Returning to Earth from Outer Planets

[JonnyBGoode]

Ok, here's a good question (I think?). I've just reacquired the skills to make an interplanetary trip to the outer planets, my first trip being to Europa and Io. Successful trip (I took the Arrow and used IMFD 5), now I want to come home. Set up IMFD for the return shot, do my burn, and I'm on my way, Jose.

Approaching Earth, I set up my Planetary Approach for a 30 degree, 300km altitude orbit. Everything seems spot-on normal. Then I notice something. The Planetary Approach computer seems to say that I need to start my Cir burn at 34.05k, which is well outside the SOI of Earth and will throw off my trajectory. Now either (a) I'm reading the computer wrong (most likely!), or (b) I am going massively too fast to ever be recaptured by Earth.

Suggestions? Maybe I need to plot my course around the Sun and come at Earth from the other direction?

I can post my return scenario, if it will help.

(I also realized a bit late that I didn't have enough fuel... oh well I'll fudge that in the scenario editor lol)

 

[Wishbone]

I suppose you've got enough fuel. Even so, prepare for aerobraking (been there, done that) with hovers firing transversally keeping you from flying away or crashing into the atmosphere. AFAIR 80k seems to be the altitude you'd like to keep during the aerobrake as long as possible. Retroapproach is also an option

(I returned from Jupiter and from that cooky 40 km/s scenario ) My final orbit was somewhat around 1000x1000, just below the first Van Allen belt IIRC.

EDIT: if low on fuel your only worry besides burning up should be getting into an elliptical orbit for a second and possibly third aerobraking.

 

[IgnoreThisBarrel]

You might be returning to Earth retrograde (although you'd probably have noticed that).

Two days ago I completed a trip from Earth to Saturn and planned to land on Mimas. It took me a little while to figure out why I was flying past Mimas at over twice its orbital velocity...

 

[JonnyBGoode]

No, I'm coming in prograde. I'm almost perpendicular, really.

I didn't even think of aerobraking. Duh. I guess I'll have to learn another MFD now, lol. And I suppose I shouldn't be aiming for 300k, I should probably make it closer to 100k or I'm not going to hit any atmo at all.

 

[Cairan]

Speaking of aerobraking, when coming from Mars the sweet target is 67.5 km for the XR2. From the outer planets I am not sure how it plays out but I guess the 75-80 km range might be more prudent indeed.

Speaking of which, I recently came back from Mars with the XR2, and tried to perform a direct reentry without circling around once or twice... Unfortunately, the longer time it takes to get captured and then to do a reentry properly to land at KSC, which prevents the deployment of the radiators, resulted in the systems overheating while I was on short final, 20 seconds from landing.

So I had to land without computers and MFDs, completely visual... Needless to say, I was very happy to pop the cabin hatch, the radiators and plug the external cooling umbilical after coming to a stop!

 

[JonnyBGoode]

Ok, haven't yet figured out Aerobrake MFD yet, but here's what I think I'm gathering (tell me if I'm close). I continue on a planetary interface of 75k PeA. When I get to around 500k from my node, I should turn prograde, roll to starboard 90 degrees (to be level with the atmosphere), and pitch up 80 degrees. At about 300k, I'll turn on full hovers, and between them and the atmo I should brake into at least an elliptical orbit?

Edit: That didn't work, hover pushed me away. I guess I have to wait until I catch air.

---------- Post added 10-17-10 at 02:37 PM ---------- Previous post was 10-16-10 at 08:38 PM ----------

Hmmm even doing that and coming in at 65k I still skipped out. Must need to do a retro burn before I get to Earth... blast this science!

Here's my return scenario... I'm slightly outside of Earth's SOI here. If anyone wants to see if I'm doing something wrong...

BEGIN_DESC
Orbiter saved state at T = 16860
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 54204.7380946874
END_ENVIRONMENT

BEGIN_FOCUS
  Ship AR01
END_FOCUS

BEGIN_CAMERA
  TARGET AR01
  MODE Cockpit
  FOV 30.00
  BEGIN_PRESET
    Cockpit:AR01:50.00
  END_PRESET
END_CAMERA

BEGIN_HUD
  TYPE Orbit
  REF AUTO
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE Interplanetary
  Scenario Old2
  MapMFD V5
  Reference earth
  Target none
  Center earth
  Data 0 1 3.038309648475988e-007 0 1 1 1 2 1 0 0 0
  MassLimit 1e+020
  CMode 0
  Config 1 1 1 1 0 0
  ExtMode 0
  Periapis Earth
  END 
  CorMFD V4
  Reference Earth
  Target Equator
  Source AR01
  ActiveProg 3 3
  DataA 0 3 0 0 0 0
  DataB 1 1 54204.73682631231 0 301160.6333335992 5.60051500170306 0.5234922655497201 0 0
  DVProg 34690.53126418863 0 0 0
  AdvConf 0 0 0 0 0
  Guidance 0
  END 
  EjectMFD V5
  Reference Auto
  Data 0 1 3 0 1 54199.77444661241 10
  Guidance 0
  END 
  BaseAprMFD V2
  Reference Auto
  Target none
  Source none
  DataA 0 0 120000 0.10821 0.366519 1 1 54199.77444661241 54199.77444661241 0
  DataB 0 3 0 1 0 1
  END 
  SlingMFD V4
  Reference Auto
  Source none
  Data 0 1 1 3 0 1 54199.77444661241 0
  END 
  LaunchMFD V4
  Target None
  Data 0 1 1 3 0 1 0
  END 
  CF1_DataA 1 0
  CF1_DataB 53145.0107469545 10 120000 2 20 150000
  CF1_SecTgt 
  mfdShare -1
  mfdProgram 4
END_MFD

BEGIN_MFD Right
  TYPE User
  MODE AeroBrakeMFD
  Reference earth
  Data 0 1.000000 1 0 0 0
  Target none
  LonLat 0.000000 0.000000 0
  AoA 0 0.000000
  Bank 0 0.000000
END_MFD

BEGIN_SHIPS
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  RPOS -4872664.52 4622046.22 386615.07
  RVEL -1053.078 -479.391 -7615.629
  AROT 33.68 81.30 28.09
  AFCMODE 7
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
GL-02:DeltaGlider
  STATUS Orbiting Sun
  RPOS -137229288345.36 14411920.13 -62293495731.43
  RVEL 38451.515 -489.164 2984.935
  AROT 175.65 -21.89 136.79
  RCSMODE 2
  AFCMODE 7
  PRPLEVEL 0:0.899958 1:0.949847
  DOCKINFO 0:0,AR01
  NAVFREQ 300 320 0 0
  XPDR 0
  NOSECONE 1 1.0000
  SKIN RED
  AAP 0:0 0:0 0:0
END
Mir:Mir
  STATUS Orbiting Earth
  RPOS 3176448.63 -3359063.39 4950033.98
  RVEL -6347.901 -4082.228 1323.410
  AROT 103.21 9.21 -46.47
  VROT 0.07 0.09 0.01
  AFCMODE 7
  IDS 0:540 100 1:542 100 2:544 100
  NAVFREQ 0 0
  XPDR 482
END
Luna-OB1:Wheel
  STATUS Orbiting Sun
  RPOS 91549538879.02 48354932185.95 147315656582.99
  RVEL -19077.414 -8247.665 15617.194
  AROT 0.26 0.08 110.12
  VROT 0.00 0.00 10.04
  AFCMODE 7
  IDS 0:560 100 1:564 100
  XPDR 494
END
AR01:UCGO\Vessels\UCGOArrowFreighter
  STATUS Orbiting Sun
  RPOS -137229288358.72 14411921.44 -62293495686.00
  RVEL 38451.515 -489.164 2984.935
  AROT 175.65 -21.89 136.79
  RCSMODE 2
  AFCMODE 7
  PRPLEVEL 0:0.345769
  DOCKINFO 0:0,GL-02
  IDS 0:320 100
  NAVFREQ 0 0
  XPDR 300
  O2_RESERVE 67.44
  SAVEVAR00 0§0,1§0.00,2§0,3§0.00,4§0,5§0.00,6§0,7§1,8§0.00,9§1,10§0.01,11§11.00,12§1,13§1,14§1,15§0.00,16§0,17§0,18§2,19§1,20§4,21§0,22§1,
  UMMUCREW Capt-Jonathan_Goode-50-70-70
  UCGO @@0,1,0,0,@@1,1,0,0,@@2,1,0,0,@@3,1,0,0,@@4,1,0,0,@@5,1,0,0,@@6,1,0,0,@@7,1,0,0,
  UCGO @@8,1,0,0,@@9,1,0,0,@@10,1,0,0,@@11,1,0,0,@@12,1,0,0,@@13,1,0,0,@@14,1,0,0,
  UCGO @@15,1,0,0,@@16,1,0,0,@@17,1,0,0,@@18,1,0,0,@@19,1,0,0,@@20,1,0,0,@@21,1,0,0,
  UCGO @@22,1,0,0,@@23,1,0,0,@@24,1,0,0,@@25,1,0,0,@@26,1,0,0,@@27,1,0,0,@@28,1,0,0,
  UCGO @@29,1,0,0,@@30,1,0,0,@@31,1,0,0,@@32,1,0,0,@@33,1,0,0,@@34,1,0,0,@@35,1,0,0,
  UCGO @@36,1,0,0,@@37,1,0,0,@@38,1,0,0,@@39,1,0,0,
END
END_SHIPS

BEGIN_ReFuelMFD
  SCENARIO_TREE 
    NAMED Computers
    END_NODE 
  END_NODE 
END

BEGIN_uap
END

---------- Post added at 03:43 PM ---------- Previous post was at 02:37 PM ----------

I think I see my problem. I'm thinking if I start from Jupiter again, and adjust my arrival date for about maybe 100 days further on, when Earth is moving away and I'm coming up from behind instead of cutting across it's orbit at a sharp angle, I might have a slower re-entry. Yes?

 

[Cairan]

I think I see my problem. I'm thinking if I start from Jupiter again, and adjust my arrival date for about maybe 100 days further on, when Earth is moving away and I'm coming up from behind instead of cutting across it's orbit at a sharp angle, I might have a slower re-entry. Yes?

You've got it right for that part, the rest is not quite ok... Basically, you want to use your vessel's wings to provide lift and drag to keep yourself from getting away from Earth while your velocity is above escape velocity, then generate as much drag as possible, in the limits of maintaining a safe altitude with lift, to slow down.

You must look at a high speed reentry as a 3-part event:

1) Initial contact
2) Capture from the object
3) Re-entry to the surface of the object

The first part, you need to come at an altitude that won't turn you into Krispy Kremated chunks at the speed you're coming in... 70-something km is a good estimate...

The next step is as much critical. During that part, you need to achieve a maximum amount of lift to prevent your ship from pulling away from the planet... The best way to do it is to have an angle of attack of 15-20 degrees, depending on the ship used, and the counter-intuitive part is that you need to be inverted, with the heat-shield facing toward space and the cockpit pointing toward the ground. However, if you were perfectly inverted, you might generate too much lift, which would cause you to sink deeper in the atmosphere and burn to a Krispy Kremation...

So, to maintain a null vertical velocity during the Capture phase at your Aerocapture altitude, you need to bank a little to the left or the right. Your Surface MFD is going to be your best friend for half an hour... Try to maintain G-forces low enough (below 3-4 G), manage the ship's hull temperature so that you keep some margin of error and most importantly, ALWAYS keep an eye on your vertical speed and vertical acceleration. If you see your vertical acceleration is going positive while your vertical speed is near-zero, that means you're going to skip back out soon if you don't generate planet-wise lift to keep yourself in the atmosphere.

During that step, your other MFD should be either Aerobrake MFD or Orbit MFD to keep an eye on your orbital parameters. When your eccentricity drops below 1 and an elliptical orbit appears, you've succeeded in getting captured. The hardest part is over...

When your perigee altitude drops into the atmosphere, the third phase of capture and reentry begins and you flip back to a heads-up attitude and do your reentry as you would from a close Low-Earth Orbit.

 

[JonnyBGoode]

Hmmm... I'm not sure that the Arrow is aerodynamic enough for lift, but I could try that. (I don't really need to land the craft though, as it isn't meant to land on Earth.)

I've set up a new approach course and when I get close to Earth again I'll see if I have any better luck.

 

[Wishbone]

With 40km/s speed and the Arrow as the vehicle, not much choice remains. Load up Topper's BTC 2.0, fire retrograde to "circularize", use linear RCS to keep PeA at 70-75 km. The aerobraking doesn't allow capture yet, so use Refuel MFD to transfer fuel from the DG, and retrobrake till Ecc drops below 0.999. At apogee, use linear RCS to raise PeA to 68-70 km and perform aerobrake as long as necessary to shed off remaining 3 km/s. Don't fire main engines, simply graze the atmosphere one orbit after another, when the apogee drops to 350 kms, circularize.

 

[JonnyBGoode]

Changing the date of re-entry changed everything. I was able to do a re-entry burn without any need for aerobraking. :thumbup:

 

[Wishbone]

Isn't that even more unrealistic than the Arrow itself? The time machine bit sounds cheesy. The only thing that can rival aerobraking on Earth is aerobraking on Jupiter

 

[Tommy]

You will have a lower Earth Insert delta-V if you plan the course so that you intercept the Earth on the opposite side of the Sun from where you departed the source planet. The closer your course is to a Hohman transfer the better.

If you follow the process given in [ame="http://www.orbithangar.com/searchid.php?ID=4142"]IMFD Full Manual/Playbacks[/ame] to adjust TEj and TIn to find the lowest dV for the trip, this will result in a near-Hohman transfer.

In other words, your transfer orbit's solar PeA should not be lower than the Earth's orbital altitude (above the Sun), and the Earth should get there at the same time you do.

 

[JonnyBGoode]

Isn't that even more unrealistic than the Arrow itself? The time machine bit sounds cheesy. The only thing that can rival aerobraking on Earth is aerobraking on Jupiter

Time machine bit? :huh:

 

[Tommy]

The Planetary Approach computer seems to say that I need to start my Cir burn at 34.05k,

The Cir value is the delta-V required to circularize the orbit, not a time-until-burn. Or did you use BTC to find a burn time based on the Cir value?

 

[JonnyBGoode]

The Cir value is the delta-V required to circularize the orbit, not a time-until-burn. Or did you use BTC to find a burn time based on the Cir value?

No, I misunderstood the Cir value. :blush: Nobody's perfect...

 

[Tommy]

Don't feel bad. I've been using IMFD for 5 years, and I'm still learning new things!

 

[TeddyJohn]

I think I see my problem. I'm thinking if I start from Jupiter again, and adjust my arrival date for about maybe 100 days further on, when Earth is moving away and I'm coming up from behind instead of cutting across it's orbit at a sharp angle, I might have a slower re-entry. Yes?

I use TransX to calculate my eject date and Cl approach. Then IMFD will perform the transfer. Also, the first planet approach burn should be done outside the Earth's SOI.