.

The reboost with the Shuttle uses the primary RCS thrusters of the Shuttle, mostly the +Y thrusters (--D)

 

[SiberianTiger]

Just forgot for a moment that the shuttle's aft looks sidewards when it's docked.

 

[Urwumpe]

Just forgot for a moment that the shuttle's aft looks sidewards when it's docked.

Thank god, this didn't happen to the shuttle astronauts during the last reboost.

If I remember correctly, the small RCS thrusters on the progress had been already enough for making Mir tumble so destruction was ensured.

 

[SiberianTiger]

If I remember correctly, the small RCS thrusters on the progress had been already enough for making Mir tumble so destruction was ensured.

BTW, what's the greatest risk in such a case? I think that would be appearing of gaps in between docking ports' surfaces, due to inertia forces overcoming the tension on hard docking hooks.

 

[Urwumpe]

BTW, what's the greatest risk in such a case? I think that would be appearing of gaps in between docking ports' surfaces, due to inertia forces overcoming the tension on hard docking hooks.

That would be the smallest concern as this appears already pretty late in the damage chain. First you would have the risk of the docking port hooks jamming (No undocking or limited undocking). Then, you would have additional stress on the structures of the modules. The weakest spot for this scenario would be the interface between Russian and US segment, as it has the highest loads. If the seal breaks there, the station is divided into two halfs. Also, the truss structure should have some huge additional torsion, don't know how this would like it.

When gaps appear in the docking port interfaces, it is already too late, it might be even possible at this point, that even cutting the thrusters would not fix the seal anymore.

 

[tblaxland]

The reboost with the Shuttle uses the primary RCS thrusters of the Shuttle, mostly the +Y thrusters (--D)

+Y thrusters in Orbiter Shuttle coordinates, -Z in real life Shuttle coordinates

 

[Urwumpe]

+Y thrusters in Orbiter Shuttle coordinates, -Z in real life Shuttle coordinates

Argh. I wasn't sure, I only remembered X being orbiters Z axis.

 

[garyw]

The shuttle thrusts "downward" (from it's local perspective). Basically, the Russian segment points into the velocity vector.

STS-126 performed a small reboost of the ISS whilst docked. The vernier thrusters were pulsed for about 36 mintues to give a 3FPS boost.

 

[Urwumpe]

STS-126 performed a small reboost of the ISS whilst docked. The vernier thrusters were pulsed for about 36 mintues to give a 3FPS boost.

Not the vernier thrusters. The term vernier is reserved to the smallest RCS thrusters, which can only be used for attitude changes - the 6 thrusters are not placed for translation.

The translation is done by the Primary RCS, but the time for the 3 fps is correct - the Shuttle has the ISS on it's back.

 

[tblaxland]

The shuttle thrusts "downward" (from it's local perspective).

Huh? The exhaust goes downward (+Z) but the thrust is upward (-Z). If it thrusted downward it would spray the ISS with exhaust.

 

[Urwumpe]

Huh? The exhaust goes downward (+Z) but the thrust is upward (-Z). If it thrusted downward it would spray the ISS with exhaust.

No, the exhaust velocity vector is used in the Shuttle for giving the directions: D = downward means, the exhaust goes down. U = upwards means, the exhaust goes up, etc.

 

[garyw]

Opps to both of you. Yes, you are both correct.

Next thing to learn is the axis of the craft....... ;-)

 

[Urwumpe]

Next thing to learn is the axis of the craft....... ;-)

Isn't there any rap as memory aid, which you can quickly toast to remember it? :lol:

After all, during my robotics lab time, you regularly saw me making strange gang signs with my hands, when I was busy converting coordinate systems.

 

[tblaxland]

Isn't there any rap as memory aid, which you can quickly toast to remember it? :lol:

After all, during my robotics lab time, you regularly saw me making strange gang signs with my hands, when I was busy converting coordinate systems.

Hehe, converting between RL shuttle and Orbiter coordinates looks a lot like :werd:. On occasion of coding AttitudeMFD, I found myself wishing I had two left hands

Anyway, the coordinates are right-handed for RL and left-handed for Orbiter and aligned thus:

[FONT=Courier New]
Shuttle | Orbiter
----------------
   +X   |   +Z (forward)
   +Y   |   +X (port)
   +Z   |   -Y (down)[/FONT]