Flight Question basic question about MAIN ENGINE BURNS

[jinchoung]

howdy,

so i would like to nail down something that seems to be the case but i haven't encountered it codified anywhere:

in regards to main engine burns, basically, do you only do it in 4 directions? and basically, i'm thinking that mebs are for significant course changes... so they're only fired:
1. prograde
2. retrograde
3. normal+
4. normal-

?

is that correct? or is it common to do mebs in arbitrary directions?
----------------------------------------------------------------------

oh, second question!

if you fire main engines to "catch up" to something ahead of your prograde trajectory, you'll also be gaining altitude as well right?

so if it's in your orbital path but you need to catch up, what is the procedure? do you angle down and "lead" the target? or do you have to resort to an MFD computed path?

thanks much.

jin

p.s. if these are really stupid questions with answers in an faq somewhere, just notify me and i'll go a huntin'.

 

[Urwumpe]

You can do such burns in any suitable directions, but these four are the preferred directions for the basic maneuvers (Hohmann transfer, plane change)

 

[tblaxland]

:hesaid: Other "suitable directions" include as required for setting up off-plane transfers and the like but these typically require an MFD to calculate the burn for you (eg, IMFD).

As for catching up to something ahead of you, lets assume you are both starting in the same orbit but separated by some distance. What you want to do is lower your orbit (retrograde burn) to lower your orbital period. This means over the course of an orbit, the angular distance between you and the target will be reduced. Eventually you will catch up and you can raise your orbit (prograde burn) to match velocity with your target. You can "wing it" but it is highly recommended to use at least Sync MFD (or for more advanced operations, Rendezvous MFD) to get a more accurate and fuel efficient rendezvous.

 

[MJR]

howdy,

so i would like to nail down something that seems to be the case but i haven't encountered it codified anywhere:

in regards to main engine burns, basically, do you only do it in 4 directions? and basically, i'm thinking that mebs are for significant course changes... so they're only fired:
1. prograde
2. retrograde
3. normal+
4. normal-

?

is that correct? or is it common to do mebs in arbitrary directions?
----------------------------------------------------------------------

oh, second question!

if you fire main engines to "catch up" to something ahead of your prograde trajectory, you'll also be gaining altitude as well right?

so if it's in your orbital path but you need to catch up, what is the procedure? do you angle down and "lead" the target? or do you have to resort to an MFD computed path?

thanks much.

jin

p.s. if these are really stupid questions with answers in an faq somewhere, just notify me and i'll go a huntin'.

On your second question. If you prograde burn you will be incresing your altitude of your orbit. If you burn at periapsis then your apoapsis will increase. If you burn at apoapsis then your periapsis will increase. If you do it in between, your orbit is gonna mess up. Since you want to catch up to whatever, you have to sync you orbits after you adjust your periapsis or apoapsis to your target.

 

[Biscuit]

Or, as Larry Niven said it:

East takes you Out,
Out takes you West,
West takes you In,
In takes you East,
Port and Starboard bring you back.

...

http://www.larryniven.org/niven_phys.shtml

If you apply a thrust eastward (in the direction of your orbit), you put yourself in a higher orbit (you move Out)
If you apply a thrust Westward (against the direction of your orbit), you put yourself in a lower orbit (you move In)
If you apply a thrust outward, you put yourself in a slower orbit, so you move West relative to everything else in your old, faster orbit
If you apply a thrust inward, you put yourself in a faster orbit and you move East relative to everything else in your old, slower orbit

If you thrust to the left or right, you move to an orbit that is at an angle to your original orbit, but which intersects your original orbit at two points, one of which is the point at which you applied the thrust - hence you come back to where to started after half an orbit.

....

"The Integral Trees" by Larry Niven ... great book, gave me my first real understanding of orbital mechanics.

 

[agentgonzo]

Or, as Larry Niven said it:

East takes you Out,
Out takes you West,
West takes you In,
In takes you East,
Port and Starboard bring you back.

I find that more compilcated than just thinking about what happens to the orbit!

 

[Biscuit]

I find that more compilcated than just thinking about what happens to the orbit!

If you already knew how orbital mechanics work, I could see that.

For me, after reading the book, that phrase stuck with me ... I didn't know how it worked at the time, so that phrase was my first real exposure to the reality of it ... I still hear at least part of it in the back of my head whenever I reach for the 'prograde' button.

... plus, any opportunity to push Niven on some poor unsuspecting soul ... hehehe

 

[TSPenguin]

Orbital Operations teached me everything I need to know about, well... orbital operations.

 

[James.Denholm]

*snip* If you do it in between, your orbit is gonna mess up. *snip*

Just to clarify this, MJR dosen't mean that your orbit would go all spastic-messed up, more that your orbit would be affected in a less predictable way.

A rule of thumb is this: Increasing your velocity (ie, burning) will "raise" the point 180 degrees from it (ie, the other side). The path you take is all about your velocity. As far as I know, there is no way to speed up towards a target without changing your velocity.

On a side note, if you're in frout of or behind your target (like, more than... maybe 100 km?), and you're on the same orbital plane, open up your Sync Orbit MFD. If your orbit intersects your target's orbit, you can use this to time your orbit (remember, the lower your orbit, the faster you go). If your orbit dosen't intersect your target's orbit, then you've got a bit of work to do. You need to work out if yor orbit is below or above your target's orbit, even if it's only by a little bit. If it's above, burn retro-grade (only a little bit! You don't want to re-enter yet, do you?). If it's below, burn pro-grade. You'll know when you've got an intersection because Sync Orbit MFD will start spitting out two columns of numbers. Now, start fiddling around with your orbit's PeA and ApA to make the two yellow numbers as close to each other as possible.

Also, a second note: For small burns, consider using your RCS thrusters on translation mode. It's a lot easier use, as you get much higher precision.

Good luck!

 

[jinchoung]

thanks much for the detailed info fellows. hmmm, interesting...

"What you want to do is lower your orbit (retrograde burn) to lower your orbital period. This means over the course of an orbit, the angular distance between you and the target will be reduced. Eventually you will catch up and you can raise your orbit (prograde burn) to match velocity with your target."

actually this confuses me...wouldn't i just end up in the same orbit that i was in previously?

!!! ohhhhhh.... i think i got it... so the retrograde burn actually unintuitively "accelerates me" and the prograde burn raises my altitude... without decelerating me in the process (which i would think would put me in the same place i was?)... dang i thought i had it there....

thanks again and thanks also for the larry niven link! this stuff is really interesting cuz as someone who's used to aerodynamic concerns within the context of flight simulators, space is anythng and everything but "intuitive".... but that makes it a really fresh subject to learn about.

jin

 

[tblaxland]

actually this confuses me...wouldn't i just end up in the same orbit that i was in previously?

!!! ohhhhhh.... i think i got it... so the retrograde burn actually unintuitively "accelerates me" and the prograde burn raises my altitude... without decelerating me in the process (which i would think would put me in the same place i was?)... dang i thought i had it there....

I don't know that I can put it into words any better than that. The best thing I can suggest is to start a scenario, bring up the Sync MFD and try it. Doing is learning .

I also recommend having a look at the beginners tutorials here:
http://orbiter-forum.com/faq.php?faq=tutorials#faq_orbitertutorials

Go Play In Space is what I learnt from, but I've heard good things about Orbital Operations too.

For a good theoretical background that is not heavy on maths, I recommend JPL's Basics Of Spaceflight:
http://www2.jpl.nasa.gov/basics/

 

[MJR]

Just to clarify this, MJR dosen't mean that your orbit would go all spastic-messed up, more that your orbit would be affected in a less predictable way.

Thank you for clarifying for me. I should have added that.

 

[James.Denholm]

^ Thanks for the thanks.

Just to clarify something I said, jin, you're sorta got the right thinking when you ask:

so the retrograde burn actually unintuitively "accelerates me" and the prograde burn raises my altitude... without decelerating me in the process (which i would think would put me in the same place i was?)

Now guys, correct me if I'm wrong, but what you're doing when you burn retrograde is you're actually decelerating. This is what puts you onto your lower orbit - because you don't have as much velocity, the Earth's gravity affects your path more, pulling your path towards it more.

Now, just to complicate things even more, as you get closer to the Earth, and closer to your Aperasis (I apologise for my poor spelling), your speed increases, and the lower your orbit, the less your orbital period is (that's the time it takes to go around the Earth once). So, if you're trying to catch up to the ISS, you want to have a lower orbit, as it's "faster", as it's orbital period is less than the ISS's. But you still want your orbit to intersect it, remember that.

And the final important point that you need to know is that everything said here can be reversed. ie:

You accelerate, and the Earth's gravity affects your path less, and your orbit becomes higher. Because of your higher orbit, the orbital period is less. Hence, if you're actualy in frount of the ISS, you can use a higher orbit to allow it to "catch-up".

Again, orbital physics interlects, correct me if I'm wrong.

 

[Omhra]

Now guys, correct me if I'm wrong, but what you're doing when you burn retrograde is you're actually decelerating. This is what puts you onto your lower orbit - because you don't have as much velocity, the Earth's gravity affects your path more, pulling your path towards it more.

Now, just to complicate things even more, as you get closer to the Earth, and closer to your Aperasis (I apologise for my poor spelling), your speed increases, and the lower your orbit, the less your orbital period is (that's the time it takes to go around the Earth once). So, if you're trying to catch up to the ISS, you want to have a lower orbit, as it's "faster", as it's orbital period is less than the ISS's. But you still want your orbit to intersect it, remember that.

And the final important point that you need to know is that everything said here can be reversed. ie:

You accelerate, and the Earth's gravity affects your path less, and your orbit becomes higher. Because of your higher orbit, the orbital period is less. Hence, if you're actualy in frount of the ISS, you can use a higher orbit to allow it to "catch-up".

Again, orbital physics interlects, correct me if I'm wrong.

You are not wrong. Except for the higher orbit having less T.
You will make life easier for yourself if you think of it all as acceleration. All of it.
Gravity accelerates you and your engines accelerate you... all of this "over time". Acceleration can be negative or positive. Your acceleration will give you a vector.. or direction of travel.
Your Vector is affected by massive objects.
Your vector will sometimes "wrap" around a planet or celestial body and if so it will have the center of this "orb-like trans-it" in line with the center of gravity of the planet. Whatever direction you use to accelerate your ship will affect your vector, some times bloating it to a hyperbola and sometimes closing it "through" a planet. In such case put on your crash helmet.
Your vector will cover a range of speeds over time.
You alter the "range of speeds at various times" by accelerating your ship.
Your thrust or Newtons when divided by your Mass will produce your acceleration in time squared. Your fuel consumption will lower your mass over time while you engines fire.
All you have to know now is the mass of the body that governs you.
From here you can start imagining which way your vector will change if you go at any speed. As you accelerate more and more your vector wants to become a straight line but it is hopelessly curved. but you can raise that curve to intercept other bodies. You just need to know how long it takes you to get there and when they will be there and make the times match.
That is the fascination with orbiter... figuring out how to figure these things out. Making your orbits ever more efficient and elegant.

 

[Andy44]

Well, this sounds wierd, but I think of it as a sort of swastika with arrowheads:

go forward you go up
go up you go back
go back you go down
go down you go forward

 

[Linguofreak]

as someone who's used to aerodynamic concerns within the context of flight simulators, space is anythng and everything but "intuitive"....

Actually, this is very similar to what happens in a plane: If you pull back on the stick, you go up, which causes you to lose airspeed, which causes you to lose lift and start going down, which causes you to speed up, which causes you to gain lift, which eventually sends you back up.

If you pull back on the throttle, you slow down, lose lift and go down, speed up, gain lift and head up, which causes you to slow down again.

The only difference is that in space the balancing forces are centrifugal force and gravity, rather than gravity and lift.

 

[HiPotOk1978]

One thing for the newbies that may be struggling in Low Earth Orbit (LEO), In orbital mechanics EVERYTHING is done in 2 steps if you want to be symmetrical. If your orbit is 200km x 200km and you want to make it say 300km x 300km, you do one burn to raise your max altitude to 300km, and at the top of your orbit, burn again. This thread as motivated me to make some tutorials I think.... Next tag, here I come