|
Lesson 4 - Ships that pass in the Night
Hopefully you have managed to shape your orbit to match that of SH-03 -
Our next task is to syncronise our orbit with SH-03, and dock with it. In
this part I'll give you some general synchronisation techniques at the end
of the document. Reading them is not neccessary to accomplish this part but
I it's always good to know how to synchronise in general situation, not just
this particular one. Addon used for the optional tips is
Arrive MFD.
You must download
This save state or its
EXE alternative. It had to be a only bit modified. Particularly I had to
add a radio trasmitter to SH-03 which will help us during docking.
We are now orbting around the Moon, chasing SH-03 almost exactly - but
how do we catch up? The first intuitve impulse is to do a prograde burn -
after all, we want to go faster so we catch up, right? But let's examine
what happens if you do this. Recall from your flight so far that a prograde
burn raises your orbit - giving you a longer path to fly, and slowing you
down. If we do a prograde burn we'll actually fall further behind!
What we need to do is take a short cut around the Moon - we need to fly a
shorter path, and fly it quicker...
...a retrograde burn.
 | Open SyncMFD in the left hand MFD and target SH-03 |
| Prepare for the upcoming manouver by engaging the retrograde autopilot.
|
SyncMFD shows our orbit and that of SH-03, the current postions of both
craft (our position as a dull green radius, and SH-03's as a yellow radius).
The bright green radius is the reference line. The reference line is the
best place to change our arrive times by doing pro- retro- grade burns,
because only burning there won't change the reference line's position. All
the calculations made by the MFD refer to this line and it can be switched
between several locations.
| Intersection 1 and 2 - the two points where the orbits cross. This is
a very precise mode |
| Sh periapsis and Sh apoapsis - Our ship's periapsis and apoapsis. |
| Tg periapsis and Tg apoapsis - the target's periapsis and apoapsis.
|
| Manual - a refernce line that can be set anywhere with the [shift ,]
and [shift .] keys. |
While it's possible to synchronise with a target using all options from
above, "Intersection 1 and 2" is generally preferred because only meeting
with target (ie. having the same longitudes) at orbits' intersection
guarantees that you will also be at the same altitudes at the moment of
interception. That said, our situation is specific in this way because we
have already equalised PeA and ApA with the our target so we are roughly at
the same altitudes on whole current orbit
The pro of 2nd set of options (Sh ap and Sh pe) is that, as you may
remember, the points in orbit at which it's most efficiently to change
altitude of the oposite one (thus trying to synchronize) are our Peri- and
Apo- apsis. These modes require some thought, ie. if we want to do a
retrograde burn to catch up with the target then the point closest to
target's orbit will be Sh apoapsis so we'll choose this one now. On the
other hand if you will be doing a prograde burn to let the target catch you,
you'll have to choose Sh periapsis.
On the right of the MFD are two lists of numbers - Sh ToR and Tg ToR (ToR
stands for "Time on Reference"). These numbers show the time in seconds
until each craft crosses the refernce line, for the current (0) and next
four (1-4) orbits. To catch up with SH-03, we need to adjust our orbit so
that we cross the reference line at (almost) exactly the same time. The two
crossings which are currently the closest are highlighted in yellow. The
DTmin is a difference in arrival times to the reference line for the
highlighted pair. Use this parameter for precise burning.
| Adjust the reference line to read "Sh apoapsis" with [left shift M] or
MOD button a few times - this reference will remain stable as we do our
burns. |
| Begin a retrograde burn (use very low power, or bursts of power), and
when the ToR for the first orbit matches, start watching DTmin. The DTmin
must be as close to 0.00 as possible. At this point less than 0.1 is
enough. Use thrusters in linear mode with combination of Ctrl ( [Ctrl 6]
and [Ctrl 9] ) to use 10% of their power. Be wary of how low our periapsis
gets (watch OrbitMFD). Normally you'd do the burn just before our ship
passes the reference line (indicated by time variable close to 0 under
Sh-ToR and orbit no. 0) but our orbit is circular already. Synch MFD
should look like this after the burn:
Sometimes it will be impossible or at least very fuel inefficient to
match the ToR for the first orbit so be prepared to choose next ones in
future. |
| Because we have "Non-spherical gravity sources" mode on, our orbit's
parameters will change slightly in time. We may have to re-align our
orbits. If RInc in SynchMFD increases above 0.00, open Align MFD in the
right display and target SH-03. Align the planes at next node for better
precision. Keep RInc at 0.00. |
| Coast around the Moon...Our craft should start slowly gain on SH-03...
|
| Notice how DTmin changed. We have to do something about it. When you
are closer than ¼ of our orbit to the reference line, turn prograde and
use translation mode with combination of [Ctrl] to get DTmin to 0.00. Also
watch the Rinc and try to minimize it if possible. |
| Once about 200 s to reference line (time variable = 200 under Sh-ToR
and orbit no. 0), switch HUD to docking mode [H] and prepare for a
prograde burn to get the orbit to its previous shape. This HUD mode
highlights the target with a box, and shows the relative velocity markers.
If you don't see them now, the directions to these markers are projected
as arrows. Swing the nose until you are facing SH-03 - the distance to the
craft will be shown on the HUD under the box. |
| Now swing the nose to face the relative velocity vector - this is a
cross, in this case closed in a circle with [target's name] and V
prefix. Under the marker is the relative velocity [m/s]. It shows the
direction and vel. at which you are moving backwards w.r.t. target. This
means that to match the velocity with a target you have to gain the given
velocity in the direction of the marker. 180 degrees around it is a marker
that looks like a simple cross with -V prefix and means
exactly opposite.
|
| Perform a short burn in direction of V indicator to
stop your relative motion. This will automatically modify your orbit to
match SH-03's one. Don't worry if you suddenly see the 2nd marker - "+".
Just zero-out its value by accelerating the in the contrary direction
using translation mode. |
Now, if you were precise enough with DTmin and Rinc, you should be within
5km of SH-03. at this distance, if we are careful and patient, just burning
straigt at SH-03 will get us closer - for the most part, common sense reigns
again. IF you happen to be much further like 40 - 200 km, you can't just
burn towards SH-03 because at such distances the change of your orbital
parameters will influence your relative motion non-linearly and you'd
eventually start receding instead of closing. You can try
Rendezvous MFD
which will get you to the target but I won't describe it here. You can also
try to resynchronise... and be more precise :)
| Accelerate towards SH-03, to 10 m/s. You will see the "+" marker -
velocity vector. You are moving directly towards SH-03 if the marker is on
the target box. Over time you may notice the marker start to slide away
from SH-03 - this is the subtle effect of your slightly differing orbital
motion. |
| If you switch the control thrusters to translation mode [/], you can
adjust your motion with the directional controls to place the velocity
vector directly on target box. Toggle between the two modes as needed. The
translation mode here works best if you are facing the target. |
| When distance to SH-03 is 1.2 km or below (also shown in Synch MFD),
cancel out relative velocity by accelerating retrograde w.r.t SH-03, ie.
face the "+" velocity vector and use [Num 9] in translation mode or the
auxillary pods. The auxillary pods on the side of the shuttle can be
rotated to serve as main, hover or retro thrusters - make sure they're in
retro mode, and use them to slow down (you'll have to use the buttons and
levers on the panel for this.) |
| We have to tune our 2nd NAV reciever to SH-03 docking port's
transmitter. Enable Docking MFD in the left display and COM/NAV MFD in the
right one. Press [Ctrl I] and from the left pull-down menu select "Vessels"
and from the right menu - "SH-03". Slide the list down to find target
docking port's IDS frequency under "Docking port status" (at the bottom).
Highlight the 2nd position in COM/NAV MFD, using SL+ button and set the
frequency using <, <<, > and >> buttons. It should look like this:
|
| Press [LShift N] and [Ctrl R] what will respectively slave Docking MFD
and docking HUD to next NAV freq., ie. the 2nd one - SH-03 docking port's
transmiter. You will see a set of rectangles moving away from SH-03's
docking port. Face the furthest one and accelerate towards it. |
| Meanwhile you can cancel SH-03's rotation due to gravity gradient
torque parameter being switched on. Press F3, select SH-03 and [Num 5]
|
| Docking MFD can reset itself to NAV 1 after switching ships. If you
can't see the indicators anymore, press [LShift N] to correct it. |
| Once at the last rectangle, cancel out relative velocity and face
SH-03. Time for some theory :) |
During docking there are three types of misalignments combined together
which you need to correct seperately. On the left I provide an easy to
understand graphical overhead view and on the right - in-cockpit view and
docking MFD's visualisation of the same situation.
Two important values in Docking MFD are:
TVEL - Tangential velocity - any velocity perpendicular to
the approach path. TVEL of 0.00 guarantees that you won't change distance
from the approach path so if you're aligned and on the app. path keep the
TVEL at 0 m/s.
CVEL - Closing velocity. Can be positive (closing) or
negative (receding).
The DST value is the distance to target.
Once you start moving your ship you'll see a yellow arrow appear in
Docking MFD (tangential velocity indicator). This neat indicator shows us in
which direction w.r.t target we are rotating / moving using translation
mode.
To align longitudinally I had to rotate left thus making the arrow point
in the direction of longitudal rotation indicator (red cross). You will
usually have to combine 2 axis rotational/translational manoeuvres to set
the proper arrow direction. Remember that the arrow shows movement
w.r.t. target and that translational [Num 1] = left, [Num 3] = right,
rotational [Num 4] = left, [Num 6] = right BUT despite their location in
NumPad - translational and rotational [Num 8] = down not up, so it can be a
bit confusing to move the arrow in the direction you want. It will point
left if you press left but will point down if you press [Num
8] (intuitive up) and up when you press [Num 2] - just in the direction
you'll move.
It's not necessary to increase the magnitude of the arrow to make it reach
the crosses. Only direction matters and besides, it would be a waste of fuel
and could lead to further inverse misalignment, saving only a little time if
it succeded.
After you have aligned longitudinally, press [Num 5] to kill rotation. After
aligning on approach path, you have to cancel out relative velocity manually.
Here is where the arrow helps. Using translation mode, try to counter its
direction so that it gets smaller and eventually dissapears and TVEL reduces
to 0.
Remember that Docking MFD operates on logaritmic scales. It means that
the smaller a value is the more sensitive the MFD becomes in displaying this
particular value.
By moving towards last rectangle you already roughly aligned yourself on
the approach path and by facing the target - longitudinally (still - roughly).
The rotational alignment can be also roughly achieved thanks to the approach
path rectangles.
| Rotate your ship so that rectangles' double lines are on top, like
this:
|
| Start from aligning longitudinally. Center the red dashed cross by
slowly rotating the ship with help of the yellow arrow by directing it
towards the cross and press [Num 5] to kill rotation once the cross is
white and in the center. Again, you can help yourself using thrusters with
10% of their power with combination of [Ctrl] |
| Align on the approach path in translation mode [Num /]. Play a bit
with the arrow to familiarise with its reaction to translational thrust.
After that direct it by using combinations of pairs of thrusters to the
green cross. Keep decreasing the tangential velocity gradually, ie.
countering arrow's direction to reduce it, as the cross approaches center.
Note the logaritmic scale - the cross can suddenly shift on the other side
if you don't decrease the velocity. After centering the cross, set the
tangential velocity to 0.00 using 10% of thrusters power. |
| You may have to kill SH-03's rotation a few times. |
| Switch back to rotation mode [Num /] and align the ship rotationally
by getting the red triangle on top-center of the outermost circle where it
will turn white. If you do it now, you probably won't have to do it again.
|
| Switch to SH-03 [Ctrl F3] and cancel its rotation again [Num 5]. Open
its docking port with [K] as well as yours ([Ctrl F3] to return to your
ship). Reset Docking MFD to 2nd NAV with [LShift N] |
| Accelerate to about 3.5 - 4 m/s using a very short burst or better -
translation mode. |
| Switch to translation mode again and keep aligning on the approach
path. If you aligned all three properly at the begining, aligning on the
app. path will be the only thing you'll have to do. If not, slow down a
bit and fine tune the remaining indicators. |
| Normally the maximal closing velocity to dock is 0.1 m/s. Keep
reducing your current closing velocity gradually to reach the mark (for
realism only really - it's possible to dock with more than 0.1 m/s) |
Keep in mind that this type of synchronisation (a retrograde one) is sane
only if the target is ¼ ahead of you max. In any other case there is a great
danger that if you start a retrograde burn to try to synchronise, the PeA
may fall below surface or at least into the atmosphere. For such cases use a
prograde synchronisation.
Next time, we return to base.
Optional part
In this subsection I'll try to teach you some general synchronisation
methods to regularise your knowlegde. Download
Arrive MFD,
extract the archive to Orbiter main directory and activate in the Launchpad
> plugins tab. Warning! It will crash your Orbiter if you try to move its
window. The correct playback for this subsection is named:
RSfD-4-ships-that-pass-in-the-night2-optional.scn
We'll start from much higher orbit than SH-03.
We are 160 degrees before SH-03. First thing to do would be orbital
alignment and after that - circulisation. Notice that our orbital period,
distance to travel are already greater, velocity is smaller and we can use
it all as adventages. It means that we don't have to equalise our orbits
from the start but we can simply ... wait for SH-03 to roughly catch up with
us and actually it's the best thing we can do if it's about saving fuel and
time, when our orbital radii are much different.
| We'll wait for SH-03 to be about 45 degrees (1/8 of orbit) from our
position and then start a retrograde burn to create an intersection of our
orbits. Be aware that waiting for this particular angular distance is not
a rule of thumb but a total guesstimation and depends strongly on orbital
velocity difference. We won't know when we'll meet with our target until
we create an intersection. An ideal situation would be if you could
simultaneously apply the retrograde burn at one of target's apses. It will
be where our velocity vectors directions are closest, thus relative
velocities smallest. We'll use Intersect option for this case for better
precision.
The most important parameter now is RAnm (top left) -
true anomaly of reference axis - the direction of the reference axis
w.r.t. the ship's periapsis direction. We will be modifying our orbit at
the ref. axis because then the axis' position won't change so to make our
synchronising burns optimal, it has to be placed as close to our (ie.)
periapsis as possible. On the other hand due to non spherical gravity
sources the RAnm can oscilate with an amplitude of about 0.4° for the
Moon, plus because we'll do one precise burn before the periapsis (thus
changing PeA and possibly reducing RAnm), it can get dangerously low and
we can eventually loose the intersection if it goes below 0°. From these
reasons you should set it a bit higher. IMHO RAnm of 10-12° is still
optimal, safe and "enough for everyone" ;)
|
| Accelerate time to reach the ref. axis. All the time keep aligning the
orbits, should the RInc change. |
| As you approach the ref. axis prepare to minimise difference in our
arrival times - DTmin (and you have to do it everytime you pass the ref.
axis).
You can anticipate the proper attitude (pro/retro-grade) of burn by doing
some logical thinking: If our highlghted Sh-ToR is bigger then the
highlighted Tg-ToR then we'll arrive DTmin seconds later, so to arrive in
the same time we have to decrease our orbital period and we can do it by
accelerating ... retrograde. Initiate the burn when first value under Sh
ToR is close to 0 s. After that help yourself with translation thrusters
to get DTmin below 0.1 s.
|
| If from some reason you happen to loose the intersection, accelerate
with appropriate attitude 180° from the former point of intersection with
low power to create it again (ie. retrograde at apoapsis). The difference
in orbital periods can get unacceptable then but it's still cool if you
are a few orbits away from the interception because you can adjust the
period greatly at intersection. Too bad if you lost the intersection at 0
orbits to interception. This would mean you have to resynchronise or use
Ref: Manual Axis which make the distance at interception bigger but
possibly still acceptable. |
| 1 orbit further DTmin changed by itself and we have to resynchronise
before reaching the ref. axis, using translation thrusters so when you are
closer to ref. axis than 1/4 of orbit turn prograde and use the thrusters
with 10% of their power to get DTmin to 0.00.
Now, if you were approaching a space station, it would be a good time
to tune your NAV reciever to station's XPDR frequency in the same manner
as you tuned to SH-03 IDS freq. You didn't have to do it before because
all the sips use the default XPDR frequency, unless told otherwise in a
scenario file, so it was already tuned. |
| It is time to learn how to finish the interception precisely. We want
to arrive close to target with zero relative velocity. There are two ways
of doing so. I'll teach you how to do it using Arrive MFD. The other way
would be using
Equation MFD (equation r=dV^2/(2*ap) ) but as long as Arrive MFD works
we're fine with it (it didn't work with Orbiter 2005). Enable Arrive MFD
in right display with [RShift F1] and [RShift V].
Press TG (target) button and type SH-03. The MFD will then display in the
central frame the relative velocity (Vel), the
acceleration which you'd need to apply now to stop at
target with zero velocity (Acc to stop). As you get
closer, the Acc to stop will increase. Engine acc is the
current engine acceleration set by you. If you want to start matching the
velocity, you have to set Engine acc equal or a bit greater then Acc to
stop without hitting the target. The third frame is concerning engines
accelerations. A parameter of our interest is Main Max acceleration.
Never let Acc to stop go beyond this or you won't be able to make it and
will hit the target even if you use engines with max power.
Arrive MFD calculates all values constantly so any errors resulting from
making wrong assumptions (ie. linearising a curved trajectory) can be
corrected by you on the fly by you by changing main thrust.
Switch on Docking HUD and face the relative velocity marker. Start
decelerating when you are close to the Ref. line and see that Acc to stop
starts to increase dramatically. Set engines power (or rather acceleration)
to be equal or a bit more than Acc to stop and keep it this way. Cut their
power when RVel = 0.
I know that using tools to cancel out 100 m/s RVel while having 30 m/s^2
of max engine acc. is like using a bazooka to kill a fly but you'll
apprieciate this knowledge once you'll start using realistic craft with 60
times less engine power (the Space Shuttle) or will have much bigger
orbital periods differences. |
| Proceed with docking as described before. The difference is that
because we used "Intersection" option we'll be so close that we'll have to
move away from the shuttle first ;) |
As you may have already figured out there is also a possibility of an
inverse situation, ie. when our radius is smaller than targer's. Then, we
are the ones who chase the target, as it was in the very first case. As I
have proven later you don't need to equalise orbital altitudes or even
create the intersection from the start, but can wait until you are closer to
the target and then create the intersection with a prograde burn.
Apart from the prograde instead of retrograde burn, the only notable
difference is that the resulting RAnm will be (180 - X) where X is the usual
value, because it always dsplays the distance to Periapsis and our
intersection is at apoapsis - 180° 'round the clock ...
... so if you want the intersection to be 11° from your apoapsis, set
RAnm to (180° - 11°) = 169°
Be aware that this method of synchronisation is widely used for eg. STS
missions
The last thing I want to say is that it's possible to actually
plan the synchronisation burn to know when you'll meet the target
using TransX, before even igniting engines. Maybe I'll cover it in another
tutorial in future. |
