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Stabilization of Interplanetary Motherships during Aerocapture


SkyFall2489
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I have a large ship made of small segments held together by docking ports, maybe 100 tons. I need to perform aerobraking maneuvers with it for its planned mission profile, so at the front end there is an inflatable heat shield. However, even with SAS and RCS set to surface prograde, the ship will flip when too deep into the atmosphere and its center of drag is likely in front of the center of mass. Flipping is a major issue with precision, but it will be even worse if the ship attempts to aerocapture at Eve and melts. How can I keep my ship stable during aerobraking manuvers?

Oh,, and as a side question, how much is aerobraking actually saving me? My planned mission is to start in low munar orbit, burn straight to an Eve encounter, capture into an elliptical orbit, transfer to gilly, capture in low orbit, drop of crew at a base, directly burn to kerbin encounter, aerocapture at kerbin and brake until I can get a mun encounter, and finally capture in low orbit at the mun. If I did not aerocaptrue at kerbin, I would use the engines to capture instead.

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15 hours ago, SkyFall2489 said:

Testing of a module with large custom airbrakes made from wing panels helps move the center of drag back enough that the reaction wheels and RCS can keep the ship pointed prograde for the most part - I'm not risking Even unless I have to though. 

That works, this eve accent craft uses it to keep balanced. 
KUH1MSDh.png
Note that its two layers for double drag, the rear tank is almost empty. 
Now this is exploiting KSP drag model, biplanes works because of two wings, layered parachutes does not double drag. 
You could also add airbrakes on the tank to adjust drag. 
6pRm0KTh.png
Here its seen with an Eve orbital tug and its rover still connected. I dropped rover first it did science and then the return rocket to get kerbal out. 
Had to do lots of passes to get into an reasonable circular low Eve orbit as the orbital velocity is high. 
 
Use drag plates a lot on ships going between Minus and LKO

Another way is rear heat shields to generate stability and drag. 
Zmb10mAh.png
Here I launched the two extra heat shields separately and docked them to the tug at the rear. I reused the RCS thrusters on the tug as I forgotten to add them :) 

Edited by magnemoe
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Several of us have tried this sort of thing and the only measure that's worked is to place additional heatshields toward the rear of the craft to cause stabilizing drag. It simply overwhelms any other methods. I'm not home right now or I would share some pics, but there are a couple of methods. 

For interplanetary transfers one generally arrives with a great deal of speed. Aerobraking is a significant assist and worth the effort it requires. 

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Aerobraking at Eve can save you a lot of delta-V, but it is definitely not for the faint-hearted due to the very high (>4km/s) velocities involved which will incinerate almost anything other than a heatshield. If you can do ISRU refuelling at Gilly, I’d say do that instead of risking an encounter with Eve’s atmosphere.

I just did an Eve landing as part of a Grand Tour and this is what it took to keep the thing pointing straight:

acpI1Nz.png

(Ignore Ike in the background…)

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25 minutes ago, SkyFall2489 said:

The other possibility is to refuel at kerbin and gilly, heading down to LKO instead of low munar orbit.

Or, refuel at Gilly then refuel at Minmus when you get back before going to the Mun. It might be worth making the mining gear detachable from the mothership and leaving that on Gilly, then setting up a separate Minmus mining operation in the meantime to have fuel ready when the mothership returns. Trying to make your ship a jack of all trades usually results in it bring mediocre at everything and heavy to boot.

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24 minutes ago, SkyFall2489 said:

Refuelling in LKO would be done with fuel from kerbin, if I were to refuel at the Mun I would set up permanent ISRU equipment that is not carried on the ship. at Gilly I have a large base already - it needs crew and that is why I have this ship. My ship is not a jack of all trades, it is a crew transport. The only point of a munar departure is to save 500 m/s (1 km/s if I don't aerobrake at kerbin)

The best way to do this is to refuel at Minmus who is more efficient or Mun orbit who is easier. set up an transfer burn node on an satelite in LKO towards Eve or other planets. 
Leave the selected moon and drop Pe down to LKO so Pe end up at the satellite node. Do this well ahead so adjust Ap to match the time of the node and avoid the Mun. 

This has the benefit of adding 7-900 m/s dV and the oberth effect  giving you an couple of hundreds m/s cost or go fast as i obviously did in the dramatic Duna aerobrake. 
That one require modification of the ship in flight as I would come in very hot and at 7 g having drag and mass at the center line is critical, the Duna base had two stages it had dropped. 

The Eve landers however was refueled in LKO and sent to Eve, mission was not time critical and the transport stage used fuel from the lander, the cone tanks is for fuel and oxidizer as the tug only used fuel as its LV-N. 
If you have an base you should also have an tug who can catch you 

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1 hour ago, SkyFall2489 said:

It's not time, it's the 900 m/s of Delta-V.

When you are in a hyperbolic trajectory, you only pass near the planet enough, and you must aerobrake all your intercept speed to get captured. else you have no capture. for this you need a more resistent ship, else you have to perform the manuever with rockets.

when you are in an elliptic orbit, you can always aerobrake. if your ship burns, just use a higher periapsis. you will brake less, but it's not a problem because you are in orbit and you will pass in the atmosphere as many times as you need. so you can circularize without any kind of thermal protection. in theory.

in practice, not so much. when you are in a high elliptic kerbin orbit, you reach periapsis at 3200 m/s. At that speed, you can aerobrake 20 to 50 m/s with an unprotected ship. as you slow down, you can take shallower passages and brake more. It may take a few dozen passages, but you can circularize your orbit in a few hours of gaming. when you are in high elliptic eve orbit, you reach periapsis at 4.7 km/s. at that speed, you burn almost instantly. you can brake maybe 1 m/s with an unprotected ship, if you're lucky. and that's not a significant braking, so you can't brake more on the next passage. so you would need a thousand aerobraking passages to circularize orbit, whch are not really feasible.

that's what I mean when I talk of time.

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6 hours ago, SkyFall2489 said:

Do you mean having a ship from the base dock to the transport before it hits eve periapsis and slow the transport down? That would be a little hard to pull off. I will have ships in gilly orbit to refuel the transport when it arrives.

The oberth manuver from mun is much more work than ejecting from mun to interplanetary - plus, when going to eve or duna, eject from mun is actually slightly cheaper. 

 

I think I have a good idea of the mission plan now, and plenty of contingencies to save fuel if needed. Thank you all for the help! I'll start with an LKO refuel, but later I'll set up ISRU at Mun and start parking the transport there. It will be mostly operating from the Gilly base, though.

Not tested direct burn from the Mun but from Minmus an burn from Minmus orbit was more expensive than an burn from LKO who is very non intuitive. 
For Eve just getting into any orbit with low Eve Pe and Ap inside the SOI lets you aerobrake over time or catch it. 

Duna is perfect for aerobraking as its low gravity don't increase your velocity at Pe much, Eve, Kerbin and Laythe cause inflatable heat shields to fail fast if you come in much faster than escape velocity with an heavy ship. Something light can aerobrake much better as it looses velocity so fast. 
 

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4 minutes ago, Vanamonde said:

Several of us have tried this sort of thing and the only measure that's worked is to place additional heatshields toward the rear of the craft to cause stabilizing drag. It simply overwhelms any other methods. I'm not home right now or I would share some pics, but there are a couple of methods. 

For interplanetary transfers one generally arrives with a great deal of speed. Aerobraking is a significant assist and worth the effort it requires. 

It's still only from Eve though, and the eve aerobrake is only if I mess up and have to save fuel. Is it really worth it then? I'm not even going down to LKO, only to the point where I can encounter the mun and park there.

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Testing of a module with large custom airbrakes made from wing panels helps move the center of drag back enough that the reaction wheels and RCS can keep the ship pointed prograde for the most part - I'm not risking Even unless I have to though. 

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5 hours ago, jimmymcgoochie said:

Aerobraking at Eve can save you a lot of delta-V, but it is definitely not for the faint-hearted due to the very high (>4km/s) velocities involved which will incinerate almost anything other than a heatshield. If you can do ISRU refuelling at Gilly, I’d say do that instead of risking an encounter with Eve’s atmosphere.

My plan is to do both, but not refuel at kerbin. I have a large base on Gilly that needs crew, and my plan is to use my mothership to transport crew from low munar orbit to gilly, refuel at gilly, and head back to low munar orbit. I've sacrificed some delta-v for TWR and looks, so I only have barely enough for the round trip. If I ditch the aerobraking stuff I could add more fuel instead. Other ships will get the crew to the mothership in low munar orbit.

 

The other possibility is to refuel at kerbin and gilly, heading down to LKO instead of low munar orbit.

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Posted (edited)
Just now, jimmymcgoochie said:

Or, refuel at Gilly then refuel at Minmus when you get back before going to the Mun. It might be worth making the mining gear detachable from the mothership and leaving that on Gilly, then setting up a separate Minmus mining operation in the meantime to have fuel ready when the mothership returns. Trying to make your ship a jack of all trades usually results in it bring mediocre at everything and heavy to boot.

Refuelling in LKO would be done with fuel from kerbin, if I were to refuel at the Mun I would set up permanent ISRU equipment that is not carried on the ship. at Gilly I have a large base already - it needs crew and that is why I have this ship. My ship is not a jack of all trades, it is a crew transport. The only point of a munar departure is to save 500 m/s (1 km/s if I don't aerobrake at kerbin)

Edited by SkyFall2489
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2 minutes ago, magnemoe said:

If you have an base you should also have an tug who can catch you

Do you mean having a ship from the base dock to the transport before it hits eve periapsis and slow the transport down? That would be a little hard to pull off. I will have ships in gilly orbit to refuel the transport when it arrives.

4 minutes ago, magnemoe said:

This has the benefit of adding 7-900 m/s dV

The oberth manuver from mun is much more work than ejecting from mun to interplanetary - plus, when going to eve or duna, eject from mun is actually slightly cheaper. 

 

I think I have a good idea of the mission plan now, and plenty of contingencies to save fuel if needed. Thank you all for the help! I'll start with an LKO refuel, but later I'll set up ISRU at Mun and start parking the transport there. It will be mostly operating from the Gilly base, though.

 

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19 hours ago, SkyFall2489 said:

 

Oh,, and as a side question, how much is aerobraking actually saving me?

You have a simple way to determine it. When you plan the manuever, where you make the close passage into eve's atmosphere? placea manuever node there, and simulate a capture burn. the simulation says you need 400 m/s before you get in orbit? then aerobraking saved you 400 m/s.

which is more or less what you actually save for capture at eve, coming from kerbin.

do notice that once you are in orbit, you can circularize by short passages in the high atmosphere, even without shields. well, ok, not really at eve, not unless you are happy with losing 0.5 m/s at every passage. but the point is, once you're in orbit you can circularize with time, and it takes little protection. so the heavy shielding only saves the capture deltaV. Not that you want to circularize a mothership anyway, you only circularize the lander

I generally don't bother aerobraking at eve, because intercept speed is relatively low and you need a lot of thermal shielding; easier to burn some fuel. On duna i aerobrake hard, instead, without needing shields.

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40 minutes ago, king of nowhere said:

You have a simple way to determine it. When you plan the manuever, where you make the close passage into eve's atmosphere? placea manuever node there, and simulate a capture burn. the simulation says you need 400 m/s before you get in orbit? then aerobraking saved you 400 m/s.

which is more or less what you actually save for capture at eve, coming from kerbin.

do notice that once you are in orbit, you can circularize by short passages in the high atmosphere, even without shields. well, ok, not really at eve, not unless you are happy with losing 0.5 m/s at every passage. but the point is, once you're in orbit you can circularize with time, and it takes little protection. so the heavy shielding only saves the capture deltaV. Not that you want to circularize a mothership anyway, you only circularize the lander

I generally don't bother aerobraking at eve, because intercept speed is relatively low and you need a lot of thermal shielding; easier to burn some fuel. On duna i aerobrake hard, instead, without needing shields.

What would you say about aerobraking at kerbin? Could I safely do that without heatshields? Assume an interplanetary speed from Eve or Duna.

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2 hours ago, SkyFall2489 said:

What would you say about aerobraking at kerbin? Could I safely do that without heatshields? Assume an interplanetary speed from Eve or Duna.

no. the only place in the stock system where you can safely aerobrake from interplanetary without shields (or without a specialized spaceplane, of course) is duna, due to being small.

on kerbin, though, you can circularize without shields in a reasonable amount of time.

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1 hour ago, king of nowhere said:

no. the only place in the stock system where you can safely aerobrake from interplanetary without shields (or without a specialized spaceplane, of course) is duna, due to being small.

on kerbin, though, you can circularize without shields in a reasonable amount of time.

It's not time, it's the 900 m/s of Delta-V.

I have a few mission plans available:

1. refuel at gilly and LKO (saves a lot of delta V)

2. refuel at gilly and munar orbit (saves even more delta v, but now I have to reload crew and fuel at the mun - which means getting that there in the first place)

3. refuel at gilly, stop in LKO (easiest and simplest, but costs the most fuel)

4. refuel at gilly, stop in low munar orbit (again, have to get stuff to the mun, but it saves some fuel)

 

For any plan I can:

Eve aerobrake (risky, saves around 400 m/s)

Kerbin aerobrake (Saves 1 km/s if stopping in LKO, saves 500 m/s if stopping in low munar orbit)

Both of these would likely require a heat shield module to be attached, which increases fuel needed due to its weight, decreases fuel available due to TWR, and removes a possible spot for a crew module. (As I am using CSMs for control and propulsion, I might be able to detach a CM's SM and put it on the front to act as a mini heat shield, but the ship will not be well protected.)

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Posted (edited)
34 minutes ago, king of nowhere said:

When you are in a hyperbolic trajectory, you only pass near the planet enough, and you must aerobrake all your intercept speed to get captured. else you have no capture. for this you need a more resistent ship, else you have to perform the manuever with rockets.

when you are in an elliptic orbit, you can always aerobrake. if your ship burns, just use a higher periapsis. you will brake less, but it's not a problem because you are in orbit and you will pass in the atmosphere as many times as you need. so you can circularize without any kind of thermal protection. in theory.

in practice, not so much. when you are in a high elliptic kerbin orbit, you reach periapsis at 3200 m/s. At that speed, you can aerobrake 20 to 50 m/s with an unprotected ship. as you slow down, you can take shallower passages and brake more. It may take a few dozen passages, but you can circularize your orbit in a few hours of gaming. when you are in high elliptic eve orbit, you reach periapsis at 4.7 km/s. at that speed, you burn almost instantly. you can brake maybe 1 m/s with an unprotected ship, if you're lucky. and that's not a significant braking, so you can't brake more on the next passage. so you would need a thousand aerobraking passages to circularize orbit, whch are not really feasible.

that's what I mean when I talk of time.

Ah. well my ship can survive 50km periapsis and apoapsis near minmus, so that reentry from eve should be fine without a heatshield. I could always install an airbrake module too.

Edited by SkyFall2489
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10 hours ago, magnemoe said:

Not tested direct burn from the Mun but from Minmus an burn from Minmus orbit was more expensive than an burn from LKO who is very non intuitive. 
For Eve just getting into any orbit with low Eve Pe and Ap inside the SOI lets you aerobrake over time or catch it. 

Duna is perfect for aerobraking as its low gravity don't increase your velocity at Pe much, Eve, Kerbin and Laythe cause inflatable heat shields to fail fast if you come in much faster than escape velocity with an heavy ship. Something light can aerobrake much better as it looses velocity so fast. 
 

Someone else did testing from Mun and found that it saved more delta v than a direct burn from LKO. I modified my mothership a bunch, sacrificing looks for performance, and now I doubt I'll need that much refuelling at all.

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