Why does it take so long to get to the ISS, IRL?

[Brainlord Mesomorph]

Granted, KSP is a ludicrous over simplification.  But when I launch to rendezvous with something in LKO, I wait until my target vehicle is rising in the west of the KSC (just like the Sun rises but in the other direction) and when my target is just over the western horizon, I launch the rocket.

My suborbital apogee is usually a very close intercept my target vehicle, and if I plot might orbital insertion burn carefully, I can usually put my second encounter on the opposite side of Kerbin.  And with that, I’m usually a launched, and docked, within half an orbit.

Why does it take SpaceX three days to get to the ISS?

What are they doing up there?

[Nibb31]

First of all, visiting vehicles typically use low-thrust RCS engines for orbital manoeuvering, so they can't afford to do a single quick burn to adjust their orbit. They usually do several orbital adjustments to reach the destination orbit over the course of several orbits.

And real-life spacecraft carry a lot less dV than in RL (Soyuz only carries 390 m/s of dV for example, and they need a good deal of that for the deorbit manoeuver), and the actual velocities are much higher than in KSP, so they need to be much more efficient. If the dV required to rendez-vous takes a 30 min burn and each orbit is 90 minutes, it's more efficient to do 1 min burns at the right time over the course of 30 orbits than to do a single 30 min burn over one third of a single orbit.

Also, typically, in KSP, you plot an encounter where you have a large speed difference with the target, and they you do a full thrust braking burn to match velocity. In RL, they approach slowly, raising their orbit in several steps so that they are never on a collision course with the ISS. They can't risk having a large speed difference with the target, nor can they do a braking burn to match velocity, because if for some reason that burn fails, they risk a high speed collision, so the approach is always slow.

Another reason that manned vehicles used to take 3 days was because the crew had a lot of work, from prelaunch activity, to launch, on-orbit check out, reconfiguration/deployment, donning/doffing suits, performing the RV and docking, and finally opening the hatch. Nowadays, procedures have been streamlined and automated, and one-day docking is the new routine, but this puts a lot of pressure on the crew, with an extremely long working day.

Real docking/undocking manoeuvers typically occur from the nadir direction of the ISS, so that if something goes wrong, the visiting vehicle is in a slightly lower orbit and it will passively move away from the ISS. This is also used for undocking so that they don't have to use thrusters in the close vicinity of the station. If it was docking from the same altitude and suddenly became unresponsive, it could find itself on a collision course. 

 

Edited January 21, 2016 by Nibb31

[Kryten]

Also, consider what would happen if launches to the ISS were similar to your example, with intercept at initial apogee; if they lost control of the vehicle at separation, as happened to Progress not too long ago, the station would be in considerable danger. Current procedures allow for vehicles to be thoroughly checked tested on orbit well before they're on any kind of intercept course, considerably reducing this kind of risk.

[HebaruSan]

I've wondered the same thing, and my guess is that it's an abundance of caution (like anything NASA does). You want to allow time to make sure the craft is working right, you don't want it to plow right into the station if there's a problem with the launch, and you want to have the final approach take place at a very low relative velocity. So you launch into a very similar orbit but with an intercept a few days out. I would guess they probably also don't use the main engines to match orbits, so it would have to be done with low TWR maneuvering engines as well, which would put an upper limit on the approach velocity.

The wikipedia page for "space rendezvous" in general lists two kinds of "drift orbit" as the first phases of a rendezvous, each taking a few days, for what that's worth. That doesn't give us the why, but it does indicate that it's not specific to SpaceX or the ISS.

https://en.wikipedia.org/wiki/Space_rendezvous#Rendezvous_phases

(Edit: Apologies for the now-redundant post. It took me longer to write that than I thought.)

Edited January 21, 2016 by HebaruSan

[KerbonautInTraining]

5 hours ago, Nibb31 said:

First of all, visiting vehicles typically use low-thrust RCS engines for orbital manoeuvering, so they can't afford to do a single quick burn to adjust their orbit. They usually do several orbital adjustments to reach the destination orbit over the course of several orbits.

[Snip]

If the dV required to rendez-vous takes a 30 min burn and each orbit is 90 minutes, it's more efficient to do 1 min burns at the right time over the course of 30 orbits than to do a single 30 min burn over one third of a single orbit.

Granted, orbital maneuvering systems are low thrust, but not that low. According to an addon I played with in Orbiter, the Soyuz can accelerate at a crushing 0.05 G's. They burn over the course of many orbits not because they have to but, as others have said, it gives more time for decision making.

[Frybert]

IRL things can malfunction. To account for this things are placed on a path to where things have to work perfectly before there is an intercept.

[Motokid600]

Because "launch to rendezvous" isn't a thing in real life. It's too dangerous. Atleast in LEO. But I'll tell you right now. I conduct my rendezvous in KSP like they do in real life and it's much easier. Its slower, but much, much easier. Especially if your trying to dock a large craft with little maneuverability. It's easier to launch into a lower chaser orbit behind the target. Then as you catch up gradually raise your orbit.

Do it right and you barely have to make any translation/speed adjustments at all. Launch to rendezvous should only be done when time is a factor. Otherwise your just making it too hard on yourself.

Edited January 21, 2016 by Motokid600

[benzman]

These two links will take you to two fascinating articles that will tell you everything that you want to know.:-

Read first:-

http://www.baen.com/rendezvous.asp

Read second:-

http://www.baen.com/rendezvous-part2.asp

 

[Brainlord Mesomorph]

OK, I think I get this.

IRL, Earth is bigger, orbits take longer, spaceships don’t have nearly the dV that we do, and IRL doesn’t get to the “revert to launch” button, so they have  these “safety procedures” and “abort procedures” that the average KSP player (me) is oblivious to. 

And speaking of safety procedures, I recently did have the orbital collision that, at the time, I blamed on poor piloting skills and poor vehicle design.  But now I realize that this high-speed intercept of mine was probably equally to blame.  Funny story:

I get a contract to build my second Moon base.  This was a little larger and heavier than Moonbase Alpha, but I put it on the same rocket.  Doesn’t quite get to orbit, and they burned a significant amount of lunar transfer fuel just getting the thing to orbit. So I send a (hastily assembled) refuel ship after it.  For some reason I put the docking port on the side, in the middle, there really wasn’t any place else.

I launch, attain orbit, and am coming in to dock.  Everything is going smoothly, we're coming straight, docking port aligned to docking port, when in the last 50 m, I suddenly realize that neither of these ships has enough RCS to slow down!

So instead of docking, the Moonbase T-bones the fuel ship at about 10 m/s.  Nothing explodes, no one is killed, but the fuel tanker just breaks up into a cloud of debris!

I have since sent a second taker and towed the Moonbase to the Moon, and I have dispatched the Orbital Sanitation Department to clean up the debris.

But clearly I need to learn more about this “approaching from the nadir” that someone mentioned.

Here I thought this was a non KSP topic, and I learned something about KSP!

Edited January 21, 2016 by Brainlord Mesomorph

[RainDreamer]

Lack of a save/load function IRL also dissuade people from rushing things and ruining stuff too.

[fredinno]

8 hours ago, Nibb31 said:

First of all, visiting vehicles typically use low-thrust RCS engines for orbital manoeuvering, so they can't afford to do a single quick burn to adjust their orbit. They usually do several orbital adjustments to reach the destination orbit over the course of several orbits.

And real-life spacecraft carry a lot less dV than in RL (Soyuz only carries 390 m/s of dV for example, and they need a good deal of that for the deorbit manoeuver), and the actual velocities are much higher than in KSP, so they need to be much more efficient. If the dV required to rendez-vous takes a 30 min burn and each orbit is 90 minutes, it's more efficient to do 1 min burns at the right time over the course of 30 orbits than to do a single 30 min burn over one third of a single orbit.

Also, typically, in KSP, you plot an encounter where you have a large speed difference with the target, and they you do a full thrust braking burn to match velocity. In RL, they approach slowly, raising their orbit in several steps so that they are never on a collision course with the ISS. They can't risk having a large speed difference with the target, nor can they do a braking burn to match velocity, because if for some reason that burn fails, they risk a high speed collision, so the approach is always slow.

Another reason that manned vehicles used to take 3 days was because the crew had a lot of work, from prelaunch activity, to launch, on-orbit check out, reconfiguration/deployment, donning/doffing suits, performing the RV and docking, and finally opening the hatch. Nowadays, procedures have been streamlined and automated, and one-day docking is the new routine, but this puts a lot of pressure on the crew, with an extremely long working day.

Real docking/undocking manoeuvers typically occur from the nadir direction of the ISS, so that if something goes wrong, the visiting vehicle is in a slightly lower orbit and it will passively move away from the ISS. This is also used for undocking so that they don't have to use thrusters in the close vicinity of the station. If it was docking from the same altitude and suddenly became unresponsive, it could find itself on a collision course. 

 

Now, they can take only 6 hours

[Trann]

I was pleasantly surprised that the first response was also the best, fact-laden, and written by a space monkey.  As it should have been.

Edited January 21, 2016 by Trann

[Hysterrics]

Well, it's real life. Like you said, in KSP everything is simplified. Ideally the Soyuz will take 6 hours to reach the ISS, but sometimes it can take 2 days.

[mikegarrison]

Launch to rendezvous is a thing in RL. Gemini 11 did it.

But to add to all the other (valid) reasons given here why it takes time to rendezvous with the ISS, there are also restrictions against firing any thrusters that might impinge against the ISS. So they are very limited in what direction they can approach the ISS from. The whole thing is very, very delicate compared to how it is done in KSP.

[Brainlord Mesomorph]

17 hours ago, benzman said:

These two links will take you to two fascinating articles that will tell you everything that you want to know.:-

Read first:-

http://www.baen.com/rendezvous.asp

Read second:-

http://www.baen.com/rendezvous-part2.asp

 

 

*&*%&%$*!!

Yesterday those links worked. TL/DR, was going to read them later 

Today :404 errors!

9 hours ago, mikegarrison said:

The whole thing is very, very delicate compared to how it is done in KSP.

Just like the LEM!

[cantab]

 

On 21/01/2016 at 11:03 AM, Brainlord Mesomorph said:

Granted, KSP is a ludicrous over simplification.  But when I launch to rendezvous with something in LKO, I wait until my target vehicle is rising in the west of the KSC (just like the Sun rises but in the other direction) and when my target is just over the western horizon, I launch the rocket.

In real life we can't do that. The ISS is in an inclined orbit and the launch time is determined by that inclination, so we can't also be choosy about exactly where the ISS is round its orbit. Some spacecraft have some wiggle room, the Space Shuttle had about a ten-minute launch window when going to the ISS, but SpaceX are working with an instantaneous launch window - it either takes off on the exact second or it waits until tomorrow. It's down partly to how much delta-V the launcher has but also to the guidance and procedures.

There still are constraints on the ISS's position, in order that the phasing orbits don't take too long, but this I believe is another major reason we don't do "direct orbital rendezvous" in real life.

[HebaruSan]

3 hours ago, Brainlord Mesomorph said:

*&*%&%$*!!

Yesterday those links worked. TL/DR, was going to read them later 

Today :404 errors!

Maybe these will work a while longer:

http://webcache.googleusercontent.com/search?q=cache:r95N3tAUSxEJ:www.baen.com/rendezvous.asp+&cd=2&hl=en&ct=clnk&gl=us

http://webcache.googleusercontent.com/search?q=cache:q3ebIXg-Hi4J:www.baen.com/rendezvous-part2.asp+&cd=1&hl=en&ct=clnk&gl=us

The title is "Rendezvous and Docking: A User’s Guide for Non Rocket Scientists" by Terry Burlison in case you need to google it again (I still have part 2 open in a tab).

[Brainlord Mesomorph]

ty!

[benzman]

Just now, Brainlord Mesomorph said:

 

*&*%&%$*!!

Yesterday those links worked. TL/DR, was going to read them later 

Today :404 errors!

 

Well, that is a major bummer!  I have had those links bookmarked for a year or more and the very day after I post them they vanish?  I must have done something really bad in a previous life.

 

[Elthy]

What would you need to develop to get the current time of min. 6 hours down? This could be important in the distant future when it comes to commercial travel to space, you need less live-support for the passengers and they dont have to sit that long in a tin-can...

[Nibb31]

Read the thread. It's not that it can't be done. It's that we don't want to take less time.

[Elthy]

I read the thread... Im asking if there could be any improvement in technology that could keep the safety up while bringin the transfer time down.