Flight Question Launch window. When? and why?

[rcd463]

Hello.

I am a new member in this forum and I had been reading a lot of post and tutorials before to ask this question but I didn't find the answer.

Basically the launch window to reach the ISS is when the orbital plane is close to the launch site and the azimuth is about 43º or 140º from Cape Canaveral. The problem for me is when to launch. Yes, about 350 sec. marked in the Align MFD, but, why this time? Is the ISS position ahead in the orbit important for launch? What could be the time window for another target?

I know it is possible to use the Launch MFD for more accurate launch but I would like to understand this before to study more MFDs.

Thank you.

 

[garyw]

Your launch window is all dependant on your propellant an energy. If you launch a shuttle it's a narrow launch window because you have to launch in plane (i.e. when the track of the ISS is over KSC) otherwise you don't have enough propellant for a plane change.

A delta-glider has enough delta-v that it doesn't really matter.

 

[vchamp]

I'll try to explain. The position of ISS in orbit doesn't matter during launch as long as it is not at the opposite side of Earth or very far away, if you don't want to spend too much time synchronizing orbits. When to launch depends only on your distance to the orbital plane of ISS, because synchronizing with target is much more fuel efficient than aligning planes.

Hope it helps and I didn't mess anything. :tiphat:

 

[Tommy]

You want to leave a bit before the actual node, 300 to 350 seconds, because it takes some time to get into orbit. During the early part of your ascent, you won't be precessing the same way the ISS is, but after a few minutes, as your velocity increases, you will start to precess the same as the ISS. The actual "best" time varies a bit with the vessel - the faster it accelerates the less "lead time" you need.

 

[SandroSalgueiro]

In the Shuttle [for example], a climb to orbit takes from 8min to 10min -- that is, from 480s to 600s. Departing when the Time to Node (Tn; point where your position on the ground intersects ISS's orbit) is 350s (in this case, ISS will pass your position before you reach orbit) will make you enter orbit behind the station but not far from it. As you may have already realized, such thumb rule is used as a way to reduce the necessary amount of time to rendevouz with the target.

The most important thing, however, is to make sure you enter orbit with roughly the same inclination as your target, hence creating a launch window.

 

[Moach]

this is because 350 seconds is a roughly fair "guess" of how long it takes you to accelerate into the same orbit as the station (with the stock DG, i believe)...

remember, the earth is spinning under the orbital plane of the ISS, so it crosses over different locations on the ground over different times of the day


wanna see it? - open up the map MFD and tarrget the station while on the ground (make sure you set the display to "orbital plane" not "ground track") - accelerate time to 100 or so and watch how the track "crawls" over the globe towards the west

this is because the earth is spinning eastwards


so 350 seconds is an estimated "lookahead" time, to keep things in check with the time you'll spend "on the way up", throughout which the station's plane ain't exactly sitting still waiting for you to reach it

different craft will require different timing - e.g. a G42-200 takes almost twice as long to reach orbit as the space shuttle or the DG do... or with a HARP gun or similar, it's a LOT faster (as long as you don't expect stuff to make it there "alive")

 

[PeriapsisPrograde]

(as long as you don't expect stuff to make it there "alive")

Pfft, who wants to go to space alive?

 

[rcd463]

I didn´t check the plane option in the Map MFD and I was using only the path mode. Thats why I couldn't understand the numbers.

I am going to start to practice checking the time to climb and using it for a better launch.

Thank you for your help!

 

[N_Molson]

You want to leave a bit before the actual node, 300 to 350 seconds

I've read something about this in a book written by a teacher at the CNES. This time delay is "time to half velocity". It's the time your vehicle need to accelerate to the half of the velocity it will have once in orbit (at a given altitude). So, for a vehicle that needs 600 seconds to reach orbit, 300 seconds seem to be the right delay.

But it is more complex than that in reality, because most vehicles (especially the ones with boosters) have an acceleration that can greatly vary during the ascent. If there is a way to calculate that, I'd like to know, that's interesting stuff.