IMFD Ejection burn costs more delta-v than initally indicated

[Marijn]

With 2000 seconds to go for an ejection burn to Mars, IMFD's Orbit-Eject programs indicates that enroute Dv=5.680 km/s.

However, measured with BurnTimeCalc, my available delta-v is lowered with 5.860 km/s after the burn, which is 180 m/s more. After a minute or so into the burn, on the values screen, the required dv starts rising.

Does anybody know whether this is normal?

I planned my voyage with Nasa's Trajectory Browser: https://trajbrowser.arc.nasa.gov/tr...n=DT&wdw_width=0&submit=Search#a_load_results

The 5.680 value indicated by the Orbit-Eject program before the burn is spot-on. I don't understand why the burn turns out to be more costly. Any ideas?

 

[Urwumpe]

A minute of burn suggests, you have quite high gravity losses along the way - did how did the pitch angle relative to Earth change?

 

[Marijn]

how did the pitch angle relative to Earth change?

The 5 minute burn starts with pitch 0 degrees in surface mode and ends with +40. Is that what you mean?

 

[turtle91]

Maybe a problem with IMFD's "Realtime Eject" feature ?
It's ok for short burns, but gives no good results for long burns.

Pitch +40, so outward....
Sounds to me, that the optimal eject-time-window has been passed during the burn, so IMFD is trying to "catch-up" the missing time using outward-burn (=time counter counts up (so much outward was not part of the initial plan)

In general, I don't have good experience with IMFD's eject feature using long burns.
Maybe a better solution might be to use TransX+BurnTimeCalc for the eject-burn.

 

[Marijn]

Maybe a problem with IMFD's "Realtime Eject" feature ?

I think so. More a limitation. When I use the infamous 'Map method' (with the delta-velocity program), I am able to reach Mars with the 5.680 km/s. That makes me think the inaccuracy is caused by the Orbit-Eject program itself.

 

[Urwumpe]

I think so. More a limitation. When I use the infamous 'Map method' (with the delta-velocity program), I am able to reach Mars with the 5.680 km/s. That makes me think the inaccuracy is caused by the Orbit-Eject program itself.

Remember, that both might be assuming a infinitesimal short burn to change your course. gravity losses could be predicted knowing your fuel consumption, but control losses (how much fuel is needed for changing attitude) are harder to predict.

 

[Marijn]

gravity losses could be predicted knowing your fuel consumption, but control losses (how much fuel is needed for changing attitude) are harder to predict.

Any idea how? I'd like to account for the difference, the 180 m/s as described above. I don't think any fuel sent on attitude control is a significant part of it?

 

[Urwumpe]

Any idea how? I'd like to account for the difference, the 180 m/s as described above. I don't think any fuel sent on attitude control is a significant part of it?

Not that much for one minute. But gravity losses could be extreme in low earth orbit. about 5 m/s per second of burn at the end, if you get to 40° FPA.

 

[boogabooga]

Always use the "Off-Axis" mode for long ejection burns.

 

[Marijn]

Always use the "Off-Axis" mode for long ejection burns.

I wasn't aware of that. I'll try that.

 

[turtle91]

>Always use the "Off-Axis" mode for long ejection burns.
That's what I have used many time in previous IMFD-versions.
But somehow, I cannot get this to work in the current version:
TEJ :about 3000+ seconds (so more than 3 seconds as explained in the manual)

When switching to "OFF-AXIS", it gives me an undefined(?) EJA-value.
So "OFF-AXIS" jumps back to REALTIME.
It stays never on "OFF-AXIS" for more than 5 seconds in my case.

 

[Marijn]

When I switch to Off-Axis, the blue line representing the solution disappears, but I can still do the burn. It also uses 180 m/s more than advertised on the Orbit-Eject screen.

 

[boogabooga]

Try to exit the scenario with the Orbit-Eject loaded, then open the current scenario. Sometimes, reloading the scenario helps.

 

[Marijn]

Try to exit the scenario with the Orbit-Eject loaded, then open the current scenario.

That did work. When doing so, the blue line reappeared. But now the Orbit-Eject screen does display the required delta-v to be 5.860 km/s, which is including the 180 m/s I lost earlier.

Since the 5.680 km/s value is exactly the same as indicated by the Nasa Trajectory Browser, I still feel being ripped off somehow..

 

[boogabooga]

Try to vary the eject time by hand in the Ejection Plan. Check the graphic to be sure that the burn is centered.

 

[Marijn]

Varying the eject time, switching to external view and quiting/resuming a scenario all may help to get a solution in IMFD.

I used an external stopwatch to measure the duration of the IMFD 297 seconds burn. It turns out to be 5:07, not 4:07. So IMFD messes with the duration of time. This can be prevented by switching, but then the required delta-v gets updated to a value including the lost 180m/s.

Then I noticed that I could not set up the Delta Velocity program with the map anymore. I had to delete all IMFD settings from the scenario before I could do that again.

So the only way to get a efficient trajectory to Mars is to use the map method without setting up the Orbit-Eject program at all.

 

[dgatsoulis]

Hi Marijn,

Would you mind posting the scenario of your vessel ~10 minutes before the TMI burn and IMFD with your plan opened in either side?
And also a screenshot showing the (pre-burn) BurntimeCalc page?

 

[Marijn]

Would you mind posting the scenario of your vessel ~10 minutes before the TMI burn and IMFD with your plan opened in either side?

Sure, here's the scenario:

BEGIN_DESC
Orbiter saved state at T = 988
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 59095.5444158044
  Help CurrentState_img
END_ENVIRONMENT

BEGIN_FOCUS
  Ship XR5-01
END_FOCUS

BEGIN_CAMERA
  TARGET XR5-01
  MODE Cockpit
  FOV 40.00
END_CAMERA

BEGIN_HUD
  TYPE Surface
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE Interplanetary
  Scenario Old2
  MapMFD V5
  Reference Auto
  Target none
  Center GravityRef
  Data 0 1 1e-006 1 0 0 0 0 1 0 0 0
  MassLimit 1e+020
  CMode 0
  Config 1 1 1 1 0 0
  ExtMode 0
  Periapis none
  END 
  CorMFD V4
  Reference Sun
  Target Mars
  Source Earth
  ActiveProg 1 1
  DataA 0 3 0 0 0 0
  DataB 1 1 59096 0 0 0.9292192059737783 0 59192 0
  DVProg 0 0 0 1
  AdvConf 0 0 1 0 0
  Guidance 0
  END 
  EjectMFD V5
  Reference Auto
  Data 0 1 3 0 0 59095.55139176848 7747.42
  Guidance 0
  END 
  BaseAprMFD V2
  Reference Auto
  Target none
  Source none
  DataA 0 0 120000 0.10821 0.366519 1 1 59095.50200494436 59095.50200494436 0
  DataB 0 3 0 1 0 1
  END 
  SlingMFD V4
  Reference Auto
  Source none
  Data 0 1 1 3 0 1 59095.50200494436 0
  END 
  LaunchMFD V4
  Target None
  Data 0 1 1 3 0 1 59095.5298887218
  END 
  CF1_DataA 0 0
  CF1_DataB 0 10 120000 2 20 150000
  CF1_SecTgt 
  mfdShare -1
  mfdProgram 2
END_MFD

BEGIN_MFD Right
  TYPE User
  MODE Interplanetary
  Scenario Old2
  MapMFD V5
  Reference Auto
  Target none
  Center GravityRef
  Data 0 1 1e-006 1 0 0 0 0 1 0 0 0
  MassLimit 1e+020
  CMode 0
  Config 1 1 1 1 0 0
  ExtMode 0
  Periapis none
  END 
  CorMFD V4
  Reference Sun
  Target Mars
  Source Earth
  ActiveProg 1 1
  DataA 0 3 0 0 0 0
  DataB 1 1 59096 0 0 0.9292192059737783 0 59192 0
  DVProg 0 0 0 1
  AdvConf 0 0 1 0 0
  Guidance 0
  END 
  EjectMFD V5
  Reference Auto
  Data 0 1 3 0 0 59095.55139176848 7747.42
  Guidance 0
  END 
  BaseAprMFD V2
  Reference Auto
  Target none
  Source none
  DataA 0 0 120000 0.10821 0.366519 1 1 59095.50200494436 59095.50200494436 0
  DataB 0 3 0 1 0 1
  END 
  SlingMFD V4
  Reference Auto
  Source none
  Data 0 1 1 3 0 1 59095.50200494436 0
  END 
  LaunchMFD V4
  Target None
  Data 0 1 1 3 0 1 59095.5298887218
  END 
  CF1_DataA 0 0
  CF1_DataB 0 10 120000 2 20 150000
  CF1_SecTgt 
  mfdShare 0
  mfdProgram 3
END_MFD

BEGIN_SHIPS
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  RPOS 5229691.399 3169014.409 -2809944.853
  RVEL 4197.7148 -6432.8538 561.8684
  AROT 75.137 46.412 147.776
  VROT -0.0247 -0.0102 -0.0371
  AFCMODE 7
  PRPLEVEL 0:1.000000
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
XR5-01:XR5Vanguard
  STATUS Orbiting Earth
  RPOS -5818776.002 -249067.596 -2922081.050
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  PRPLEVEL 0:0.544267 1:0.989244
  IDS 0:199 100
  NAVFREQ 94 524 84 114
  XPDR 193
  SECONDARY_HUD 2
  LAST_ACTIVE_SECONDARY_HUD 0
  ADCTRL_MODE 0
  TAKEOFF_LANDING_CALLOUTS 7410.681959 1542.999647 1543.000554 0.000000 -8.870155
  APU_FUEL_QTY 0.877361
  LOX_QTY 1.000000
  CABIN_O2_LEVEL 0.209000
  CREW_STATE 0
  INTERNAL_SYSTEMS_FAILURE 0
  COGSHIFT_MODES 0 0 0
  MWS_ACTIVE 0
  COOLANT_TEMP 38.286470
  DMG_0 1.000000 Left Wing
  DMG_1 1.000000 Right Wing
  DMG_2 1.000000 Left Aileron
  DMG_3 1.000000 Right Aileron
  DMG_4 1.000000 Landing Gear
  DMG_5 1.000000 Docking Port
  DMG_6 1.000000 Retro Doors
  DMG_7 1.000000 Top Hatch
  DMG_8 1.000000 Radiator
  DMG_9 1.000000 Airbrake
  DMG_10 1.000000 Left Main Engine
  DMG_11 1.000000 Right Main Engine
  DMG_12 1.000000 Left SCRAM Engine
  DMG_13 1.000000 Right SCRAM Engine
  DMG_14 1.000000 Fore Hover Engine
  DMG_15 1.000000 Aft Hover Engine
  DMG_16 1.000000 Left Retro Engine
  DMG_17 1.000000 Right Retro Engine
  DMG_18 1.000000 Forward Lower RCS
  DMG_19 1.000000 Aft Upper RCS
  DMG_20 1.000000 Forward Upper RCS
  DMG_21 1.000000 Aft Lower RCS
  DMG_22 1.000000 Forward Star. RCS
  DMG_23 1.000000 Aft Port RCS
  DMG_24 1.000000 Forward Port RCS
  DMG_25 1.000000 Aft Star. RCS
  DMG_26 1.000000 Outboard Upper Port RCS
  DMG_27 1.000000 Outboard Lower Star. RCS
  DMG_28 1.000000 Outboard Upper Star. RCS
  DMG_29 1.000000 Outboard Lower Port RCS
  DMG_30 1.000000 Aft RCS
  DMG_31 1.000000 Forward RCS
  DMG_32 1.000000 Bay Doors
  DMG_33 1.000000 Elevator
  IS_CRASHED 0
  MET_STARTING_MJD 59095.519724
  INTERVAL1_ELAPSED_TIME -1.000000
  INTERVAL2_ELAPSED_TIME -1.000000
  MET_RUNNING 1
  INTERVAL1_RUNNING 0
  INTERVAL2_RUNNING 0
  ACTIVE_MDM 3
  TEMP_SCALE 2
  CUSTOM_AUTOPILOT_MODE 0
  AIRSPEED_HOLD_ENGAGED 0
  SCRAM0DIR 0.000000 0.000000 1.000000
  SCRAM1DIR 0.000000 0.000000 1.000000
  HOVER_BALANCE 0.000000
  MAIN0DIR 0.000000 0.000000 1.000000
  MAIN1DIR 0.000000 0.000000 1.000000
  GIMBAL_BUTTON_STATES 0 0 0 0 0 0
  ATTITUDE_HOLD_DATA 0.000000 0.000000 0 0 0.000000
  DESCENT_HOLD_DATA 0.000000 -3.000000 1
  AIRSPEED_HOLD_DATA 192.870145
  OVERRIDE_INTERLOCKS 0 0
  TERTIARY_HUD_ON 1
  CREW_DISPLAY_INDEX 0
  GEAR 0 0.0000
  RCOVER 0 0.0000
  DOCKINGPORT 0 0.0000
  AIRLOCK 0 0.0000
  IAIRLOCK 0 0.0000
  CHAMBER 0 0.0000
  AIRBRAKE 0 0.0000
  RADIATOR 1 1.0000
  LADDER 0 0.0000
  HATCH 0 0.0000
  SCRAM_DOORS 0 0.0000
  HOVER_DOORS 0 0.0000
  APU_STATUS 0
  EXTCOOLING_STATUS 0
  TRIM 0.021688
  LIGHTS 0 0 0
  PARKING_BRAKES 0
  PAYLOAD_SCREENS_DATA 0.2 0 1 0
  PAYLOAD_BAY_DOORS 0 0.0000
  RCS_DOCKING_MODE 0
  ACTIVE_EVA_PORT 0
  CREW_ELEVATOR 0 0.0000
END
XR5-01_Bay:XRPayloadBay
  STATUS Orbiting Earth
  RPOS -5818770.543 -249064.963 -2922057.996
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:36,XR5-01
  AFCMODE 7
END
CSA_XR5_ER_LOX-03-1:CSA_XR5_ER_LOX
  STATUS Orbiting Earth
  RPOS -5818777.526 -249067.658 -2922081.533
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:2,XR5-01
  AFCMODE 7
  PRPLEVEL 2:0.289916
  NAVFREQ 0 0
END
AIA_Logistics-08-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818775.210 -249067.012 -2922075.931
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:7,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
AIA_Logistics-13-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818772.894 -249066.365 -2922070.329
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:12,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
AIA_Logistics-18-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818770.578 -249065.719 -2922064.727
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:17,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
AIA_Logistics-17-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818770.558 -249068.742 -2922064.387
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:16,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
AIA_Logistics-19-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818770.599 -249062.644 -2922065.074
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:18,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
AIA_Logistics-12-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818772.874 -249069.388 -2922069.989
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:11,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
AIA_Logistics-14-1:AIA_Logistics
  STATUS Orbiting Earth
  RPOS -5818772.915 -249063.290 -2922070.676
  RVEL 3529.5265 -648.4142 -6953.3411
  AROT -173.419 22.328 -89.589
  VROT -0.0478 -0.0024 -0.0001
  ATTACHED 0:13,XR5-01
  AFCMODE 7
  NAVFREQ 0 0
END
END_SHIPS

BEGIN_ExtMFD
END

I don't know how to upload screenshots :embarrassed: so I made a clip:

To use the 'map-method' using the Delta-Velocity program, it seems necessary to delete everything between the MFD Left and Right tags in the scenario file before that works again.

Edit: The Orbit-Eject programs shows 5.677km/s initially in realtime mode, then after some juggling with the buttons the same in Off-Axis mode and after quiting and resuming the current state, the required burn is 5.845 km/s. If you execute the 5.680 km/s burn and time it with a stopwatch, it would not match.

 

[Linguofreak]

The 5 minute burn starts with pitch 0 degrees in surface mode and ends with +40. Is that what you mean?

If you're starting the burn at pitch zero and the burn isn't very short, you will end up spending a fair amount of extra deltaV. This applies to TransX or IMFD, and to Orbiter, KSP, or reality. For burns where you expend a fairly low fraction of your total mass in propellant, you need to have approximately half of the burn before the pitch zero point, and half after, so your pitch should be negative at the beginning of the burn, and positive to about the same amount after. If you expend a large fraction of your mass in propellant, so that your acceleration increases significantly during the burn, you will need to start the burn correspondingly earlier, as you won't accelerate as much during the first half of the burn as you will during the second.

 

[Marijn]

If you're starting the burn at pitch zero and the burn isn't very short..

That may all be true. I let IMDF figure out how the vessel should be oriented. I am not going to worry about that myself. I just want to hit the AutoBurn button and be sure I can thrust the values displayed. The problem can be avoided using the Delta Velocity program together with the Map program, so the burn is definitely possible.

My conclusion at this moment is:
- The accuracy of the 'map-method' is higher than the Orbit-Eject program (which shouldn't be surprising)
- The Orbit-Eject program initially displays the optimal delta-v value which only can be achieved with the map-method. After quiting and resuming, it shows the correct, but higher value the Orbit-Eject program will actually use.