How much fuel to Bop?

[Jeb-head-mug kerman]

Hello! Recently I've been working on my Aquarius rocket, meant to go to Bop and back, and I just wanted to know how much m/s of fuel I need and of how to get to Bop. My rocket is a single laugh vehicle based slightly on SpaceX's starship, and has 8295 m/s of fuel in orbit, and 5 NERV motors.

Thanks for any help!

Edited October 12, 2019 by Jeb-head-mug kerman

[AHHans]

The short answer is: 8295 m/s in LKO will get you into orbit of any other planet or moon. Depending on the target and your flying you might not have enough fuel to also get back.

The slightly longer answer is: have a look at: Community Delta-V Map There it says that you need:

• to leave Kerbin's SOI: 930 m/s
• to get to Jool's SOI: 980 m/s plus a possible 270 m/s for a mid-course plane change
• to capture in Jool's SOI: 160 m/s (If you don't aerobrake or use a Tylo assist)
• to get to Bop: 220 m/s plus a possible 2440 m/s for a plane change
• to capture into a low Bop orbit: 890 m/s
• (to land on Bop from said orbit: 230 m/s)

So in total a minimum of 3180 m/s up to a maximum of 5890 m/s. In my experience you cannot fully avoid the penalty for plane changes, the 2440 m/s inside the Joolian system seem a tad excessive though. If you capture on Jool in the right way you should be able to avoid most of it. So with decent flying and aerobraking on the return to Kerbin you should be able to land on Bop and get back on 8295 m/s.

[Jeb-head-mug kerman]

Thanks AHHans! My plan was to land on Bop and mine there as I thought that would be necessary, so you think that might not be so? And also, would I have to use gravity assists to land on Bop with the amount of fuel I have and get back?

12 hours ago, AHHans said:

The short answer is: 8295 m/s in LKO will get you into orbit of any other planet or moon. Depending on the target and your flying you might not have enough fuel to also get back.

The slightly longer answer is: have a look at: Community Delta-V Map There it says that you need:

• to leave Kerbin's SOI: 930 m/s
• to get to Jool's SOI: 980 m/s plus a possible 270 m/s for a mid-course plane change
• to capture in Jool's SOI: 160 m/s (If you don't aerobrake or use a Tylo assist)
• to get to Bop: 220 m/s plus a possible 2440 m/s for a plane change
• to capture into a low Bop orbit: 890 m/s
• (to land on Bop from said orbit: 230 m/s)

So in total a minimum of 3180 m/s up to a maximum of 5890 m/s. In my experience you cannot fully avoid the penalty for plane changes, the 2440 m/s inside the Joolian system seem a tad excessive though. If you capture on Jool in the right way you should be able to avoid most of it. So with decent flying and aerobraking on the return to Kerbin you should be able to land on Bop and get back on 8295 m/s.

 

[k00b]

[Guest]

1 hour ago, Jeb-head-mug kerman said:

And also, would I have to use gravity assists to land on Bop with the amount of fuel I have and get back?

Depends on your transfer windows but gravity assists are highly advisable in the Jolian system. You can save > 1000 m/s in the capture, depending on where you're going. They're also not all that hard to plan, just take a bit of time and fiddling to get a moon in the correct phase. Passing in front of a moon slows you down, passing behind it accelerates you. 

Gravity assists on the way out of the Jolian system are much harder, I eventually just gave up on them as a matter of fact. You need a particular phase angle and velocity relative to Jool, which means you need to have your starting moon and the assisting moon in exactly the right phases for that, and then do an Oberth burn to get the ejection right. That was too hard for me so I ended up just doing an Oberth burn to eject directly from Laythe orbit, which is where I was.

Bop is not easy to reach though. If you're new to the Jolian system, visiting Laythe or Vall is much easier.

[Jeb-head-mug kerman]

1 minute ago, Brikoleur said:

Depends on your transfer windows but gravity assists are highly advisable in the Jolian system. You can save > 1000 m/s in the capture, depending on where you're going. They're also not all that hard to plan, just take a bit of time and fiddling to get a moon in the correct phase. Passing in front of a moon slows you down, passing behind it accelerates you. 

Gravity assists on the way out of the Jolian system are much harder, I eventually just gave up on them as a matter of fact. You need a particular phase angle and velocity relative to Jool, which means you need to have your starting moon and the assisting moon in exactly the right phases for that, and then do an Oberth burn to get the ejection right. That was too hard for me so I ended up just doing an Oberth burn to eject directly from Laythe orbit, which is where I was.

Bop is not easy to reach though. If you're new to the Jolian system, visiting Laythe or Vall is much easier.

Thanks! In that case I might go to Vall, and how could I go about doing gravity assists?

[Guest]

Just now, Jeb-head-mug kerman said:

Thanks! In that case I might go to Vall, and how could I go about doing gravity assists?

First plot a plain old encounter with Jool, as close to the orbital plane of the moons as you can.

Then, when you're on your way but still pretty far from Jool, set a manoeuvre node. Grab a cup of tea and spend some time tuning it. If you're targeting Vall, you'll want to grav brake off Tylo or Laythe. Make very tiny and meticulous adjustments to your orbit so that you're passing in front of Tylo or Laythe. This is the braking part. The tricky part is that your ideal goal is that when you're ejected from Tylo or Laythe, going much slower than when you arrived, you'll get a nice, tangential, low-energy encounter of Vall, and only need a pretty small burn to get captured there.

Things are rarely entirely ideal however, so expect to spend a bit on a retrograde burn at Tylo or Laythe periapsis to get the Vall encounter. 

Expect to spend easily a half an hour tuning orbits if you've never done it before. You'll get a feel for it soon enough. Start by just planning an encounter on Tylo or Laythe, and observing how different encounters change your trajectory. Once you've got a feel for that, target Vall. It gets easier after you've done it a few times. 

(I had one campaign where I set up bases on Laythe, Vall, and Bop, and ended up doing a quite a lot of gravity pinball around the system. Bop was... not easy.)

[Jeb-head-mug kerman]

Thanks! Can you mini for ore on Vall?

[Jeb-head-mug kerman]

Oh gods, I've just launched my rocket, and I am thinking 'thus is gonna be so easy!' This is because I still will have 7500 m/s of fuel left after the Jool encounter!

I think my rocket could do an Eeloo mission....

[AHHans]

9 hours ago, Jeb-head-mug kerman said:

My plan was to land on Bop and mine there as I thought that would be necessary, so you think that might not be so?

Hmm... So I wasn't as clear as I thought to be. So: with 8295 m/s in LKO you can get to Bop, land there, and get back to aerobrake and land on Kerbin. (Well, yes, it is always possible to screw things up, but I don't expect you to.)

9 hours ago, Jeb-head-mug kerman said:

And also, would I have to use gravity assists to land on Bop with the amount of fuel I have and get back?

You should not need to, but as @Brikoleur said: they are easy to do when entering the Joolian system, can give large benefits, and are good training.(*)

One note about Bop and Val: they are not in the same plane around Jool. So if you plan to land at both of them then you need to plan on having to use most of the 2440 m/s for a plane change when going from one to the other. (You should also try to avoid having to use all that, but planning for the worst case is always prudent.

7 hours ago, Jeb-head-mug kerman said:

Oh gods, I've just launched my rocket, and I am thinking 'thus is gonna be so easy!' This is because I still will have 7500 m/s of fuel left after the Jool encounter!

Ah, yes, the good old "Here goes nothing!" feeling when you launch your first interplanetary mission and don't know yet if it will end in triumph or disaster. I guess you'll do just fine, and probably sooner than later will miss that feeling when you "know" that you can master everything that Isaac Newton throws at you. (Well, except Eve. Eve is a challenge on its own.)

P.S. (*) And not just in the army version of "good training" but also in the regular version.

[Jeb-head-mug kerman]

27 minutes ago, AHHans said:

Hmm... So I wasn't as clear as I thought to be. So: with 8295 m/s in LKO you can get to Bop, land there, and get back to aerobrake and land on Kerbin. (Well, yes, it is always possible to screw things up, but I don't expect you to.)

You should not need to, but as @Brikoleur said: they are easy to do when entering the Joolian system, can give large benefits, and are good training.(*)

One note about Bop and Val: they are not in the same plane around Jool. So if you plan to land at both of them then you need to plan on having to use most of the 2440 m/s for a plane change when going from one to the other. (You should also try to avoid having to use all that, but planning for the worst case is always prudent.

Ah, yes, the good old "Here goes nothing!" feeling when you launch your first interplanetary mission and don't know yet if it will end in triumph or disaster. I guess you'll do just fine, and probably sooner than later will miss that feeling when you "know" that you can master everything that Isaac Newton throws at you. (Well, except Eve. Eve is a challenge on its own.)

P.S. (*) And not just in the army version of "good training" but also in the regular version.

Ah, well! I unfortunately completely screwed up my gravity assists (As I've never done them before) and gave up, but I shall soon try again! I had TONS of fuel but I was just going so fast I couldn't slow down. If you could tell me how to go about them, I would be very grateful!

Edited October 13, 2019 by Jeb-head-mug kerman

[AHHans]

23 minutes ago, Jeb-head-mug kerman said:

I unfortunately completely screwed up my gravity assists (As I've never done them before) and gave up,

Which is why I said to not count on them. Just because they are easy when you know how to do them doesn't mean that it will be easy the first time you try.

23 minutes ago, Jeb-head-mug kerman said:

If you could tell me how to go about them, I would be very grateful!

Well, the main advice was is what @Brikoleur wrote: set up the maneuver node(s) and fiddle around with them until the resulting orbit - after the encounter with Tylo or whatever you are using - is to your liking. By doing a combination of radial-in/-out and pro-/retro-grade you can adjust the time when you cross the orbit of another body, and thus if and at which angle you encounter this body. (You also need to get the plane right, but that shouldn't be news to you.) What makes gravity assists when capturing at Jool easy is that you can combine that burn with your (probable needed) plane-change burn during your transfer to Jool. Doing that burn far away from Jool (and thus its moons) gives you a large amount of leverage so that even small burns have a large effect on when you encounter your target.

My suggestion is: next time you set up a transfer to anywhere: play around with the transfer, pass "in front of" or "behind" the target at different distances, and observe the resulting orbit after the encounter. Not really to actually do that, but to see what kind of encounter will have what effect on your resulting orbit.

One exercise is to set up one burn in LKO that will take you to the Mun, fly by the Mun, and get you back into Kerbin's atmosphere for an aerobrake, with only that single burn in LKO. (That's how NASA did it for the Apollo missions, and why Apollo 13 could get back to Earth after their main craft failed.)

Edit: Once you have some understanding how an encounter affects the orbit, try to set up the mid-transfer burn on the way to Jool to a) encounter Tylo and b) get captured at Jool by that encounter.

Edited October 13, 2019 by AHHans

[Guest]

52 minutes ago, Jeb-head-mug kerman said:

Ah, well! I unfortunately completely screwed up my gravity assists (As I've never done them before) and gave up, but I shall soon try again! I had TONS of fuel but I was just going so fast I couldn't slow down. If you could tell me how to go about them, I would be very grateful!

There isn't any magic trick to them really, just plan the encounter and tune the orbit. Most likely you'll end up braking off Tylo.

With interplanetary transfers, if you have that much dV you can certainly brake. It just takes a while. You might have to split the burn into several more manageable ones to keep control of your trajectory. 

I once did a Moho capture using an ion drive craft with a stupidly low TWR. I think I split the burn into five segments of about four minutes each, then adjusted the following one to correct the error in the previous one. It worked and was very satisfying when I was safely in orbit, but it's not something I'd want to do every day.

[Jeb-head-mug kerman]

thanks

[magnemoe]

On 10/13/2019 at 1:48 AM, Jeb-head-mug kerman said:

Thanks! In that case I might go to Vall, and how could I go about doing gravity assists?

My tricks is to first do the burn to Jool. then set Tylo as target, set up an trajectory who intercept Tylo orbit on the right side of jool just there you would do an circulation burn. 
Now you need to change your arrival time to Jool so Tylo is at this position then you arive, this will change your flyby so you would also have to adjust that. 

Here is 4 bases done with the Tylo gravity assist and on Pol intercept trajectory. 
Note that this trajectory of the 3 other bases are not optimal, better to just lift Ap to Pol orbit then raise Pe to get an intercept. However I had fuel enough and this was cheaper.