Flight Question XR5 - Is a round trip from Earth to Neptune Possible?

[TG]

Like many of you, I had a successful voyager style flight to Neptune many months ago. I did this in an XR5 and had enough fuel to barely return to the inner planets.

The problem, of course, was that the return orbit had extremely large speeds as it approached the periapsis, which I had no direct way of lowering for a return to Earth. The speeds approached something on the order of 60km/s, maybe a lot more, and lowering that speed even with magically refueled engines is impossible for the XR5. I then exited orbiter, put my feet up on the desk, and had a beer, consoling myself that I couldn't have done any better. But this problem continued to bug me, and so I finally turn to the orbiter forum to consult more experienced pilots.

My first question is the same as the title, is return to earth's surface (in one piece) possible or has it been done by anyone with an XR5?

My second question is a bit more specific. If it is possible, what maneuvers could you suggest to do this? Even if it is impossible, it should be an interesting discussion for advanced slingshooters in these forums, and anyone interested in heliocentric orbits in general.

Personally, I understand the concept of slingshots relatively well. But for free-return type slingshots, I'm more rusty, and my own feeling is some sort of gravitational braking (?) would be necessary for this sort of return voyage. I've done free-returns with the moon, but have trouble visualizing its effects on highly elliptical orbits such as moon transfer orbits and comet-like heliocentric orbits. It's precisely in mid-orbit slings (perhaps with jupiter or saturn) that the famous figure-of-eight diagram faces limitations as an accurate description. In any case, I could be wrong in that gravitational braking applies in this kind of a situation. Especially because I just made up that phrase, I'm skeptical that it even exists as a possibility. Here is a crude mspaint drawing attempt of what I mean (I realize the planets aren't ideally aligned for this, it's more of an illustration of the concept of reducing your ApA with a sling):

Another helpful choice is to intercept the Earth while its tangential velocity with the sun is closest to yours. In other words, meeting the Earth while it is orbiting in the same direction as you, rather than coming at you from the 'front' or the 'side'. Perhaps this could take 30km/s off your relative velocity with the earth.

Whatever the case, I hope this thread generates a bit of a discussion on these troubling topics, and beginners and newcomers shouldn't feel shy to post their thoughts or questions so that they can try this stuff out too!

 

[Mister Mxyzptlk]

Have you considered an aerobraking maneuver? I am not sure if it is feasible at 60+km/s, but I am curious to know. There are many on the forum who might answer that question.

 

[dgatsoulis]

My first question is the same as the title, is return to earth's surface (in one piece) possible or has it been done by anyone with an XR5?

Yes it is possible, but not directly from Neptune. Even a perfect Hohmann transfer from Jupiter to Earth results at an encounter velocity ~14.5 km/s when you hit the atmosphere (100 km). If I'm not mistaken, that's at the edge of what the XR5 can take. (No cargo, only the pilot, very little RCS fuel and an inverted aerocapture).

Dropping directly from Neptune (~30AU) to Earth would mean that you'd have a velocity of ~41.5 km/s at perihelion = 11.5 km/s excess velocity relative to Earth. (Best case scenario)
That translates to 16 km/s when you hit the atmo at 100 km which is way past what the XR5 can handle.

The best strategy would be to "climb down the ladder" again. Jupiter is the most important step and if you arrange your slingshot around it well enough you could end up with a relatively low velocity encounter with Earth. Then sling the Earth to reduce your speed even more and land at the second encounter, when your velocity has dropped to something the XR5 can handle.

Mars could also prove to be useful, not so much because of it's mass but because of it's atmosphere. (Aerosling) You can dip inside to lose a little bit of speed and then continue with the rest of your journey.

 

[TG]

Words

Thanks for the reply. I'm glad you suggest that it's at least theoretically possible. The Mars aerodip in particular is something that I never considered.

I do have a question about 'climbing down the ladder' as you put it, and maybe you or others could give me some advice:

When you sling around Jupiter to increase your aphelion, not only does your orbit's semi-major axis change, but the entire orbit's direction is changed. What would the character of the sling have to be to lower the energy of your ship towards the sun, or 'climb down the ladder'.

I have a pretty hard time getting an intuitive grasp of what would do this. Tiny mid-course corrections aside, I tend to only do burns during periapses/apoapses, and don't really understand what directional velocity change mid-orbit would squash your orbit into a lower level.

For example, if you did a U-turn type sling around Jupiter back towards the direction you approached from, aside from being difficult that would seem to put you back in the same high energy orbit, despite the fact that you sped up opposite to your original direction (which means you should have slowed down). And if you slung towards jupiter's orbit's retrograde, that wouldn't really slow you down because it's at a right angle to your pre-sling direction, it would just change the orbit and probably add speed. And if you slung towards the sun, that would probably add even more energy to your orbit.

If anyone can explain to me how these slow-down slings work, I'd appreciate it.

---------- Post added at 21:53 ---------- Previous post was at 21:48 ----------

Have you considered an aerobraking maneuver? I am not sure if it is feasible at 60+km/s, but I am curious to know. There are many on the forum who might answer that question.

I'm afraid it's not feasible for two reasons. If flying inverted, the heat you would get for trying to anchor yourself in Earth's atmosphere is much too great for that speed. Also, because you're speeding through the atmosphere at 60km/s, you would need an angle of attack that's higher than the wings' limit just to keep your vertical velocity stable.

If you were trying a traditional non-inverted aerobrake, you'd also have problems with the impossible heat, and because of your speed, you'd shoot through the atmosphere like a bullet and pop out before you even had a chance to do any significant braking.

 

[dgatsoulis]

It's pretty difficult explaining these things without drawing some short of picture. Even then, it's a 2D representation of a 4D situation. With that disclaimer, I'll try to explain what's going on.
The important thing to keep in mind is that there are two "types" of trajectories. One is relative to the sun and the other is relative to the planet.

Let's take the "climbing down" example, with three steps: A→B→C

In this pic, you want to get from A to C. Don't worry about the planets yet, just imagine you have a spacecraft at point A and you want to get to point C. In order to do that, you must make a retrograde burn at point A (green arrow) to get you to the green transfer orbit and then another one at point B (red arrow) to get to the red transfer orbit.

Now imagine that point A is a planet. If you place a spacecraft around it, the exit point of it's trajectory must have the same direction as the retrograde burn in the first pic.

Now imagine that point B is also a planet. If you want to get to the red transfer trajectory -relative to the sun- , the direction of the exit point must be the same as the second retrograde burn of the first pic.*

Is this making any sense?

*(Ofcourse the trajectory of the spacecraft at B is impossible, since it shows a 180° turn, but you get the idea of the approach and departure trajectories relative to a planet.)

 

[ADSWNJ]

Is this making any sense?

A beautifully elegant description, Dgat ... thanks! I've never done a reverse slingshot like this, but it looks like a great thing to attempt. Can you set these up with TransX?

 

[dgatsoulis]

Yes you can. You usually need a "outward angle" smaller than -150° or greater than +150° but that depends on your initial approach. I think flytandem has an example or two on his page.

 

[Cairan]

I once tried something quite interesting on a return trip from Saturn's moon Titan... Instead of coming direct to Earth, I used Venus to aerobrake myself after a sling from Earth into the inner solar system, then back to Earth for final reentry... it added a few months to my journey but by timing everything correctly, I had a blast!

 

[TG]

Is this making any sense?

Absolutely. The concept is a lot simpler than I thought it would be. Thanks a lot for the description!

I guess where I got confused before was that I thought the sling at point B on your images would reverse your heliocentric orbit, when it would in fact only lower its speed relative to the sun.

The practical challenge of this would be to get a tight enough sling to lower your aphelion, but if you played with Transx enough to get to Neptune, you should be able to figure out a way to get back to Jupiter at a good point and direction.

As a side note, what program did you use to come up with those images, if you made them yourself? After seeing those, I really don't want to use mspaint for orbit plotting anymore.

---------- Post added at 13:45 ---------- Previous post was at 13:09 ----------

I once tried something quite interesting on a return trip from Saturn's moon Titan... Instead of coming direct to Earth, I used Venus to aerobrake myself after a sling from Earth into the inner solar system, then back to Earth for final reentry... it added a few months to my journey but by timing everything correctly, I had a blast!

That sounds like a very useful way to get back from the outer planets. Dgatsoulis suggested doing this with Mars, but Venus not only gives an additional braking point, it also has a shorter orbital period and a thicker atmosphere. I suppose the challenge with Venus is that your encounter velocity will be much higher than with Mars, but the fact that you used this to get back to Earth is very encouraging.

 

[dgatsoulis]

As a side note, what program did you use to come up with those images, if you made them yourself? After seeing those, I really don't want to use mspaint for orbit plotting anymore.

Those are vector drawings made with CorelDraw. Then I took JPG snapshots and posted them. Ripley has informed me that inkscape is free and has similar features with Corel, but I haven't used it.

I once tried something quite interesting on a return trip from Saturn's moon Titan... Instead of coming direct to Earth, I used Venus to aerobrake myself after a sling from Earth into the inner solar system, then back to Earth for final reentry... it added a few months to my journey but by timing everything correctly, I had a blast!

:thumbup: Interplanetary pinball should be an Olympic sport.

 

[Cairan]

The trick is that you do not do a "passive" sling with Earth, you do an "active" sling... Let me explain:

We all know that from the standpoint of maximizing impulse from an engine firing, it's better to do it close to a gravitational source than farther out, so here's the trick, and it requires perfect timing!

You get into the SOI of Earth retrograde... barely minutes before the encounter, as close to the atmosphere as possible without contacting the atmosphere, you do a retrograde burn to slow down a bit... since your slingshot is already reducing your sun-relative velocity, by reducing your Earth-relative velocity at the first flyby you are setting yourself up for a much tamer Venus encounter. At Venus, you do the same, reducing your relative velocity a few notch, then doing an aerobrake so that you align yourself for the second and last Earth encounter. However, your second burn at Venus and the aerobrake maneuver there is much more effective than farther out in the solar system...

The catch is that you have to plan before hand how much dV you want left after the burns at encounter time to plan out the next encounter using an hypothetical dV... if there was any residual from the slingshots and the venusian aerobrake, you correct it right away after a sling maneuver, don't wait to be farther out of the SOI...