Aerobraking with interstellar ships...

[Biggen]

How is aerobraking being handled with larger intersteller ships (~150t+) when arriving either at a destination with an atmosphere or Kerbin? I successfully tested aerobraking my ship coming from an orbit height of Minimus to Kerbin at a 37km Pe. Speed was 3200m/s at Pe and it took me a few orbits to slow down enough to drop my Ap close enough to reach my fuel depot in LKO.

However, Im at the end of my Moho mission and Im about to bring back my 4 kerbals on that same ship. Ill be coming in around 5500m/s from Moho. I have plenty of fuel to do a retro burn but it just seems wasteful to have to budget ~2000+ extra dV for each mission just to capture burn back at Kerbin or send a rescue tanker to "top up" that amount.

Ive looked at the inflatable heat shields but those cant be reinflated so Id have to ferry up a new one from Kerbin after each return. This isnt a deal breaker though as I also have to ferry up new full drop tanks and a new lander (landers are jettisoned to save weight on return).

How would I even attach that inflatable to the rear with 4 nukes taking up space? Offset tool I guess? I could put the inflatable between the 4 nukes and attach it to the fuek tank with a docking clamp so it can be replaced?? Just thinking out loud.

Here are a couple of pics of the mothership before I left for Moho.

Edited February 13, 2017 by Biggen

[Foxster]

Rather than asking how you aerobrake a monster craft, you should ask "Why is my craft a monster?"

Rarely is the answer to anything in KSP to add more stuff. It is more likely to be to take stuff away. 

If you minimise your lander and get rid of the crew cabin you would be able to get the whole craft to a fraction of its current size. It would then be easier to figure out how to do the aerobraking.   

[Biggen]

11 minutes ago, Foxster said:

Rather than asking how you aerobrake a monster craft, you should ask "Why is my craft a monster?"

Rarely is the answer to anything in KSP to add more stuff. It is more likely to be to take stuff away. 

If you minimise your lander and get rid of the crew cabin you would be able to get the whole craft to a fraction of its current size. It would then be easier to figure out how to do the aerobraking.   

Thats all well and good, but the dV requirements to get to Moho, land, and get back require a lot of fuel. A bare minimum of 4500dV is required just to get there and get a capture burn. Still have to budget for landing, return, etc...

Sure I could send a flotilla there but that has its own issues as well...

Edited February 13, 2017 by Biggen

[Foxster]

OK. What are you trying to do? Get a man to Moho and back? Anything else?

[Agustin]

From the images You can see he is traveling with more than 3 kerbals. anyway, would like to see a light craft example from foxster to make his point.

Edited February 13, 2017 by Agustin

[Snark]

The usual answer to this problem is:  Heat shields, plus a good transfer window.

If you use a tool like http://ksp.olex.biz, you can see that arriving in Kerbin LKO from Moho will put your Pe velocity at only ~1700 m/s above circular LKO speed, in other words, you'd be hitting atmosphere at around 4000 m/s.  That's eminently easy to handle via aerobraking... as long as you have a heat shield.

If you're coming in a lot faster than 4000 m/s, it means your transfer window wasn't great; the solution is to use a better window for the trip.

If you don't like spending ablator, you can use the inflatable heat shield, which has no ablator and can be re-used.

1 hour ago, Biggen said:

Ive looked at the inflatable heat shields but those cant be reinflated so Id have to ferry up a new one from Kerbin after each return.

Or just leave it inflated all the time. 

1 hour ago, Biggen said:

How would I even attach that inflatable to the rear with 4 nukes taking up space?

...the above design happens to use some modded parts, but you get the idea.  Discussion here:

 

[Foxster]

39 minutes ago, Agustin said:

From the images You can see he is traveling with more than 3 kerbals. anyway, would like to see a light craft example from foxster to make his point.

But why is he taking 3 Kerbals? That's the stuff that leads to large craft. Every extra bit of payload means a lot more fuel and engines to get it to the destination and back. 

Here are some examples of low mass Moho craft...

...or one of my own with 13,304dV in LKO...

Edited February 13, 2017 by Foxster

[Biggen]

42 minutes ago, Snark said:

The usual answer to this problem is:  Heat shields, plus a good transfer window.

If you use a tool like http://ksp.olex.biz, you can see that arriving in Kerbin LKO from Moho will put your Pe velocity at only ~1700 m/s above circular LKO speed, in other words, you'd be hitting atmosphere at around 4000 m/s.  That's eminently easy to handle via aerobraking... as long as you have a heat shield.

If you're coming in a lot faster than 4000 m/s, it means your transfer window wasn't great; the solution is to use a better window for the trip.

If you don't like spending ablator, you can use the inflatable heat shield, which has no ablator and can be re-used.

Or just leave it inflated all the time. 

 

...the above design happens to use some modded parts, but you get the idea.  Discussion here:

 

I didn't even think about keeping it inflated...  I'll play with some designs in VAB and see what I can come up with.  Thanks, Snark!

23 minutes ago, Foxster said:

But why is he taking 3 Kerbals? That's the stuff that leads to large craft. Every extra bit of payload means a lot more fuel and engines to get it to the destination and back. 

Here are some examples of low mass Moho craft...

...or one of my own with 13,304dV in LKO...

The question isn't why but how...

  I get that lighter crafts are better.  But I've been playing for less than a month so you will have to pardon me if I don't have all the expertise some do around here about building rockets...

[Foxster]

18 minutes ago, Biggen said:

  I get that lighter crafts are better.  But I've been playing for less than a month so you will have to pardon me if I don't have all the expertise some do around here about building rockets...

Isn't that why you posted here, to get a bit of help from those with a little more experience? 

My answer to your question is along the lines of not getting into the difficult situation in the first place. So, rather than putting a lot of effort into thinking how are you going to get a massive ship to Moho and back to Kerbin, you are better putting the effort into how to make the craft efficient.  

 

Edited February 13, 2017 by Foxster

[Jarin]

2 hours ago, Foxster said:

Isn't that why you posted here, to get a bit of help from those with a little more experience? 

 

People play in different ways. You've aptly communicated that a smaller craft would be easier to handle here, and posted a useful example pic. However, some people LIKE building big, and will still come here for advice specific to those designs.

@Biggen what's your opinion on some light modding? A simple module manager file makes the heat shield able to inflate/deflate repeatedly.

[Biggen]

2 hours ago, Foxster said:

Isn't that why you posted here, to get a bit of help from those with a little more experience? 

My answer to your question is along the lines of not getting into the difficult situation in the first place. So, rather than putting a lot of effort into thinking how are you going to get a massive ship to Moho and back to Kerbin, you are better putting the effort into how to make the craft efficient.  

 

 Thanks for continuing to NOT answer my OP... Very helpful...

8 minutes ago, Jarin said:

 

@Biggen what's your opinion on some light modding? A simple module manager file makes the heat shield able to inflate/deflate repeatedly.

Actually I like this. Ive seen some posts about doing this. Ill check it out. Im not sure I like the "look" of the inflatable shield always being... inflated. 

[Foxster]

1 minute ago, Biggen said:

 Thanks for continuing to NOT answer my OP... Very helpful...

Instead of being snitty about it, how about a little friendly dialogue where you explain why you are hauling what you are? I personally wouldn't do it but I'm guessing you have a good reason for it. Are you making a fully re-usable craft maybe and so don't want to discard anything on the trip?

[Jarin]

3 hours ago, Biggen said:

Actually I like this. Ive seen some posts about doing this. Ill check it out. Im not sure I like the "look" of the inflatable shield always being... inflated. 

Spoiler

//Allow deflating inflatable heat sheield
//Author: Chris97b
@PART[InflatableHeatShield]:AFTER[Squad]
 {
    @MODULE[ModuleAnimateGeneric]
     {
        @allowAnimationWhileShielded = true
        @disableAfterPlaying = false
     }
 }

Other simple tweaks can be found in @Alshain's Community Database of MM Patches.

[Aegolius13]

Various and sundry thoughts:

• Aerocapture is usually the limiting factor, since you only get one pass.  Once you're captured, you can take as many passes as necessary, which means you can use less severe breaking.  
• A ship's mass and surface area do not scale evenly.  For example, if you double the scale of a ship, mass increases by a factor of 8, but surface area only increases by a factor or 4.  This is bad for aerobraking, since mass correlates with kinetic energy (what you need to get rid of), and surface area correlates with drag (what you use to get rid of it).  Thus, it's inherently harder to aerobrake with a larger craft, even with similar part selection and design choices.  
• A heat shield would be necessary to aerocapture from Moho.  With a big ship, you're probably best off with the inflatable, due to its wide coverage.  Since it does not require ablator, it also tends to work well for multiple passes to bring your apoapsis down. 
• But the inflatable heat shield is very draggy, which can make ships tippy.  You can mitigate this by having lots of attitude control, by concentrating mass at the end of the ship that will have the shield, or by adding drag at the opposite end.
• You can, as you say, do part of your braking with engines and then aerobrake the rest.  But coming from Moho, without a heat shield, that's probably going to mean lots and lots of engine.  
• It is feasible to aerocapture from Duna or Eve without a shield, but it may take a little design work.  And if you're coming from further away but using engines to help, the same design techniques will help you lean more heavily on the aerobraking. 
  • You'll want to choose more heat resistant parts whenever possible. I.e., use 1x6 retractable solar panels instead of Gigantors.  Use Venors or linear RCS thrusters instead of the 4 way blocks (or better yet, get rid of RCS if you don't need it).   Use the small RGU probe core instead of one that can only take 1200 degrees.  
  • Parts that are inherently heat-prone, like science instruments, should probably be stowed in a service or cargo bay.  
  • If your ship tends to be somewhat long and narrow, you will get the best braking performance by flying it sideways to maximize your drag.  Leaving SAS on normal or antinormal works well.  But aerodynamic forces may try to push it out of this alignment.  For example, if you have heavy engines at the back, that side may tend to drift forward.  To get around this, I like to put my engines amidships, on radially-mounted nacelles.  As a side bonus, this frees up the rear node for a docking port.  

 

[Aegolius13]

10 hours ago, Biggen said:

 when arriving either at a destination with an atmosphere or Kerbin?

Also - tolerances vary considerably from place to place.  Thinner atmospheres are more forgiving, since you can go go deeper in atmosphere, and thus spend longer braking more gradually.  I think smaller planets are more forgiving too, since their gravity will not accelerate you quite as much.  

Duna is very forgiving.  As long as you don't have any highly exposed 1200 degree tolerance parts, you'll probably be fine here.  The trick is to dip very low into the atmosphere.  

Kerbin is probably the next easiest body. 

Laythe is, I think, a little tougher than Kerbin.  This is probably because the atmosphere gets dense more quickly as you descend.  But it's fairly similar.  Fortunately, you can use gravity assist around Laythe or Tylo as an alternative way to save energy.  

Eve and Jool are probably auto-death if you don't have a heat shield.  Eve aerobraking is not generally recommended, since there's usually an easier option in the form of those gravity assists.  Eve does not offer alternatives, though. 

Edited February 14, 2017 by Aegolius13

[Biggen]

45 minutes ago, Aegolius13 said:

Various and sundry thoughts:

• Aerocapture is usually the limiting factor, since you only get one pass.  Once you're captured, you can take as many passes as necessary, which means you can use less severe breaking.  
• A ship's mass and surface area do not scale evenly.  For example, if you double the scale of a ship, mass increases by a factor of 8, but surface area only increases by a factor or 4.  This is bad for aerobraking, since mass correlates with kinetic energy (what you need to get rid of), and surface area correlates with drag (what you use to get rid of it).  Thus, it's inherently harder to aerobrake with a larger craft, even with similar part selection and design choices.  
• A heat shield would be necessary to aerocapture from Moho.  With a big ship, you're probably best off with the inflatable, due to its wide coverage.  Since it does not require ablator, it also tends to work well for multiple passes to bring your apoapsis down. 
• But the inflatable heat shield is very draggy, which can make ships tippy.  You can mitigate this by having lots of attitude control, by concentrating mass at the end of the ship that will have the shield, or by adding drag at the opposite end.
• You can, as you say, do part of your braking with engines and then aerobrake the rest.  But coming from Moho, without a heat shield, that's probably going to mean lots and lots of engine.  
• It is feasible to aerocapture from Duna or Eve without a shield, but it may take a little design work.  And if you're coming from further away but using engines to help, the same design techniques will help you lean more heavily on the aerobraking. 
  • You'll want to choose more heat resistant parts whenever possible. I.e., use 1x6 retractable solar panels instead of Gigantors.  Use Venors or linear RCS thrusters instead of the 4 way blocks (or better yet, get rid of RCS if you don't need it).   Use the small RGU probe core instead of one that can only take 1200 degrees.  
  • Parts that are inherently heat-prone, like science instruments, should probably be stowed in a service or cargo bay.  
  • If your ship tends to be somewhat long and narrow, you will get the best braking performance by flying it sideways to maximize your drag.  Leaving SAS on normal or antinormal works well.  But aerodynamic forces may try to push it out of this alignment.  For example, if you have heavy engines at the back, that side may tend to drift forward.  To get around this, I like to put my engines amidships, on radially-mounted nacelles.  As a side bonus, this frees up the rear node for a docking port.  

 

Wow, this is great! Ill have to read it a few times to absorb it all but it all makes sense. After the recommendations, Ill be adding the inflatable for sure.

I did some heat testing in my initial design.  No rcs block thrusters, shielded docking port, basically everything out in the open is rated for 2000k+. But this will be my 1st aerobrake/capture from beyond Minimus. Ill probably end up burning half retro and half radial out to slow down some and see how that goes. Have to F5 for sure.

 

[Vanamonde]

This is how I dealt with the issue. Interplanetary tractor ship empty, 

 

and loaded with cargo. 

 

The heat shields are disributed so that drag helps keep the ship nose-first through atmo, with the engine mass also up front and firing around the cargo mounts to also help with stability. In fact, it stays oriented through aerobraking even with SAS turned off. 

[Blaarkies]

6 hours ago, Foxster said:

Instead of being snitty about it, how about a little friendly dialogue where you explain why you are hauling what you are? I personally wouldn't do it but I'm guessing you have a good reason for it. Are you making a fully re-usable craft maybe and so don't want to discard anything on the trip?

Maybe he is doing a role-playing playing mission, adding some story to what he is doing and why he is doing. Kudos to @Biggen , its a lot more challenging that way. Playing career with the minimalist approach of sticking a kerbal alone in a mercury capsule for years at a time really feels like cheating, i mean even official hard mode career is laughably easy when doing that.

As for OP, try a gravity assist at Eve. Sure gravity assists are finicky, but the one you need is for this isn't all that advanced. The end result will be a Pe at Eve, and Ap/Intersect at Kerbin...thats roughly 3300m/s re-entry at Kerbin, a lot easier since you only need to slow down about 150m/s to reach an elliptical orbit around Kerbin

[Biggen]

7 hours ago, Vanamonde said:

This is how I dealt with the issue. Interplanetary tractor ship empty, 

 

and loaded with cargo. 

 

The heat shields are disributed so that drag helps keep the ship nose-first through atmo, with the engine mass also up front and firing around the cargo mounts to also help with stability. In fact, it stays oriented through aerobraking even with SAS turned off. 

Wow, that ship is impressive! You just leave them inflated and carry on with your missions?

7 hours ago, Blaarkies said:

Maybe he is doing a role-playing playing mission, adding some story to what he is doing and why he is doing. Kudos to @Biggen , its a lot more challenging that way. Playing career with the minimalist approach of sticking a kerbal alone in a mercury capsule for years at a time really feels like cheating, i mean even official hard mode career is laughably easy when doing that.

As for OP, try a gravity assist at Eve. Sure gravity assists are finicky, but the one you need is for this isn't all that advanced. The end result will be a Pe at Eve, and Ap/Intersect at Kerbin...thats roughly 3300m/s re-entry at Kerbin, a lot easier since you only need to slow down about 150m/s to reach an elliptical orbit around Kerbin

Yeah, playing with the big parts are a lot more fun, at least for me. Sending a single Kerbal on a several year journey to Moho and back just doesnt make sense from a realistic standpoint. He needs friends and a place to sleep/work/play for that!

I looked at an Eve intercept, but I have no idea what that would do to my orbit after the flyby to slowdown. Ive never down gravity assists before so I was gun shy to try that.

 

[Cubfan]

10 hours ago, Blaarkies said:

@Biggen  As for OP, try a gravity assist at Eve. Sure gravity assists are finicky, but the one you need is for this isn't all that advanced. The end result will be a Pe at Eve, and Ap/Intersect at Kerbin...thats roughly 3300m/s re-entry at Kerbin, a lot easier since you only need to slow down about 150m/s to reach an elliptical orbit around Kerbin

Exactly what I was thinking. The gravity assist from Eve gets you into a nice circular orbit. It makes getting to Moho pretty easy (with enough fuel, of course). I haven't tried it coming back yet, but I think it would work just as well. As far as big ships; I like large ships and landers. I like mining and sending MPL's everywhere. It makes things tough sometimes, but I don't play this game cuz it's easy. Plus, I really like the look of the MK1-2 command module. It just looks awesome. Heavy, but awesome.

[Biggen]

Well I made it back from Moho this morning. I ended up bleeding off ~1000dV by burning part radial out and part retrograde so I could keep my Pe around 38km once I was back within Kerbins SOI. I was able to get the capture in one orbit and it took a couple more orbits to aerobrake my Ap down to manageable levels to make docking with my fuel depot easier.

ll try using Eve next time and see if I can save some fuel. Never tried a gravity assist so that should be fun. First thing is to add some heat shields though so gotta dry dock this sucker...

From the reading Ive done, to use a planet as a gravity break and thus reduce my orbit and slow my speed,  I need to pass in front of it's respective prograde movement, correct?

Edited February 14, 2017 by Biggen

[DerekL1963]

On 2/13/2017 at 8:59 AM, Biggen said:

Sure I could send a flotilla there but that has its own issues as well...

Just out of curiosity, what issues do you have with flotillas?   And if you're going to Moho, xfer windows happen often enough that you can pre-position too.

[Spricigo]

44 minutes ago, Biggen said:

From the reading Ive done, to use a planet as a gravity break and thus reduce my orbit and slow my speed,  I need to pass in front of it's respective prograde movement, correct?

I may be misunderstanding what you are saying but look like you want to plot a transfer from Moho to Eve and use Eve's  gravity to raise you orbit to Kerbin. In this case what you need its a gravity slingshot (passing behind Eve)

 

[Biggen]

11 minutes ago, DerekL1963 said:

Just out of curiosity, what issues do you have with flotillas?   And if you're going to Moho, xfer windows happen often enough that you can pre-position too.

Just more craft to keep up with. I generally already launch to planets/moons in three sets as follows:

1. Three relay sats launched (in one vehicle) and put into evenly spaced orbits for 100% communication coverage.

2. Autonomous science lander with a rover in its cargo hold is next launched and landed in a nice flat spot.

3. Mothership is sent last with Kerbals so they can carry about on the surface with stuff to do. Only thing returned from the surface is the lander that brought the Kerbals down to begin with. Its jettisoned upon them re-boarding the ship.

I guess this is kind of a flotilla in of itself. But I still think building a decent size ship seems more realistic than flinging out one poor Kerbal for a multi-year mission in a MK1 capsule upon a 20 ton rocket.

Edited February 14, 2017 by Biggen

[DerekL1963]

1 minute ago, Biggen said:

Just more craft to keep up with.

Makes sense.  Everyone has their tolerance for that.