I'm curious what others here can do, and how. :thumbup:
I am finally sitting down to try this. I will be flying 3 versions of this flight and I'll be keeping a detailed log including pics of the progress of each mission.
1. A direct Earth > Ganymede flight with 2 main burns. One to leave Earth and the second to enter a low orbit around Ganymede. (plus corrections).
2. This version will make use of the Oberth effect, so there will be 4 main burns. TJI, capture burn at low Jovian altitude, Ganymede rendezvous setup at the apoapsis of the capture burn and finally the GOI burn. The TJI itself will be broken down into a series of two or three periapsis 'kicks', in order to minimize gravitational losses. The first burn will need to take place about 3 days before the indicated July 11th 2011 date posted by the OP.
3. The final version will be this: At the front end, I'll have a series of gravity assists to reach Jupiter. Once I'm there, I'll utilize the Oberth effect again and at the back end of the mission I'll add a series of gravity de-assists, to enter a low orbit around Ganymede. The launch date will be as close as I can get to July 11th 2011, but it may have to be a few months before or after that.
Pre-flight setup: Non-spherical gravity switched on. IMFD configured like this:
The reason for this setup is this: First, IMFD's map works best if you have non-spherical gravity switched ON. Yes, there is an option for IMFD to auto-detect the setting, but as far as the map's predictions go, you get the best results if you have it switched on. (Not to mention the benefit of the added realism.) The rest of the settings are selected for maximum accuracy, except the last one in the map's configuration page (error tolerance).
There, I have found that using a value of 4.000 is a good compromise between the accuracy of the map and the speed at which connected IMFD's 'talk' to each other.
You can setup IMFD like this from within Orbiter, or you can copy the lines below and paste them in your OrbiterRoot\Config\IMFD5.cfg file. This way, you won't have to set it up again each time you start a scenario.
Code:
// Configuration file for BaseSyncMFD, I-MFD, AeroBrakeMFD
Color_01 0x0000DD00 // Current Ship orbit, Source Orbit [Bright green]
Color_02 0x00008800 // Additional Map lines [Dark green]
Color_03 0x0000BB00 // Base Text color [green]
Color_04 0x0000EEAA // Hilighted text
Color_05 0x0000CCFF // Target orbit, Some Warnings [Orange]
Color_06 0x00EEEEEE // Selected Item [White]
Color_07 0x00FF5555 // Planned trajectory [Blue]
Color_08 0x00666666 // Planets [Dark Grey]
Color_09 0x000000DD // Warning lights [Red]
Color_10 0x00EEEEEE // [White]
Color_11 0x00999999 // Headlines, Some planets [Grey]
Color_12 0x0000BB00 // Adjustable Items
// Multibody predictor configurations
Rectification 0.005 // Rectification constant for trajectory engine
NonSpherical 2 // Use Non spherical gravity on low orbit prediction in Map Program, 0=Disabled, 1=Auto, 2=Always ON
LegSize 1 // Legsize factor used in Multibody predictor
LegsPerFrame 64 // Number of legs calculated each time step
Celbody 1 // Use Orbiter's Celbody Interface to improve accuracy of the map (1=On, 0=Off)
Adaptive 0 // Use Adaptive step-size control (1=On, 0=Off)
AdapTol 4 // Error tolerance (Lower value = higher accuracy)
Integrator 0 // Default Integrator 0=RK5(6), 1=RK6(7), 2=RK7(8)
// General Configurations
RefAltitude 120e3 // Reference altitude for atmospheric entry interface
ReEntryAngle 5.5 // Default ReEntry in degrees
DateFormat 0 // Default Date Format 0=MJD, 1=GET
UseRegres 1 // Use Nodel Regression Estimation (1=On, 0=Off)
IgnState 1 // Use numerical ignition state propagation
FastUpdate 1 // Use Fast Update of MFD Screens
// Key Setup
BaseSyncKey 0x30 // StartUp key used by BaseSyncMFD
IMFDKey 0x17 // StartUp key used by IMFD
RotationKey1 0x26 // Rotation Key "L"
RotationKey2 0x2C // Rotation Key "Z"
// Auto-burn options
AB_Rate 10.0 // AutoBurn MaxRate (degrees/s)
AB_RCS_Th 2.0 // RCS Level Treshold m/s
AB_Thr_Th 20.0 // Throttle Down Level Treshold m/s
I will be changing some of these settings to suit my needs as the missions progress, but as far as the initial setup goes, this is how I start my scenarios.
Off to fly the first version of the mission.
Since the challenge scenario permits a 'magical' fuel re-supply at LEO, I'll start at a 300x300 km orbit around Earth with the Shuttle-A fully fueled. The inclination and LAN will be selected to fit the plane of the TJI burn, but I will have an approximate 1/10th of a degree misalignment of the orbital plane to maintain some realism. Usually my launches with the DeltaGlider are within 0.01°, but I can't even remember the last time I rode the aerodynamically 'challenged' Shuttle-A into LEO. This is how I'll start all 3 versions of the mission.
First task is to set the date, get the ship in LEO and check my delta-v budget.
Including the RCS, I start out with 14630 m/s.
Firing up IMFD and setting up a Jupiter intercept course. Since I will be going directly to Ganymede, I am selecting the course with the minimum arrival velocity at 5655 m/s
Preflight calculations:
I'm starting from the end. Arrival V∞ = 5655. This means a periapsis velocity at Ganymede's distance of 16394 m/s. Ganymede is orbiting Jupiter at a velocity of 10881 m/s; allowing for a ~10° off-plane intercept, I get an arrival V∞ relative to Ganymede equal to 5984 m/s, This gives a periapsis velocity @ 20 km above Ganymede equal to 6578 m/s which means an orbit insertion burn of 4647 m/s.
TJI burn (impulsive) = 6440 m/s, Burntime = 571.2 seconds. This burn is
huge, covering more than 10% of a full orbit. Before continuing, I want to address this by figuring out the gravitational losses.
I know that the grav. losses are calculated by
[math] \int_{t_1}^{t_2 } g \; sin \gamma \; dt [/math] where g is the gravitational acceleration at the altitude of the burn, t2-t1 is the duration of the burn and γ is the flight path angle at the beginning/end of the burn. This is known as the finite burn problem.
Unfortunately I don't know how to solve integrals, so I'll just plug it in to wolframalpha and get the result: 842 m/s.
Hmm... from experience with the DG, I know that a burn this big has significant losses, but I don't think it's going to be
that high. Besides, IMFDs AB feature doesn't use the method shown in the pic above, TransX does though (start burning to the "inside" of the prograde direction and end the burn to the "outside" of the prograde direction.)
No other option than to use a quicksave at this point and let the burn happen with IMFD's AutoBurn feature in the Orbit-Eject program, while keeping track of the ΔV. By the end of the burn, I am left with 7496 m/s.
Wow, the impulsive burn was 6440 m/s but the actual burn cost 14630 - 7496 = 7134 m/s. That's 694 m/s lost on IMFD's AutoBurn trying to correct the finite burn problem.
Fortunately, I know a couple of ways to minimize this loss. The first one is instead of leaving Earth in a single burn, using a series of smaller burns, each time raising the apoapsis of the trajectory until escaping after an x amount of revolutions. I will use this method in version #2 of this flight.
The other way to do it, is to use IMFD's Delta-Velocity and Map programs, instead of letting the autopilot of the Orbit-Eject program do the burn. All I have to do, is to try and arrive at Jupiter at the same date as the one shown in the Target Intercept program (this should result in an arrival velocity close to what the Target Intercept program shows), while adjusting my burn variables (x,y,z and time) keeping the ΔV as close as I can to the impulsive burn. This way, I should be able to keep the losses under 100 m/s.
Back to the quicksave and the calculations:
GOI burn ≈ 4650 m/s
TJI = 6450 + ~100 ≈ 6550 m/s
1% room for corrections ≈ 110 m/s
Total ΔV = 11310 m/s , let's round it to 11.3 km/s
I should be able to make it directly to Ganymede low orbit with ~3300 m/s left in the tanks.
Starting the Delta-Velocity and map programs. Since these are connected, I need to make a note of the Target Intercept program's arrival date, which is 56660.
Then I select an unshared IMFD and enter the burn shown in the orbit eject program. Once that's done, I share the other IMFD with the one showing the Delta-Velocity program, open the map, target Jupiter and press "plan" to see the trajectory prediction.
Right from the start, I run into a problem. The map's prediction doesn't extend all the way to Jupiter.
I can fix that by changing a few settings in the map's configuration page.
Period Limit: No
Hyper Limit: No
Adaptive: Yes
The journey will take ~910 days which is ~80M seconds. I'll let the prediction extend out to 100M seconds by setting the Time Limit to 100M (typing 100M or 100e6 or 100000000 is the same thing)
Now the prediction extends all the way to Jupiter.
It is time to start fiddling with the burn variables to get the encounter when/where I want it. My aim is for an arrival close to 56660 at a PeD of ~1 G away from Jupiter (Ganymede's distance) and as low an equatorial inclination as I can get. (Ganymede is just 0.15° inclined relative to Jupiter's equator).
After ~5 mins of adjusting the burn variables:
I am spending less than my prediction and arriving 20 days early. But the PeV seems ok and the minimum equatorial inclination I can get is slightly more than 7°. I'll go ahead and burn this plan.
Here is the trajectory after the burn.
TJI cost: 6514 m/s
T to Jupiter periapsis: 76.51M seconds
Time-warping to the MCC at ~ 40M seconds away from Jupiter.
I switch the Map to reference Jupiter and targeting the equator. I center the periapsis at Jupiter (CNT : p-Jupiter) and I wait until Ganymede is near the periapsis of the predicted path. I need to setup my MCC with these characteristics:
Arrival @ Ganymede's distance from Jupiter, with the PeT being a multiple of Ganymede's orbital period around Jupiter. (that's why I waited for Ganymede to be near the periapsis). I also need to get the line of nodes exactly at the periapsis.
On the left is the pre-burn path and on the right the path after the MCC
Notice how Ganymede 'right now' is almost exactly at the prediction's periapsis, the line of nodes is also there (but it did raise my Eq Incl to 11.74°) and the PeT is 64 times Ganymede's orbital period.
MCC cost: 37 m/s. Burning and time-warping to 20M seconds away from perijove.
The drill for the second MCC is the same as before; wait for Ganymede, get the periapsis and node exactly at Ganymede and the PeT a multiple of Ganymede's orbital period. Cost: 12 m/s
Time-warping 'til the PeT is in the single M digits away from perijove, to repeat the process once more.
Turns out, I don't have to perform the 3rd MCC. My path is exactly where I want it, with the PeT being 16 times Ganymede's orbital period.
Time-warping 'til I'm inside Jupiter's strong SOI. Once there, I'll check my map to see how I am doing.
Ok, I am deep nside Jupiter's grav. well (G = 0.61) but I am not sure which prediction to trust. With the Map's "Adaptive" set to "On" I am doing really well, almost exactly on the path I want, but setting it to "Off" shows I will be falling deeper than Ganymede's orbit. I'll fire up TransX, target Ganymede and see what kind of prediction it shows.
Ok, so with the map's "Adaptive: On" I am getting almost the same prediction as TransX. At this distance from Jupiter, this is somewhat trustworthy. I'll time-warp 'till I am ~1 Ganymede orbit away and keep a close eye to the map.
For my next (and final) MCC I'll keep the "Adaptive" to "On". I'll also lower the Time Limit to 5M seconds. I'll setup an Offset Target Intercept plan (target Ganymede) and watch the Map's prediction with Ganymede also targeted. Anything after that will be minor linear RCS corrections.
Turns out I made the right call. The Ganymede intercept burn costs only 9 m/s.
Finally, the last leg of the mission. I'll make minor linear RCS adjustments to keep an orbit insertion altitude of ~20km above Ganymede and perform the GOI burn once I arrive.
Half a day away from Ganymede periapsis.
The GOI burn is a substancial one, at 4773 m/s. I'll set it up as a maneuver, to minimize grav losses again.
Welcome to Low Ganymede Orbit. ΔV left: 3254 m/s
In a next post, I'll try the 2nd version of the journey with the Oberth effect at the back end, and a series of periapsis kicks at the front end.