• 0
MPDerksen

Planning a Mun mission (rocket design basics)

Question

I think I can knock out 3 missions in a single flight.  2 request science data be transmitted or recovered from Kerbin and the Mun, respectively.  For those, I think I just take a thermometer and an antenna, take a reading from orbit, and then send it back.  I have a level 2 tracking station, so I think a simple communitron-16 will cover it.

The 3rd one is asking me to put an unmanned probe into Mun orbit, with specific numbers, and has me puzzled.

1)  Unmanned means using an OKTO, but I won't have control on the dark side of Mun, so I wonder if it's best to carry the probe and launch it from a manned rocket?  Or put some relays around Mun first?  (that would be an entirely different challenge, I think).  Just put the probe on top of the capsule and separate at the right time?  Put some RCS stuff on there in case I need to tweak it?

2)  The orbit requested says "Longitude of Ascending Node. 177.1 degrees".  I understand what it's asking, but I'm not clear how to know the Longitude as I move it.

3)  Here is the fundamental question:  planning a mission, I tend to think down.  I design and build the top that will accomplish the goals (in this case the probe), then put enough horsepower below it to get where I want to go.  In this case, I'm not clear HOW MUCH dV I'm going to need for all this.  Rather than telling me how much for this one, can you help me learn how to figure it out myself?  This way, I'm not asking for EVERY mission I attempt.  

4)  Bonus question:  I also have a 4th mission of rescuing Edtop from the orbit of Mun.  Now that I've done this successfully 3 times from Kerbin orbit, maybe I can try this one before/after my probe launch?  

I'm generally thinking that I can take the same rocket I've been using for the last 3 rescues, stick a probe on top (need help with design), and then add enough dV to complete everything.  That would be the over-all mission plan.

Share this post


Link to post
Share on other sites

25 answers to this question

Recommended Posts

  • 1

A fairing would definitely help, not only for drag reduction but to protect from heat on a decent launch trajectory. Unless you want to waste a ton of dV by going straight up for awhile then pitching hard over at 30 or 40km high.

More important, I see that those orbits are going in different directions. The most efficient place to change direction would be at a high (edge of SOI is best) AP, or to land and do the surface rescue and then launch in the other direction. Alternately, on the way to the Mun, you could release the sat to match orbits on the one side, then tweak the trajectory so the rescue ship is lined up to match orbits on the other side. The earlier you do the tweak, the less dV it will take.

Share this post


Link to post
Share on other sites
  • 0
2 hours ago, MPDerksen said:

1)  Unmanned means using an OKTO, but I won't have control on the dark side of Mun, so I wonder if it's best to carry the probe and launch it from a manned rocket?  Or put some relays around Mun first?  (that would be an entirely different challenge, I think).  Just put the probe on top of the capsule and separate at the right time?  Put some RCS stuff on there in case I need to tweak it?

As I understand these sorts of missions, the mission is accomplished when a vessel with the correct specifications enters the correct orbit and stays there for ten seconds.  As such, I don't think there is any specific prohibition against taking it on the nose of a manned rocket.  Don't forget any antenna or other accoutrements that the mission might require--some of them are terribly specific.

2 hours ago, MPDerksen said:

2)  The orbit requested says "Longitude of Ascending Node. 177.1 degrees".  I understand what it's asking, but I'm not clear how to know the Longitude as I move it.

There should be a target orbit in the map view; the correct longitude of ascending node is wherever the ascending node is on that target.  KSP doesn't provide a good way to know that orbital parameter.  Zero longitude is a specific direction in the game, but since the skybox is essentially wallpaper painted on the face of infinity, it doesn't correspond to any actual celestial object.  You'll either need to use a mod that gives you the information or else get comfortable with flying from map view.

2 hours ago, MPDerksen said:

3)  Here is the fundamental question:  planning a mission, I tend to think down.  I design and build the top that will accomplish the goals (in this case the probe), then put enough horsepower below it to get where I want to go.  In this case, I'm not clear HOW MUCH dV I'm going to need for all this.  Rather than telling me how much for this one, can you help me learn how to figure it out myself?  This way, I'm not asking for EVERY mission I attempt.

Your design approach is still correct; it's just that you need to add up a bit more delta-V at the top end, which of course will have exponential increases down the rocket in terms of necessary fuel.  But delta-V adds:  if you need, for example, one hundred for the probe release and five hundred for the rescue, then take six hundred for that part of the mission.  The Mun transfer is going to cost the same no matter what you take; you'll just need to account for the extra mass.  Of course, you'll need to take care because of drag for the launch from Kerbin, but I assume you know how to handle that.

The trick for these sorts of missions is knowing which parts can be combined with other parts and which are best done independently; anything combined saves fuel.  For example, it may pay off the most to capture with a perispsis close to the Mun and an apoapsis at the probe orbit; you simply leave the probe with enough fuel of its own to fix its orbit.  Then you can continue to burn down for the rescue (assuming the rescue is closer to the Mun than the probe orbit) at the next periapsis and you won't need to carry any extra fuel beyond what you need to loft the probe to that point in the first place. This works because it's trivially easy to give a probe the hundred (or whatever) metres per second it will need to do its mission, as opposed to changing orbits with a more massive rocket up high where you don't get any free efficiency.  But the best part is that so long as you burn at the near-Mun periapsis, it doesn't cost you extra fuel to drop off the probe; the only expense is time.

As far as knowing what the delta-V costs will be if you've never been in the neighbourhood, you'll need either a map or a good calculator.  But I got the impression that you can plan for a single mission just fine; it's the two-for-one that you've not yet done.

3 hours ago, MPDerksen said:

4)  Bonus question:  I also have a 4th mission of rescuing Edtop from the orbit of Mun.  Now that I've done this successfully 3 times from Kerbin orbit, maybe I can try this one before/after my probe launch?

There's no reason not to do so, especially if you can cut the probe mass to near-trivial levels.  I caution you to remember the extra required delta-V for the return and the higher speed of moon return re-entry.  Also double-check your extra seat; nothing inspires imaginative cursing like taking a rescue ship that has no room, but without your knowledge until you get there.

3 hours ago, MPDerksen said:

I'm generally thinking that I can take the same rocket I've been using for the last 3 rescues, stick a probe on top (need help with design), and then add enough dV to complete everything.  That would be the over-all mission plan.

Probe design is fairly straightforward:  you need the core, a battery, antenna, and solar panel or four, plus fuel and an ant engine if you want them (hint:  yes you do), and whatever else the mission specifies.  Sometimes the mission asks for certain science experiments.  Sometimes, it wants a docking port.  Sometimes, it asks you to take certain other parts (like landing gear, for some reason I don't ken), so be sure that you add them.  Then add a separator (not a decoupler), a fairing, and stick it on top of your rocket's nose.  For these kinds of probes, you should keep the size down to .625-metre parts.

Don't forget that a probe on your nose means that a parachute won't be.  Take radials.

Good luck!

Share this post


Link to post
Share on other sites
  • 0

Very cool.  Here is a picture of the 2 target orbits.  The green is my goal for leaving the probe for mission #3.  I'm thinking I will hit that first and leave the probe behind (leaving my rocket lighter) and THEN move out to intercept the Derelict/stranded Kerbalnaut in the yellow, higher orbit.

9awpkw.jpg

(You can also see the stranded Kerbal on the dark side that I will land and rescue in the next mission)

Below is my first guess at a craft design.  This is TOTAL shooting in the dark.  The bottom is the empty seat for my rescue, above is my pilot (radial chutes).  I've attached this via a docking port because I also have a mission to dock 2 vessels above the Mun (which will be in the next mission when I rescue Phowell from the surface).  I have some science stuff in the bay, along with a single monoprop can.  I didn't know where else to put it.  I have 2 x 1x6 panels on the side and a battery below the OKTO.  My tracking station is level 2, so the pair of communitron-16s will be enough, right?  can you please explain why I would want the ANT engine instead of just the RCS jets?  If I add that, I can't have a docking port.

10f8d3a.jpg

In your comments above, you mention the higher return velocity from Mun.  I still haven't done enough of this to know better, so how much dV will I need once I'm on my way back to loop around and land?  This might be the perfect mission to try and land at the pole for getting science from that biome, but I'm not bringing any science modules home so all I would get is the EVA and surface sample?

Do i need a fairing around this thing?  I can always just test fly it and see.

And finally, if I have a good 1st and 2nd stage saved, do I have to load that, and then build on top of it?  Or is there a way to pull it out and stick it under the probe I just built?

Share this post


Link to post
Share on other sites
  • 0
4 minutes ago, StrandedonEarth said:

More important, I see that those orbits are going in different directions. 

I TOTALLY missed that.  Wow, that would have been frustrating.  I'll skip the rescue on this trip, then, and just get the probe stable and get my science missions done.  This means I don't have to come back, which changes the math and the design quite a bit, right?

So that I have solid control, what do you recommend as a command unit for this?  Just the OKTO?

Edited by MPDerksen

Share this post


Link to post
Share on other sites
  • 0

I generally use the best probe core available. You may want to stack two probes, and send one with a relay antenna well past the mun to act as a relay. So you may as well do the orbital rescue anyways; there are many ways to tweak the kerbin departure orbits to get to the different orbits. Or combine the surface rescue with the sat launch.

Edited by StrandedonEarth

Share this post


Link to post
Share on other sites
  • 0

3aayt.jpg

So I put a probe in orbit a while back (it doesn't have an antenna, I don't believe).  This is my concern with using an un-manned craft.  Once it goes "red", I can't control it anymore.  I'm still new to Career mode, so should I take some time to put some relays in my system?  Why isn't my probe "relaying" my signal to Mun?  Lack of antenna? (which was a stupid oversight).

This is what I have to choose from.  My 2 pilots each only have a single star.

4jkshe.jpg

Share this post


Link to post
Share on other sites
  • 0

Well, you certainly don't want the Stayputnik, so that leaves the OKTO.  Bear in mind that for a relay, you have to put a relay antenna on the craft. The foldable dish relay should be plenty within the Kerbin system, especially since I tend to mount them in pairs for balance.

I don't know what level your pad and VAB are at, but I often stack two pods together and let them renter one by one. Once I even did this, to get two rescue contracts done (or was it 3?):

4PAtKbX.png

That was in 1.0.4, I never turn reentry heat off, but it managed to reenter like that. It must been from LKO, and maybe a shallow entry profile. But I could have separated them, de-orbited separately,  and re-entered just the pods.

Share this post


Link to post
Share on other sites
  • 0

Since you have "no control without signal" turned on in your settings, your case is different from what most people face. With the default setting (off), you could do this mission with no kerbal and no relay.

As it is, you need one or the other, or you won't be able to match the proper satellite orbit.

Once you have matched that orbit, if you have a kerbal onboard -- you can just EVA the kerbal, and then the contract will complete. (But yes, you still need the probe core.) If you use a relay, I'd put it really high over the Mun, and time my mission for when the relay is over the back side.

You can try doing math to calculate deltaV requirements, but I think that flying and reverting test missions is more fun than doing math. I just create a craft with a huge amount of deltaV and do the entire mission, and see how much I have left or how much I was short.

If your commnet lines are red, then you still have signal and control. It's when they turn black that you've got nothing.

I tend to do my rescue missions after the satellite missions are complete, because of the "unmanned" requirement in the satellite missions.

Getting back from the Mun to LKO takes the same amount of deltaV as getting to the Mun, unless you want to try something crazy and dangerous like aerobraking. If all you want to do is just throw away your rocket and land one guy in a little capsule with a heatshield, that takes very little deltaV at all.

Your original design with the RCS and no Ant would have worked nicely, if you had enough deltaV -- especially if you made a few small changes. (Adding a small aero nosecone, getting rid of the service bay -- you don't need fairings or service bays sometimes.) You also don't need to leave the satellite there. Your contract will complete after 10 seconds, and then you can do anything you like with the whole craft. Like take it and complete another satellite mission.

KSP is supposed to be educational and make you think, and trying to complete many contracts with the same craft is one of the best ways to stretch your mind. It conflicts with roleplaying though, so if you think roleplaying is more fun, then don't worry about it.

 

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, StrandedonEarth said:

Well, you certainly don't want the Stayputnik, so that leaves the OKTO.  Bear in mind that for a relay, you have to put a relay antenna on the craft. The foldable dish relay should be plenty within the Kerbin system, especially since I tend to mount them in pairs for balance.

I haven't unlocked the relay antenna yet.  I just checked, and only have the 2 "16"s and the HG-5.  I need more science to move forward on the tree.

1 hour ago, bewing said:

Since you have "no control without signal" turned on in your settings, your case is different from what most people face. With the default setting (off), you could do this mission with no kerbal and no relay.

I didn't MEAN to change that setting, in fact, I don't even know where it is.... lol.  

I don't think I'm in the right place, or maybe I just started the game that way and now I'm locked in.  

2r4n03l.jpg

I'm totally having fun with this, but should probably just ask less and try more.  Linking a few missions together IS a neat challenge, but stretching too far would be more frustrating than educational.  

Share this post


Link to post
Share on other sites
  • 0
20 minutes ago, MPDerksen said:

I didn't MEAN to change that setting, in fact, I don't even know where it is.... lol.

It's in your "Difficulty options" under the "Advanced" tab. And no, you can change it at any time -- you're not locked in.

Share this post


Link to post
Share on other sites
  • 0
4 minutes ago, bewing said:

It's in your "Difficulty options" under the "Advanced" tab. And no, you can change it at any time -- you're not locked in.

Found it.  This is how it's set right now.  I left it until you confirm I should click anything :). 

148kw1l.jpg

Share this post


Link to post
Share on other sites
  • 0

OK, you have it turned off. So that's good the way it is. You can still partially control your crafts even when you have no commnet signal. That means you don't need relays or pilots if you don't want them.

Share this post


Link to post
Share on other sites
  • 0

More success!  I got my Mun orbit data and was able to rescue the derelict around Mun on the first try.  Any tips on getting a capture to control the direction of orbit?  I got lucky.

Second, now that I'm coming from further away, is there a window of acceptable approach vector/speed to keep me from burning up on re-entry?

Share this post


Link to post
Share on other sites
  • 0
52 minutes ago, MPDerksen said:

Any tips on getting a capture to control the direction of orbit?  I got lucky.

In general, as you are creating your close approach, you are burning prograde. You start out with a very high Munar Pe and a prograde capture. As you burn more, your Pe decreases, and decreases, and decreases, and goes below the surface of the Mun. If you burn some more, your Pe goes to zero, then pops out the other side of the Mun with a retrograde capture, then starts increasing. So that's usually the best way to control the direction of the captured orbit. If you are coming in from Minmus or someplace stranger, you may start out with a retrograde capture. At that point you have to fiddle around with all the handles on a maneuver node to find out what turns it into a prograde capture. Worst case, you actually enter the Mun's SOI, capture to a super high Ap, and then reverse your orbit at the Ap for a very low cost.

Quote

Second, now that I'm coming from further away, is there a window of acceptable approach vector/speed to keep me from burning up on re-entry?

It depends on what parts you are using in your craft, and how much drag you can deliberately create. High tech, extremely heat resistant parts have a wide window for a direct reentry with no retro burn in the Kerbin atmosphere. Low tech MK1 parts don't, and usually require a big retro burn. Even if you do a full retro burn to circularize back into LKO, you still may have to retro burn some more before you reenter.

If all you have is a MK1 command pod, parachute, and a heatshield, then you can aim for a reentry Pe of 30km from the Mun. For anything else you need to experiment to find out a safe and effective reentry Pe.

Edited by bewing

Share this post


Link to post
Share on other sites
  • 0
On 2/17/2019 at 8:10 PM, bewing said:

In general, as you are creating your close approach, you are burning prograde. You start out with a very high Munar Pe and a prograde capture. As you burn more, your Pe decreases, and decreases, and decreases, and goes below the surface of the Mun. If you burn some more, your Pe goes to zero, then pops out the other side of the Mun with a retrograde capture, then starts increasing. So that's usually the best way to control the direction of the captured orbit.

Not sure I follow you.  Here is my current situation.  I need to match a counter-clockwise orbit to complete this mission (as well as learn something).  Where would I "burn more"?  Or should I have moved my maneuver node around to catch it AFTER I hit the Ap of my approach?  I still have 1,400 dV in the probe, which should be plenty.

24ce4d1.jpg

Share this post


Link to post
Share on other sites
  • 0
28 minutes ago, MPDerksen said:

Not sure I follow you.  Here is my current situation.  I need to match a counter-clockwise orbit to complete this mission (as well as learn something).  Where would I "burn more"?  Or should I have moved my maneuver node around to catch it AFTER I hit the Ap of my approach?  I still have 1,400 dV in the probe, which should be plenty.

24ce4d1.jpg

Heh. OK, you have a special situation there. You are going to crash into the Mun! :D  You can tell, because the display is not showing a Pe.

A counter-clockwise rotation around anything in the Kerbin system is prograde. So you actually need a little bit less oomph here.

So, click on the Mun and say "focus view". Look at your projected orbit, and note that it has you plowing into some crater somewhere. You also may be above the Mun's North Pole, or below the South pole -- it's hard to tell from that one pic.

In any case, you are going to need to either burn west, or retrograde. You may also need to add a little normal/antinormal to approach a little closer to the equator -- or closer to your target orbit around the Mun.

 

 

Share this post


Link to post
Share on other sites
  • 0
11 minutes ago, bewing said:

Heh. OK, you have a special situation there. You are going to crash into the Mun! :D  You can tell, because the display is not showing a Pe.

A counter-clockwise rotation around anything in the Kerbin system is prograde. So you actually need a little bit less oomph here.

So, click on the Mun and say "focus view". Look at your projected orbit, and note that it has you plowing into some crater somewhere. You also may be above the Mun's North Pole, or below the South pole -- it's hard to tell from that one pic.

In any case, you are going to need to either burn west, or retrograde. You may also need to add a little normal/antinormal to approach a little closer to the equator -- or closer to your target orbit around the Mun.

 

 

 

I mis-spoke.  My target orbit (seen in red) is clock-wise, but yes, I'm destined to "land" in a crater at very high speed.  Not ideal.  The good news is that I have learned to launch equatorial and not have to adjust my AN and DN.  I think I corrected about 0.2 degrees to create this.  

So what should I have done differently at the original burn?  Wait just a little longer to start the burn to enter the SOI leading my target?  And, more importantly, what do i do about it now?  Push my orbit "left" with the normal/anti-normal maneuver?  You can see from my previous picture that I've just left Kerbin.

rk8qjd.jpg

Share this post


Link to post
Share on other sites
  • 0
25 minutes ago, MPDerksen said:

So what should I have done differently at the original burn?  Wait just a little longer to start the burn to enter the SOI leading my target?  And, more importantly, what do i do about it now?

No, in that case you are in good shape. Just burn a little bit more prograde. You could have done it during your ejection burn too.

The theory is: burning prograde now will make you arrive at your encounter sooner. So the Mun will not have moved as far in its orbit. So you will arrive "in front" of it. Which will put you in a retrograde orbit.

Keep this view that you are showing in this picture. Lock SAS to prograde, and burn very gently while watching your orange projected orbit. You will see it move west over the surface of the Mun. Then it will hit the center of the Mun dead on. Then it will continue moving west until it emerges out the west side of the Mun and gives you a Pe orbiting in the other direction. Just watch it, and it will probably suddenly make sense.

For bonus points, stop burning when your projected orbit is tangential to your target satellite orbit.

 

Edited by bewing

Share this post


Link to post
Share on other sites
  • 0
32 minutes ago, bewing said:

No, in that case you are in good shape. Just burn a little bit more prograde. You could have done it during your ejection burn too.

The theory is: burning prograde now will make you arrive at your encounter sooner. So the Mun will not have moved as far in its orbit. So you will arrive "in front" of it. Which will put you in a retrograde orbit.

Keep this view that you are showing in this picture. Lock SAS to prograde, and burn very gently while watching your orange projected orbit. You will see it move west over the surface of the Mun. Then it will hit the center of the Mun dead on. Then it will continue moving west until it emerges out the west side of the Mun and gives you a Pe orbiting in the other direction. Just watch it, and it will probably suddenly make sense.

For bonus points, stop burning when your projected orbit is tangential to your target satellite orbit.

 

Do I get bonus points ?  So yes, that was very helpful in actually understanding the goal.  

Thank you.

 

esw7sk.jpg 

Share this post


Link to post
Share on other sites
  • 0

Yup, that counts for bonus points! Just gotta brake properly into that target orbit now and you get the big kerbucks.

Share this post


Link to post
Share on other sites
  • 0
43 minutes ago, bewing said:

Yup, that counts for bonus points! Just gotta brake properly into that target orbit now and you get the big kerbucks.

Before I close this out (mission was successful), a general question about creating Mun/Minmus encounters:  should I have done my burn a little later and had the same effect?  Once I create the node, I can slide it around my current orbit, but I see so many different colored lines and encounter circles, I can't really tell what i'm shooting for.  So I end up just settling for getting on my way, then refine halfway along.  Doesn't seem a very efficient way to do this.

Share this post


Link to post
Share on other sites
  • 0
4 hours ago, MPDerksen said:

Before I close this out (mission was successful), a general question about creating Mun/Minmus encounters:  should I have done my burn a little later and had the same effect?  Once I create the node, I can slide it around my current orbit, but I see so many different colored lines and encounter circles, I can't really tell what i'm shooting for.  So I end up just settling for getting on my way, then refine halfway along.  Doesn't seem a very efficient way to do this.

Yes, starting the burn a little later around would have worked. Still, to get a retrograde orbit you will always have to burn a little harder than to get a prograde orbit.

Picture what is happening when you reach the "top" of your orbit around Kerbin. You are far out on an eliptical orbit, so going slowly. If your Ap is slightly closer in to Kerbin than the Mun is (you didn't quite burn hard enough), the Mun comes sailing past above you and picks you up with its gravity. You therefore get dragged in and whipped around behind the Mun... giving a prograde flyby. You then burn against the direction the Mun is dragging you, to circularise into a prograde orbit.

If, on the other hand, you burn harder out from Kerbin, so that your Ap is higher than the Mun's orbit, the opposite happens. The Mun comes racing up behind and below you, between your ship and Kerbin. It picks you up and drags you along again, but you've already passed in front of the Mun by the time it arrives. As the Mun drags you behind it, this time you're going clockwise.

So yes, you need to start a bit further ahead to get a retrograde intercept, but the most important thing is getting your Ap slightly higher than the Mun to start with. Remember that whatever happens, you will always be going very slowly out there as you approach your Kerbin-relative Ap, so you are basically just waiting around in the right bit of space so that the Mun can swing by and scoop you up the right way.

Share this post


Link to post
Share on other sites
  • 0

I don't know if you're on PC or console, but if you are on PC, I (and probably many others here) recommend the mod "Precise Maneuver" or "Precise Node" (two variants of the same mod). It makes it much easier to fiddle with the node and see  how the changes affect the trajectory (I hate trying to drag nodes around).

 

Share this post


Link to post
Share on other sites
  • 0
5 hours ago, MPDerksen said:

Before I close this out (mission was successful), a general question about creating Mun/Minmus encounters:  should I have done my burn a little later and had the same effect?  Once I create the node, I can slide it around my current orbit, but I see so many different colored lines and encounter circles, I can't really tell what i'm shooting for.  So I end up just settling for getting on my way, then refine halfway along.  Doesn't seem a very efficient way to do this.

Your efficiency is determined by 1) the time it takes to get there, 2) the dV you spend getting there, 3) personal aesthetics.

So, burning a little later is bad for 1), you have to look at your dV numbers for 2) (but it's usually worse to wait too long), and for 3) you have to decide for yourself.

Just getting an intercept and then tweaking it en route is actually a perfectly fine way to do it. The only times you need to be really careful about your intercept are when you are doing a gravity slingshot, and not just a simple capture.

Share this post


Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.