Heavy ATV Must Learn to Apply the Brakes Before Docking with the ISS
Written by Ian O'Neill
Although ESA's Automated Transfer Vehicle (ATV) will be approaching the International Space Station (ISS) at a rate slower than tortoise-pace, what would happen if the 20 tonne space truck didn't slow down as it docks with the station? It wouldn't be pretty. In all likelihood, the large mass of supplies and metal would cause significant structural damage to the ISS and could be life-threatening to the astronauts on board. To avoid a very big dent in the manned outpost, the ESA's partners insist that the ATV carry out some practice runs of the Collision Avoidance Manoeuvre (a.k.a. the "emergency brake")…
The ATV "Jules Verne", still sitting in an orbital holding pattern awaiting the departure of Space Shuttle Endeavour from the ISS, still must prove its robotic worth. The unmanned supply vehicle is the most advanced spaceship the ESA has ever launched into space and it appears to be performing well. Recent engine problems were quickly and neatly solved and the re-supply mission of the ISS appears to be progressing nicely.
Worked into the schedule of the ATV's orbit of Earth are some practice manoeuvres – after all, the robot has a lot of time on its hands, a bit of activity should be welcomed.
First up is the spaceship equivalent of an emergency brake. The ATV project will have never been allowed near the space station without an emergency procedure should there be a problem during docking. Although the relative speed between the station and approaching ATV will be exceedingly slow, the orbital velocity of both will be approximately 27,000 km/h, so any unforeseen collision or misalignment could be highly dangerous.
So, the Collision Avoidance Manoeuvre will be carried out on Friday, before the ATV is anywhere close to the station to make sure the operation is successful at preventing a mock collision.
The ATV carries countless failsafe measures; critically the robot runs three parallel flight-control computers with an independent computer overseeing them. If something should go wrong, the flight-control computers can be overridden and an avoidance manoeuvre enacted. Also, mission control in Toulouse, France can manually initiate the Collision Avoidance Manoeuvre and so can the ISS astronauts inside the docking module watching events as they unfold. A big red button has even been installed in the Russian Zvezda module to raise the alarm and force the ATV to stop and reverse at 5 km/h.
Source: BBC
The astronauts do it by hitting a big red button on a panel positioned in the Russian Zvezda module.
Filed under: Space Flight, Space Station
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March 16th, 2008 at 5:40 am
The orbital velocity of the ATV and the station has *nothing* to do with how dangerous or otherwise the docking manoeuvre is. If that were true, the poking someone with your little finger would be incredibly dangerous thanks to the combined velocity of the Earth, the Sun, the local neighbourhood, the Orion Arm, the Milky Way, etcetera etcetera.
It is only relative velocity that matters, and the relative velocity during docking—because the people controlling it are not insane—will be very low.
The reason that the manoeuvre is dangerous is that the ATV masses 20 tonnes (according to the article), and therefore has not insignificant momentum even at low velocities. As a result, it could easily do significant damage to the station if something went wrong.
March 16th, 2008 at 7:09 am
Agreed with the commenter above. Not sure why orbital velocity was mentioned in the post. The real threat is that something weighing 20 tons can do a lot of damage even if it hits 'you' at 2 km/h, especially if 'you' (the station) weigh a lot more.
March 16th, 2008 at 12:13 pm
To alastair and Ted:
Although the relative speed between the station and approaching ATV will be exceedingly slow, the orbital velocity of both will be approximately 27,000 km/h, so any unforeseen collision or misalignment could be highly dangerous.
I am fully aware that the relative speed is the important thing, and not the orbital speed. I added the orbital velocity of the station as it gives a clue as to how extreme this docking procedure will be.
After all, flying 27,000 mph is a pretty cool thing to mention
Cheers, Ian
March 16th, 2008 at 5:40 pm
But Ian, there is no other way to read your sentence without the implication that the orbital velocity is what makes the docking maneuver dangerous. While the 27,000 mph orbital velocity is indeed cool, it doesn't belong in that sentence.
Also, these objects are not "flying" either.
March 16th, 2008 at 7:18 pm
Ever heard of the term "Space Flight"?
March 16th, 2008 at 7:19 pm
Actually, "falling" around the Earth would be more correct.
March 16th, 2008 at 10:21 pm
I don't know, but if I was falling at terminal velocity in Earth's atmosphere, and a partner also falling at terminal velocity, but at a small velocity relative to me, failed to transfer the parachute to me, the results would certainly be horrific in spite of the slow relative velocities, and amplified by the Earth-centered velocities.
Huge velocities make everything more difficult, even between objects moving at small velocities relative to each other.
I used the word "velocity" way too many times, but I'm pressed for time time, and have to type at a considerable velocity (yes, typing has no directional vector, and as such cannot be called a velocity).
March 16th, 2008 at 11:40 pm
Well, ok, perhaps "momentum" should be dropped into the sentence rather than the "orbital velocity":
Although the relative speed between the station and approaching ATV will be exceedingly slow, the 20 tonne spacecraft momentum will be very high, so any unforeseen collision or misalignment could be highly dangerous.
But as pointed out by autumn, I'm travelling at high velocity and don't have the time to correct it.
Regardless, I think I emphisize the point that docking in space can be a bit risky
Cheers, Ian
March 17th, 2008 at 12:45 am
I learned, that bodies fall around the earth on an elliptic path because they have the exact velocity that is linked to this path by the laws of Johannes Kepler.
Now, if the ATV has to break and reverse at 5 km/h due to an incident during docking manouver, does this mean, that it does no longer fit into the path of the ISS? So, does it loose heigth and follow a lower orbit immediatelly?
March 17th, 2008 at 4:19 am
If the ATV applied thrust purely in the reverse direction then initially it would move away from the ISS along what was their common (roughly) circular orbit. The ATV would now however be at apogee in a new elliptical orbit and so it would start to lose height and in the process gain speed and so it would move below and ahead of the ISS. It would be the same as the first burn in a Hohmann transfer. However there is no reason why the ATV should carry out such a simple manoeuvre and so there is no reason why the collision avoidance manoeuvre should not reposition the ATV behind the ISS at a safe distance but on the same orbital path. Take a look at the ESA website for details. http://www.esa.int
March 17th, 2008 at 6:33 am
Once again, poor planning on somebody's part. What's wrong with an 'external' docking module that momics and extends the normal dock by only allowing a dock when detached from the SS? The docking module can then be positioned to the SS with the ATV attached.
March 17th, 2008 at 7:49 am
It really struck me how this image is eerily similar the artistic work that used to adorn the pages of science fiction works such as Asimovs and other authors short stories from the 70's.
Unfortunately it also reminded me of the images that were utilized in the 80's Star Wars program under Reagan.
Jamie
March 17th, 2008 at 11:18 am
Autumn said:
"I don't know, but if I was falling at terminal velocity in Earth's atmosphere, and a partner also falling at terminal velocity, but at a small velocity relative to me, failed to transfer the parachute to me, the results would certainly be horrific in spite of the slow relative velocities, and amplified by the Earth-centered velocities."
Your example of a parachute transfer is a bad example because your downward motion will eventually end with contact with the ground. You and your partner are likely to have different terminal velocities which will tend to move you apart. Variations in air resistance will also, on average, tend to move you apart. Falling at terminal velocity through an atmosphere is NOTHING like traveling at similar velocities through a vacuum/space.
To put it another way, do you have trouble parking your car in parking space because (at the equator) the earth surface is traveling at 1000 mph due to its rotation?
Autumn said:
"Huge velocities make everything more difficult, even between objects moving at small velocities relative to each other."
Actually they don't. That is the whole point. Earth and everything in orbit around travels more than 2 million kilometer per hour as the solar system is dragged around the Milky Way. And it has absolutely no discernible effect on any of these space docking maneuvers. The only things things that count are the relative velocities of the two objects, the mass, and the physics of orbital mechanics.