Direct transfers

JonnyBGoode

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I'm not sure if this is the right forum for this question; feel free to move it if it should go elsewhere...

Back in Orbiter 2010, there was a very lovely and ambitious project called To The Moon in 24 Hours, which proposed nuclear powered rockets that would fly directly toward the moon and directly back, rather than following the slower Hohmann Transfer orbit. The sci-fi series The Expanse uses a similar method of flight to the planets (as their rockets are much more powerful than today's rockets and they can afford to), flying toward their target until they get to a halfway point, then turning and burning the other way to slow down (and using the thrust to produce artificial gravity, as well).

Is there a way, using the MFDs we currently have for Orbiter, of calculating such direct routes?
 

Urwumpe

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The technical term you mean is accelerated transfers in case of the "To the Moon in 24 hours", which is based on this NASA paper *. For an accelerated transfer, you use a much higher apogee than needed, resulting in a much higher velocity at the encounter and a off-tangent departure from the moon. Both vastly increases DV and thus propellant demand.

You can produce such trajectories with IMFD and TransX, both are easily capable of this.

In case of The Expanse, you need continuous thrust trajectories, there was one MFD around that was able to navigate such routes, but I have forgotten its name.

* One of the add-ons I really miss in 2016, would be great to restart it again.
 

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The direct transfer in The Expanse, using the Epstein drive, is called brachistochrone transfer.
There is some information on the AtomicRockets page.

In Orbiter you can basically switch on planetarium mode [F9], look for your target planet, point the nose to it and accelerate. On halfway flip over and decelerate. :)
 

JonnyBGoode

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In Orbiter you can basically switch on planetarium mode [F9], look for your target planet, point the nose to it and accelerate. On halfway flip over and decelerate. :)

Will that work? The target moves, after all...

Also, how would you calculate the halfway point? EDIT: Never mind, that link you gave me said how. I just need to figure the distance between points before I burn.
 
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Urwumpe

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Urwumpe I really miss that add-on too. Actually I think it worked in 2006 and stopped working in 2010. :(

Yeah, sadly. It also had a good 2D cockpit and lots of scenery to enjoy. Would be more great now with 3D terrain and VCs. I really should learn Blender...
 

francisdrake

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In The Expanse the ships run the Epstein drive at a constant acceleration of 1 g = 10 m/s².
If my 'back of an envelope' calculations are correct this brings you to Mars in just 2 days.
The planet does not move far in this short time.

Point the ship, pull a little lead (aim in the moving direction), burn 1 day, flip, burn another day, arrive.
For 100 million km this takes 2.3 days, reaching a maximum speed of 1000 km/s!
 

macVsog

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Yeah, sadly. It also had a good 2D cockpit and lots of scenery to enjoy. Would be more great now with 3D terrain and VCs. I really should learn Blender...
I think the perfect team to create add-ons is one artist and one programmer
 

Urwumpe

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In The Expanse the ships run the Epstein drive at a constant acceleration of 1 g = 10 m/s².
If my 'back of an envelope' calculations are correct this brings you to Mars in just 2 days.
The planet does not move far in this short time.

Point the ship, pull a little lead (aim in the moving direction), burn 1 day, flip, burn another day, arrive.
For 100 million km this takes 2.3 days, reaching a maximum speed of 1000 km/s!

What if the target is not in opposition or conjunction, but for example, somewhere between the two extremes? How do you calculate when to turn around? You would need to know where you are relative to the reference trajectory line.
 

Urwumpe

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I think the perfect team to create add-ons is one artist and one programmer

Its at least the best way to get the best of both worlds, but its not perfect. Perfect is a team or community that covers the following skills:

  • Orbiter add-on architecture
  • World creation
  • Software development
  • 3D modelling
  • Scenario design
  • Tutorial video production
  • methodic testing and test design
You need many different skills to get a great add-on done, just programming and GFX is a good starting point.
 

JonnyBGoode

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What if the target is not in opposition or conjunction, but for example, somewhere between the two extremes? How do you calculate when to turn around? You would need to know where you are relative to the reference trajectory line.

I think his point is that since your transfer is measured in days, rather than weeks or months, you don't need to be as accurate because the planets won't have moved very far. Just burn to the midpoint, flip around (and probably do a short burn to recalibrate your trajectory) and then burn the other way to slow down.
 

Urwumpe

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I think his point is that since your transfer is measured in days, rather than weeks or months, you don't need to be as accurate because the planets won't have moved very far. Just burn to the midpoint, flip around (and probably do a short burn to recalibrate your trajectory) and then burn the other way to slow down.

Yes, but that requires you to know the transfer time precisely enough - the simple formula \[ t = 2 \sqrt{s/a} \] for example means, you need constant acceleration, not constant thrust.
 

JonnyBGoode

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Yes, but that requires you to know the transfer time precisely enough - the simple formula \[ t = 2 \sqrt{s/a} \] for example means, you need constant acceleration, not constant thrust.

Ouch. Then it wouldn't be as easy as just setting the throttle. You'd need some sort of autopilot to ramp up the acceleration over time.
 

Urwumpe

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Ouch. Then it wouldn't be as easy as just setting the throttle. You'd need some sort of autopilot to ramp up the acceleration over time.

Down - since the spacecraft becomes lighter during the burn, acceleration will go up at constant force.

Also, such a "point and fly" trajectory wouldn't be very efficient, since you always accelerate away from the target location at your arrival. It takes maximal 5 days to Mars at 1 G, Earth moves a bit less than 5° during that time - that is a lot of velocity building up moving you AWAY from Mars, meaning you need a lot more DV at the turning point.
 

JonnyBGoode

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Well if you had magical Epstein drives, you wouldn't have to worry much about efficiency. ;)
 
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