SyncMFD Required orbit dips into the atmosphere, and dealing with circular orbits

[Star sailer]

Imagine this scenario: In order to get DTmin to 0 you must do a retrograde burn but this burn dips your orbit into the atmosphere. What do you do? This is for a rendezvous w/ a station that is at about 200 kilometers above the surface of Earth. Im in a space shuttle, so limited fuel is available

 

[Shifty]

Is this for rendezvous? What's the situation?

 

[Cras]

Don't do it.

If you are trying to rendezvous, and you are getting a solution that requires you to burn retrograde and lower your perigee so low that you re-enter the atmosphere, you are doing something very very very wrong.

What exactly are you trying to do that is giving you this?

 

[Star sailer]

Don't do it.

If you are trying to rendezvous, and you are getting a solution that requires you to burn retrograde and lower your perigee so low that you re-enter the atmosphere, you are doing something very very very wrong.

What exactly are you trying to do that is giving you this?

Okay so I have my apoaopsis and periapsis. When my target passes over my periapsis I check its altitude. Then I go to my apoapsis and do a burn to equalize my periapsis altitude w/ target altitude at that time. Planes are aligned already. Then I go to my periapsis and it just happens Im required to do a retrograde burn to lower DTmin to 0. This burn dips the orbit of my ship into atmosphere.

 

[Shifty]

If you end up slightly clockwise from the station (the station is "in front" of your vessel), I'd think you burn prograde and orbit for a long time. If you push your apogee out to 1000km, you can lose about 35 degrees of arc (relative to the target) per orbit. Then just pull it back in with a retro burn at perigee when you start to get closer

 

[n122vu]

In this situation, the only safe solution I see is to burn prograde until your apoapsis is much higher and your orbit is significantly more eccentric than your target. Eventually DTmin will start going back down. Not the most efficient, especially since you will essentially be rendezvousing at or near your periapsis and will likely be going much faster than your target, requiring another significant burn for rendezvous. But at least the ship will be intact...


**EDIT** :ninja: 'd!

 

[Star sailer]

In this situation, the only safe solution I see is to burn prograde until your apoapsis is much higher and your orbit is significantly more eccentric than your target. Eventually DTmin will start going back down. Not the most efficient, especially since you will essentially be rendezvousing at or near your periapsis and will likely be going much faster than your target, requiring another significant burn for rendezvous. But at least the ship will be intact...


**EDIT** :ninja: 'd!

Im in a space shuttle so I have limited fuel

 

[garyw]

Shuttle tends to fly to the ISS which is around the 350km mark. That means it can fly into an orbit below the ISS and catch up.

You can still fly higher than your target, even an orbit 10km higher than your target means that your target will catch up with you.

 

[Shifty]

If you increase your apogee by 100 km, which should be a very quick burn, you'll close by about 4 degrees per orbit. Could take a really long time, but you'll get there eventually.

 

[RisingFury]

Don't burn all the way to into the atmosphere. Only lower your pariapsis to a save altitude, then wait. You'll catch up with the station a few orbits later. It'll take more time. Then later on you can again adjust your periapsis, but you won't have to adjust it much.

Another trick is to not adjust the orbit to exactly DTmin = 0. It can be off by a few seconds for a successful rendezvous, but you'll have more work later in closing in. Instead of bringing you to within 1 km or so as it would on an accurate intercept, it'll bring you to within 50 km from the target.

 

[Star sailer]

Don't burn all the way to into the atmosphere. Only lower your pariapsis to a save altitude, then wait. You'll catch up with the station a few orbits later. It'll take more time. Then later on you can again adjust your periapsis, but you won't have to adjust it much.

Another trick is to not adjust the orbit to exactly DTmin = 0. It can be off by a few seconds for a successful rendezvous, but you'll have more work later in closing in. Instead of bringing you to within 1 km or so as it would on an accurate intercept, it'll bring you to within 50 km from the target.

Thanks it worked! I didnt do the burn all the way but orbit after orbit, the DTmin and my distance from target shrank. Is it normal for the number of orbits remaining to stay at 4? After a while it went down 3,2,1,0 orbits remaining.

 

[Cras]

you can increase the number of orbits forward SyncMFD displays.

And also...why are you looking to rendezvous at target perigee? It is more proper, and a better idea in general, to look to rendezvous at target apogee.

 

[Star sailer]

you can increase the number of orbits forward SyncMFD displays.

And also...why are you looking to rendezvous at target perigee? It is more proper, and a better idea in general, to look to rendezvous at target apogee.

I didnt think it really mattered at the time, but now I just realized that when I did the retrograde burn, my perigee and apogee flipped so technically I did the rendezvoue at my apogee.

 

[blixel]

Imagine this scenario: In order to get DTmin to 0 you must do a retrograde burn but this burn dips your orbit into the atmosphere. What do you do?

I remember running into this problem when I was first learning to rendezvous. In my case, the problem was that the object I was trying to rendezvous with (the ISS) was pretty much on the other side of the planet. Even when I set Sync Orbit to display 18 orbits I found that I still needed such a large change to my DTmin that it wasn't workable. (See image below)

What I eventually came to realize was that I should simply use time warp to close the distance between my vessel and the ISS - with no regard to Sync Orbit MFD until I was within at least 1/4 of an orbit from the target. (See image below)

Once you are within a 1/4 of an orbit from the target, you should be able to use Sync Orbit MFD without having to worry about dipping into the atmosphere. Also keep in mind that the closer your orbit matches your target, the closer you can get before having to set up Sync Orbit MFD. In these image examples, my ship's total time for one orbit (T) is 5,433 seconds. My target's T is 5,501 seconds, so our orbits are pretty closely matched already. This means I can get quite a bit closer to the target before setting up Sync Orbit MFD. (See last image below.)

 

[Shifty]

In these image examples, my ship's total time for one orbit (T) is 5,433 seconds. My target's T is 5,501 seconds, so our orbits are pretty closely matched already. This means I can get quite a bit closer to the target before setting up Sync Orbit MFD. (See last image below.)

But this means you're only gaining 68 seconds on the target per orbit. That's about 4.5 degrees per orbit. If you're 180 degrees out of phase, it'll take 40 orbits to catch up. And even at 100x time acceleration, that's 36 minutes in real time of just waiting to get close. (2.5 days of orbiting for your poor astronauts.) If you're 180 degrees out of phase, you should push your apogee out so that you have some relative phase velocity. You can always pull it back in when you're closer if you want to approach at low relative speed.

 

[garyw]

Shuttle took around 2.5 days to catch up. That gave those poor astronauts time to configure the shuttle for docking, check the heat shield, get some sleep, etc.

 

[Shifty]

Roger that. And Soyuz takes about 2 days as well. I had originally written the post for a full 355 degree catch-up, which was what was proposed by the OP. I edited it to match blixel's example. Should have removed the 'poor astronaut' comment when the time was reduced from 5 days to 2.5.

 

[garyw]

Well, Soyuz generally now does a 'fast rendezvous' of just a few hours and it's a lot of work but the astronauts/cosmonauts prefer that to the cramped Soyuz.

 

[Cras]

But this means you're only gaining 68 seconds on the target per orbit. That's about 4.5 degrees per orbit. If you're 180 degrees out of phase, it'll take 40 orbits to catch up. And even at 100x time acceleration, that's 36 minutes in real time of just waiting to get close. (2.5 days of orbiting for your poor astronauts.) If you're 180 degrees out of phase, you should push your apogee out so that you have some relative phase velocity. You can always pull it back in when you're closer if you want to approach at low relative speed.

depends on how quick and realistic you wanna be.

Part of finding your proper launch window is to make sure you dont find yourself in this position where you gotta orbit the earth for a week before you are in a position to rendezvous.

If you are looking to fly the Shuttle to the ISS, liftoff in plane when the station is somewhere over the south pacific. From there you should be able to space it out for about a 2 day catch up.

If you want to rendezvous right away, launch in plane when the station is right in front, somewhere over greenland.

If you really want to get there fast, use Launch MFD and do a direct ascent.