Flight Question Saturn to Home

[EnDSchultz]

Well, I just got Deepstar 2 back into Earth orbit after over a decade away from home. The only problem is the amount of fuel I used to get home. Excluding the ejection from Saturn, I probably had a total delta V of at least 20k for the trip home. 7k of this was at my periapsis around the sun to lower my orbital apoapsis to coincide with Earth, and 12k was just slowing down to be captured by the blue planet. The rest was spent for various correction burns and the like. This was actually the most efficient path I could find, given the circumstances, since not doing a burn at my solar periapsis would have resulted in an encounter velocity at earth of over 30k. Is there a more efficient way of going from one of the outer planets back to Earth that doesn't involve so much manual deceleration? Had I chosen any of my other interplanetary ships that didn't have such a ridiculously high specific impulse as Deepstar 2, I don't think I would have had enough fuel to make it home. I used TransX for navigation.

 

[n72.75]

Remember that saturn is a long way up the suns gravity well. also, deepstar is very heavy, so it takes a lot of impulse to get you to where you need to go and stop you when you get back

 

[flytandem]

Ways to be more efficient might be...
1. Combine dropping the Ap and the capture at Earth into the same burn both being done in Earth's gravity well.
2. Make the transfer from Saturn to Earth fairly in plane with the ecliptic. Doesn't have to be perfect but a great angle off from that would cause a high arrival speed at Earth.
3. Time the transfer from Saturn so that it is fairly hohman and arrives tangentially with Earth's orbit to reduce encounter speed.

BTW i just downloaded the Deepstar and tried it. The Deepstar craft itself seemed to work fine but the lander in the Jupiter Io scenario has no RCS controls. I couldn't see anything in the manual activate it.

 

[the.punk]

BTW i just downloaded the Deepstar and tried it. The Deepstar craft itself seemed to work fine but the lander in the Jupiter Io scenario has no RCS controls. I couldn't see anything in the manual activate it.

To activate the RCS of the lander press SHIFT and *

 

[flytandem]

thanks! worked fine. I managed a soft manual landing at some random spot and relaunched into orbit fairly close to alignment with deepstar. 16% fuel left to rendezvous with the deepstar ship.

 

[the.punk]

Good that it works.

 

[EnDSchultz]

BTW i just downloaded the Deepstar and tried it. The Deepstar craft itself seemed to work fine but the lander in the Jupiter Io scenario has no RCS controls. I couldn't see anything in the manual activate it.

Yeah, I just posted a question about that maybe a week or two ago. The thread (and solution) is still on the first page of the "addon support" subforum. =P

Thanks for all the suggestions, guys, though I have one question about the issue of the Hohmann transfer. If I understand what you mean correctly, then a Hohmann transfer from Saturn would mean I would reach Earth's orbit (and Earth itself, if timed properly) at the periapsis of my orbit around the sun. Wouldn't that mean that's the point at which I would be going fastest and therefore the worst place to intercept Earth? I'm probably just misunderstanding what you mean or missing an important concept here...

 

[flytandem]

If does seem strange that to intercept the Earth at the fastest point would turn out to be the best. But the faster you are going the more the exhaust is doing work for you. Work is force times distance. The exhaust is the same force on you whether you are fast or slow so best is to do burns as much as possible in gravity wells so the force is over a greater distance.

Often I find it tough to decide whether to push part way out like from Titan to a high Ap above Saturn, then a short slowing at Ap so that the Pe is a close shave of Saturn surface then at the Pe do a prograde push to eject in interplanetary flight, or just push out from Titan to Earth. I don't have the equations to show the best way but usually if one has the time to doddle with such high Ap coasts and has the ability to plan the direction and date that the final Pe burn is pointed, this way is more efficient. Any rocket scientists out there able to explain when it's best to go direct or a double push. BTW is this the Bi-Elliptical I've often heard of? I made a challenge once relating to this gravity well drop.

Anyway, even if you just do a surface push from a low moon orbit, try to do it when the moon's motion is the direction you want to go (much like going east when launching from Earth surface) or like a pitcher throwing a fastball, the ball is flicked by the fingers at the end of a moving arm. And you should plan to arrive at your Pe of the sun, meeting the Earth at that point (and heading east on Earth surface) and then, if possible, aerobrake. If your can't aerobrake, then engine brake. If you don't have enough fuel that way then, you probably don't have enough fuel to do the trip.

I used to take almost no time at all to slop-slam my way all around the solar system. But the more I practiced the more planning it took to go on shorter and shorter trips. If I extrapolate this trend eventually I will be taking forever to get nowhere. :rofl:

 

[EnDSchultz]

Often I find it tough to decide whether to push part way out like from Titan to a high Ap above Saturn, then a short slowing at Ap so that the Pe is a close shave of Saturn surface then at the Pe do a prograde push to eject in interplanetary flight, or just push out from Titan to Earth. I don't have the equations to show the best way but usually if one has the time to doddle with such high Ap coasts and has the ability to plan the direction and date that the final Pe burn is pointed, this way is more efficient. Any rocket scientists out there able to explain when it's best to go direct or a double push. BTW is this the Bi-Elliptical I've often heard of? I made a challenge once relating to this gravity well drop.

Anyway, even if you just do a surface push from a low moon orbit, try to do it when the moon's motion is the direction you want to go (much like going east when launching from Earth surface) or like a pitcher throwing a fastball, the ball is flicked by the fingers at the end of a moving arm. And you should plan to arrive at your Pe of the sun, meeting the Earth at that point (and heading east on Earth surface) and then, if possible, aerobrake. If your can't aerobrake, then engine brake. If you don't have enough fuel that way then, you probably don't have enough fuel to do the trip.

Alright, thanks again for all the great pointers. My initial plan was to start my trajectory home simply by doing a single burn right from Titan, but for some reason I couldn't get TransX to plot my course on the interplanetary scale so my only choice was to break orbit from Titan and then plan my Saturnian ejection burn from there.

As for doing the burn when the moon is moving in the direction I want to go, just to make sure I understand properly, in this case that would be when Titan is on the 'inside' part of its orbit, IE, between Saturn and the sun? And I would make my burn prograde when I'm on the sunny side of Titan assuming I'm orbiting west to east (and assuming Titan does the same for Saturn!)? So Titan would be moving in a direction that would help me speed up in relation to Saturn in order to break orbit, and slow down in relation to the sun to lower my periapsis towards Earth's orbit.

 

[Tommy]

If I understand what you mean correctly, then a Hohmann transfer from Saturn would mean I would reach Earth's orbit (and Earth itself, if timed properly) at the periapsis of my orbit around the sun. Wouldn't that mean that's the point at which I would be going fastest and therefore the worst place to intercept Earth?

Actually, its the opposite. It's the best time, not the worst. The Oberth effect makes the burn more efficient, since you are near a large mass. In general, the best time to change your velocity is when you are going the fastest, and the best time to change your plane is when you are going the slowest.

 

[Linguofreak]

Thanks for all the suggestions, guys, though I have one question about the issue of the Hohmann transfer. If I understand what you mean correctly, then a Hohmann transfer from Saturn would mean I would reach Earth's orbit (and Earth itself, if timed properly) at the periapsis of my orbit around the sun. Wouldn't that mean that's the point at which I would be going fastest and therefore the worst place to intercept Earth? I'm probably just misunderstanding what you mean or missing an important concept here...

There are several things involved here:

First of all, out of all the orbits that touch or cross both Earth's and Saturn's orbits (and thus can possibly be used to get between the two planets), the ones that just touch Saturns orbit at apohelion and just touch Earth's orbit at perihelion will be the ones that require the least delta-v when departing from Saturn.

The second issue is that orbits which cross Earth's orbit instead of just touching it at their perihelion will meet it at a greater angle, and thus quite likely a greater velocity relative to Earth, even if the velocity relative to the sun is lower.

The third issue is that burns at periapsis are more efficient, because even though you're going faster, the difference between your velocity at periapsis and escape velocity is lower than at any other point on the orbit.

 

[flytandem]

Had a couple of days out recreating. Nothing like a relaxing electric canoe trip from a vacation cabin to a beach resort/bar with live music and wine. Anyway getting home and reading this it inspired me to plant a ship on Titan and see about how to find a way back in one single climb to orbit with a single prograde burn within 1 orbit.

First I used scenario editor to place a surrogate ship on Saturn surface. TransX, escape plan, to see where as projected in ecliptic that the ship would have to be relative to Saturn when orbiting Saturn on a plan to go to Earth. The plan was mostly a retrograde away from Saturn (relative to its orbit around the sun) with a bit of inward to arrive at Earth at the perihelion. Also a bit of plane change was tossed in to help make for a close encounter and slow 10K arrival at Earth. In the first stage one gets to choose the Pe height so I opted for one that about matched the orbit of Titan. It showed the location that Titan should be for the ship on Titan to do such a planned eject. It was just a visual but FWIW was about the 5 o'clock position in Titan's orbit around Saturn. I used today's date as a starting date.

Then, looking at the ship on Titan, and TransX, escape, to see it orbiting Saturn, I advanced the date (not with scenario editor, but warping ahead) so that Titan was about where the ship on Saturn had showed it should be for the Saturn Earth flight (about 5'oclock position). I was now ready for launch.

Now to get the direction for launching I figured I should first place the Saturn surface ship (using scenario editor) into an orbit that about matched Titan height and more importantly, had the exact INC and LAN as shown in the TransX plan. Then for the ship on Titan I targeted that ship, and I knew I would be doing a gob of prograde away from Saturn so I tossed in 5K and then visually adjusted the plane change to get it to make a fairly circular view of this ship. It showed my departure direction of about 165 degrees (surprising but who am I to argue).

Once in orbit I canceled all plan velocities, changed my target from the ship to escape Saturn and target Earth, turned on a maneuver, tossed in a first guess of 5K prograde, advanced the date to find out how it would make for for a perihelion and saw quickly that it was a fair bit inside Earth. I backed the prograde burn down to somewhat less, and after going back a forth a couple times between prograde and date I had a very clean maneuver of a bit under 4200 m/s of only prograde had a nice soft 9800 m/s encounter speed at Earth. That's soft for a trip back from Saturn. And much to my pleasure it looked a lot (almost identical) like the original plan that had been set up for the surrogate ship sitting on Saturn.

I knew that I couldn't save my orbit and plan and had to do the burn and get on my way first. Something about scenario saves near Saturn that don't work. Hey has that ever been fixed?

And in hindsight I should have tweaked the orbit of the surrogate ship so that it's node with Titan was where Titan sat at launch date (the current date). And then move, with the editor, the ship opposite Titan to the other node and brought the TransX eject orientation around to sit on top of this ship and then burned a plane change to match the required surrogate plan (rel inc to zero). This would have allowed a more precise launch direction for the Titan ship I suppose but since I still had to guess at the approximate prograde burn getting a launch heading is not terribly important to be exact. After all if I am going to burn 4200 m/s then being even as much as 45 degrees off plane and going just a fraction of that while in orbit, I would only be adding another few hundred m/s. Not huge considering the dv of the entire flight.

So after I was done, I sat for a moment wondering if it would at all be possible to explain what the heck I had just done to figure my way home from Titan in one climb to orbit and pro-burn. I think I found my way back far better (took 15 minutes) than I was able to explain it (30 minutes).