On transfers.

[ArmchairPhysicist]

In KSP today I realized something, anytime in the past that I’ve performed a interplanetary burn, I’ve always thought of it as simply changing my trajectory. Nothing special, just orbital mechanics stuff.

My current space craft.

Exploration 2

A fusion powered Daedalus/2001 Discovery inspired spacecraft. It has life support to last it 75 years, fusion fuel to last it about 36 years at full nuclear awesome..... I mean full power. 

Oh and it has about 750,000 ms D/v, nothing special.

Todays mission entailed visiting and landing on Galileo Planet pack’s Icarus, basically mercury. After transferring down to Icarus and doing science I then decided to visit Leto. I’ve never been out that far and I finally built a starship with the guts to make the journey. 

Heres where I realized I know nothing about space travel. I set up my burn to send me to Leto, only to see it would take 21 years to get there. That’s a long time but not unmanageable. However because I have the D/v I can simply burn straight towards Leto with a little lead and make it in 4 and a half years. 

All this time I believe I’ve been using something called a Hohmann transfer, which seems to be a very specific maneuver. My current journey will use something else but I’m not sure what the maneuver’s name might be.

So if all this time my simple transfer was a very specific maneuver, are there other well defined maneuvers that might make my life easier? 

 

Tl:Dr

I found out there are more than one way to get from Planet a to Planet b. This means there could be others. What might they be?

Edited April 2, 2018 by ArmchairPhysicist

[Zeiss Ikon]

What you're doing, burning to ridiculous velocity, coasting for a while, then braking for capture, is what's commonly called a "cometary orbit" -- because your orbit will resemble the path of a comet, which might (due to gravity assist when it left the Oort Cloud) escape the solar system.

If you had the "nuclear awesome" to burn continuously (a vessel referred to in the science fiction of the 1950s as  "torch ship"), you could travel on a "brachistochrone" -- that is, shortest time -- trajectory.  For destinations within the solar system (the real one) these are virtually straight lines.  It's only a week from Earth to Mars at a constant 1 g burn, and even Neptune is only about 6-7 weeks.  Burn harder, and if you don't wind up with compressed disks and fallen arches, you get there faster.  Remember Heinlein's book, Have Space Suit, Will Travel?  One week, Luna to Pluto, at 8 g constant (with a flip turnover in the middle).

[sevenperforce]

Keep in mind: the longer the injection burn, the longer the insertion burn.

[ArmchairPhysicist]

7 minutes ago, Zeiss Ikon said:

What you're doing, burning to ridiculous velocity, coasting for a while, then braking for capture, is what's commonly called a "cometary orbit" -- because your orbit will resemble the path of a comet, which might (due to gravity assist when it left the Oort Cloud) escape the solar system.

If you had the "nuclear awesome" to burn continuously (a vessel referred to in the science fiction of the 1950s as  "torch ship"), you could travel on a "brachistochrone" -- that is, shortest time -- trajectory.  For destinations within the solar system (the real one) these are virtually straight lines.  It's only a week from Earth to Mars at a constant 1 g burn, and even Neptune is only about 6-7 weeks.  Burn harder, and if you don't wind up with compressed disks and fallen arches, you get there faster.  Remember Heinlein's book, Have Space Suit, Will Travel?  One week, Luna to Pluto, at 8 g constant (with a flip turnover in the middle).

Interesting, I’ve read tunnel in the sky, which is why i know the term Torchship, but never Have spacesuit, will travel. I suppose what I’m doing right now does sound a lot like this a cometary orbit, so thanks. I’ve yet to try the one where you burn the during the whole transit though, I think I’ll try it.

[YNM]

Hohmann transfers are transfers that takes only one stable orbit, with the smallest dV possible within one synodic period.

 

There's no limit if you want to use high-elliptic or parabolic or even hyperbolic transfers. In fact, there is a name (which I forgot) for continuous-thrust transfers.

[tater]

31 minutes ago, YNM said:

Hohmann transfers are transfers that takes only one stable orbit, with the smallest dV possible within one synodic period.

 

There's no limit if you want to use high-elliptic or parabolic or even hyperbolic transfers. In fact, there is a name (which I forgot) for continuous-thrust transfers.

Brachistochrone trajectory.

 

[YNM]

15 minutes ago, tater said:

Brachistochrone trajectory.

Yes, that. Thanks !

 

The point is that trajectory problems are nearly infinite - you can't tell the trajectory just by knowing the start and end positions, but you can easily do the reverse.

[magnemoe]

8 hours ago, sevenperforce said:

Keep in mind: the longer the injection burn, the longer the insertion burn.

This, and the added dv cost to the brake burn might well be higher than the extra you used during injection. 
On the other hand going to minmus tend to be 925 m/s and 9 days, 940 m/s and its less than 4. braking might cost up to 400 m/s 

[p1t1o]

11 hours ago, ArmchairPhysicist said:

<snip>

You might find this link interesting:

https://en.wikipedia.org/wiki/Interplanetary_Transport_Network

 

[sevenperforce]

7 hours ago, YNM said:

7 hours ago, tater said:

8 hours ago, YNM said:

Hohmann transfers are transfers that takes only one stable orbit, with the smallest dV possible within one synodic period.

There's no limit if you want to use high-elliptic or parabolic or even hyperbolic transfers. In fact, there is a name (which I forgot) for continuous-thrust transfers.

Brachistochrone trajectory.

Yes, that. Thanks !

The point is that trajectory problems are nearly infinite - you can't tell the trajectory just by knowing the start and end positions, but you can easily do the reverse.

A Hohmann transfer is one end of the spectrum; a brachistochrone is the other end. The Hohmann transfer is the lowest-energy transfer which always exists; the brachistochrone is the fastest transfer possible.

A Hohmann transfer is not necessarily the cheapest transfer in all cases. Depending on the orbits you are transitioning between, it may be possible to do a cheaper (though much longer) bi-elliptic transfer. However, bi-elliptic transfers are not possible for all possible origins and destinations, and they are not discrete, whereas there is only one solution to the Hohmann problem for any given system.

A brachistochrone requires a minimum TWR to be viable; if you have very high specific impulse but low thrust, you end up in a spiral rather than a proper brachistochrone.

[Bill Phil]

Do you want to minimize time or delta V cost? Minimizing time through constant acceleration basically maximizes delta V. Minimizing delta V basically maximizes time. The question is which one do have more of?

[mikegarrison]

And of course, so far we've only been talking about simple orbital transfers where only the spacecraft itself is involved. When you add in gravity assists and aerobraking, you can further complicate things.

[YNM]

9 hours ago, sevenperforce said:

The Hohmann transfer is the lowest-energy transfer which always exists...

What, I can transfer to Mars from Earth when in conjunction and call that "hohmann" ?

It's probably true for orbit-to-orbit (ie. GTO injection); not quite true for planet-to-planet (might as well call it "point-to-point").

Edited April 3, 2018 by YNM

[sevenperforce]

3 minutes ago, YNM said:

What, I can transfer to Mars from Earth when in conjunction and call thar "hohhman" ?

A Hohmann transfer is a transfer between two orbits, not between two objects.

[YNM]

1 minute ago, sevenperforce said:

A Hohmann transfer is a transfer between two orbits, not between two objects.

Yeah. But then there's the question about transferring between elliptic orbits.

Edited April 3, 2018 by YNM

[Gargamel]

 

On 4/2/2018 at 7:31 PM, ArmchairPhysicist said:

Oh and it has about 750,000 ms D/v, nothing special.

Nobody asked about this!?  I'd love to see this

[PB666]

On 4/3/2018 at 7:58 AM, sevenperforce said:

A Hohmann transfer is one end of the spectrum; a brachistochrone is the other end. The Hohmann transfer is the lowest-energy transfer which always exists; the brachistochrone is the fastest transfer possible.

A Hohmann transfer is not necessarily the cheapest transfer in all cases. Depending on the orbits you are transitioning between, it may be possible to do a cheaper (though much longer) bi-elliptic transfer. However, bi-elliptic transfers are not possible for all possible origins and destinations, and they are not discrete, whereas there is only one solution to the Hohmann problem for any given system.

A brachistochrone requires a minimum TWR to be viable; if you have very high specific impulse but low thrust, you end up in a spiral rather than a proper brachistochrone.

Technically speaking the fastest possible trajectory is the path light would take between two positions in space. The dV required would >600,000,000 m/s (technically infinite for matter but we can set some limit as the potential energy required would be so great as to form a black hole at the point of the space ship).

Therefore it would have to be a practical limit of velocity, and the problem that we should all know by now, to get higher dV requires lower mass and higher power, there for power/mass ratios go up (ISP goes up) and since humans or equipment are fragile little beasts that can yet convert 100% heat to directional velocity, we can only provide that power slowly . . . . .accelerations are very low.

[sh1pman]

35 minutes ago, PB666 said:

technically infinite for matter but we can set some limit as the potential energy required would be so great as to form a black hole at the point of the space ship

This is interesting. If E=mc^2, can you make a black hole out of your space ship by just accelerating it a lot? At some point its kinetic energy will become so high that its equivalent total energy (relativistic mass) gets high enough to form an event horizon around it. 

[YNM]

1 hour ago, PB666 said:

Technically speaking the fastest possible trajectory is the path light would take between two positions in space.

... called a "null" trajectory.

But in a world where thrust are limited, brachistochrone are your choice.

[sevenperforce]

1 hour ago, sh1pman said:

This is interesting. If E=mc^2, can you make a black hole out of your space ship by just accelerating it a lot? At some point its kinetic energy will become so high that its equivalent total energy (relativistic mass) gets high enough to form an event horizon around it. 

Good question, but no. You can't accelerate yourself into a black hole because the fabric of space has no universal coordinate system. Space does not notice how fast you are going; your gravitational interactions with space are the same no matter how much you accelerate.

If you think about it, it HAS to be this way, because all motion is relative. Your kinetic energy has to be measured with respect to some outside object; you can be moving at 200 km/s relative to Earth but only 2 km/s relative to some other object way out in space. If relativistic kinetic energy caused the formation of an event horizon, then you could be a black hole w/r/t Earth but not w/r/t another object, and that's obviously very problematic.

[YNM]

1 hour ago, sh1pman said:

If E=mc^2, can you make a black hole out of your space ship by just accelerating it a lot?

Acceleration are not part of the stress-energy tensor I think ? Though given that any change in the metric (curvature) tensor would result in a change in the other tensor (stress-energy), this makes for some... interesting properties.

But I bet the limit you'd have a "black hole"is c : ie. for all purposes, impossible. And even then it could be a "timelike" black hole, so basically the null trajectories experienced by photons.

 

EDIT  : Really useful footnote :

Spoiler

The bumbling buffoon I might be at remembering stuff now completely irrelevant to my field, please look at an actual book for reference. I recommend Introduction to Cosmology by Barbara Ryden. Or look up "Equivalence Principle".

 

Edited April 4, 2018 by YNM

[sevenperforce]

5 minutes ago, YNM said:

Acceleration are not part of the stress-energy tensor I think ? Though given that any change in the metric (curvature) tensor would result in a change in the other tensor (stress-energy), this makes for some... interesting properties.

But I bet the limit you'd have a "black hole"is c : ie. for all purposes, impossible. And even then it could be a "timelike" black hole, so basically the null trajectories experienced by photons.

It's impossible even in theory. Simply put: there is no universal reference frame, so spacetime has no idea how fast you're going, so it can't make a black hole around you.

[sh1pman]

Just now, YNM said:

Acceleration are not part of the stress-energy tensor I think ? Though given that any change in the metric (curvature) tensor would result in a change in the other tensor (stress-energy), this makes for some... interesting properties.

But I bet the limit you'd have a "black hole"is c : ie. for all purposes, impossible.

Yeah, it's not even remotely realistic, but my question is more about "does your kinetic energy affect the curvature of spacetime around you?".

[YNM]

1 minute ago, sh1pman said:

"does your kinetic energy affect the curvature of spacetime around you?".

Only acceleration though, oddly.

https://en.m.wikipedia.org/wiki/Equivalence_principle

4 minutes ago, sevenperforce said:

Simply put: there is no universal reference frame, so spacetime has no idea how fast you're going, so it can't make a black hole around you.

You can with accelerations though.

[sevenperforce]

9 minutes ago, sh1pman said:

Yeah, it's not even remotely realistic, but my question is more about "does your kinetic energy affect the curvature of spacetime around you?".

Right. The answer is no; it does not. As long as you are in an inertial reference frame, your kinetic energy does not curve spacetime.

If you are in a non-inertial reference frame, all bets are off. A rotating object, for example, has centripetal acceleration and thus produces frame-dragging effects.