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i just realized the devil is purple, please help me with eve


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so i sent my first unmanned mission to eve today, and well, i guess i hadnt planned accordingly.

the mothership is in a circular 100km orbit with 8 probes intended to land on the surface and radio back science.......but for hte life of me it seems their is absolutely no way to land these.

i think i drastically underestimated EVE's atmosphere.

any tips on how to get these down to the surface in one peice?
gZh235I.png

it has a heat sheild, but it doesnt really help, the sci jr blows up first(or the airbrakes if i forget to spam them open and close) they each have around 800dv, 2 drogues, 2 regular chutes, and four airbrakes.

and why cant i imbed pictures?

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1 hour ago, putnamto said:

any tips on how to get these down to the surface in one peice?

A larger heat shield would have been nice, but in any case I think part of the reason you're losing the Science Jr. first is because the exposed Twitch engines are conducting heat to it from the atmosphere.  The air brakes and Science Jr. both have a thermal tolerance of 1200 K; the engines have 2000 K.  The fuel tanks match the engines in thermal tolerance; try attaching the engines to those, or try putting the engines and fuel tanks on a decoupler so they can deorbit the probe and then go away, leaving the heat shield alone to manage the thermal load--if you distribute the masses correctly, you won't need a reaction wheel to keep the heat shield facing forward.

The picture appears to have embedded without any trouble.

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2 hours ago, putnamto said:

any tips on how to get these down to the surface in one peice?

A wider heat shield. It's not uncommon for a heat shield to not wholly cover a part of it's nominal diameter. The solar panels, airbrakes, and engines will be exposed in any event. Even if the shield covered the Science Jr, it's likely that the first part to go would upset the balance and send the craft tumbling.

Starting the descent with tanks full and using the engines to decelerate might work -- though I'm not sure if 800m/s will suffice. You want to slow down to 1400m/s by 45km or thereabouts, that means slowing by something like 2km/s. So your airbrakes would need to be good for more than 1km/s of braking in the upper atmosphere. I don't think it will work.

 

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On 4/9/2019 at 12:46 PM, Not Sure said:

Try multiple passes, dip below the atmo and come back out until you drop in entirely with the least amount of energy

Tried  that, eventually  got  fed  up  and  cheated  a  copy  of  the  ship  tgrir  with  bigger  shields.

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13 hours ago, putnamto said:

Tried  that, eventually  got  fed  up  and  cheated  a  copy  of  the  ship  tgrir  with  bigger  shields.

I remember when I sent a half-ablator shielded probe to Eve and still had to burn off over 900m/s to survive entry. That was a fun mission in the end though.

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On 4/9/2019 at 3:00 AM, putnamto said:

any tips on how to get these down to the surface in one peice?

Landing on Eve for a small unmanned probe is either practically impossible or very easy.  ;)  Mainly comes down to whether it's heat-shielded and aerodynamically stable, or not.

On 4/9/2019 at 3:00 AM, putnamto said:

2 drogues

First of all, you absolutely do not need drogue chutes.  Eve's the one place in the solar system where you're guaranteed not to need them.  They're completely pointless.  Reason:  For Eve reentry and landing, it's the reentry heating that's the killer.  If you can survive that, i.e. until you get down under 1200 m/s or so, then you've got it made, because Eve's atmosphere is so thick that it's a parachute's best friend.

It's almost physically impossible to need drogues on Eve:

  • They're irrelevant during the hot part of reentry (because you're going way too fast for them to be able to open anyway).
  • They're also irrelevant during landing, because regular chutes work just fine.  The atmosphere is so thick that you're basically guaranteed to drop under 300 m/s long before you get close to the ground, which means that good ol' aerodynamic drag will easily slow you down to speeds where a regular (non-drogue) parachute can open.
On 4/9/2019 at 3:00 AM, putnamto said:

airbrakes

For little probes like this, you also don't need airbrakes.  For pretty much the same reason as drogue chutes.

You can't use them much during the hot part of reentry, because they have a very low heat tolerance relative to other parts-- they'll blow up.  And once you get past the hot part of reentry, you don't need them because the atmosphere will slow you down just fine.

Now, one situation where airbrakes can be useful on Eve is if you're landing something a lot bigger (like a crewed lander that needs to ascend to orbit again) and having stability problems because it keeps wanting to flip around without the heat shield in front.  In a case like that, you can use airbrakes on the "tail end" (i.e. the end opposite the heat shield) to generate drag there to help keep the ship pointed in the right direction.  It works because the heat shield does protect them from heat in its "shadow", but doesn't shield them from drag.

But that only works if the airbrakes are completely hidden behind the heat shield's "footprint", which only works if the heat shield is really big (for example, the inflatable one).  For a little craft like yours, it's pretty hard to design it in a way that keeps the airbrakes safely hidden behind the heat shield even when extended, so they're almost not worth the bother.

On 4/9/2019 at 3:00 AM, putnamto said:

it has a heat sheild

Ding ding ding ding ding.  :)  This is the answer.  A heat shield is what you need to get you through the hot part of reentry.  As long as you've got a heat shield-- and you have a craft that's aerodynamically stable with the heat shield in front-- then you'll do just fine.  Beyond that, all you need for a little probe is a single Mk16 parachute and you're good.

On 4/9/2019 at 3:00 AM, putnamto said:

the sci jr blows up first

So, depending on how you have things set up, there are a few possible ways this could be going wrong.

Possibility #1 is that you have the heat shield mounted directly to the Science Jr.  If that's the case, that's a problem.  The reason is that the Science Jr., like other science instruments, has a really low temperature tolerance, just 1200 K.  Heat shields get hot during reentry, even when they're burning off ablator.  They don't get hot enough to explode most spacecraft parts (that's why heat shields are effective)... but they do get above 1200 K.  So if you've got the Science Jr. attached directly in direct contact with the heat shield, then one thing that can happen is that the Science Jr. simply overheats a bit too much from being in contact with the hot shield.

If that's the problem, fortunately it's pretty easily solvable.  Most spacecraft components can handle 2000 K or more.  So just put something in between the Science Jr. and the shield-- for example, a Z-1K battery-- as a sort of "buffer".  That way, the heat shield heats up the buffer part (which can take the heat, and doesn't explode)... and then you slow down below the hot part of reentry before the buffer part can in turn heat up the Science Jr. to the kaboom point.

Possibility #2 is that you haven't switched the navball to surface mode.  By default, the navball auto-switches to "orbit" at very high altitudes, and to "surface" farther down.  If you haven't manually tinkered with it, it's possible that you're going through the hot part of reentry while still in "orbit" mode.  That's a problem because the airflow is traveling (relative to your ship) in surface-relative :retrograde: direction, not orbit-relative :retrograde: direction.  So if you're still in "orbit" mode and have SAS set to hold :retrograde:, you may be tilted a teeny bit away from where you should be, which means the side of the Science Jr. may be catching some heat.

Possibility #3 is that your reentry profile is too shallow.  It's a pretty common thing for KSP players unfamiliar with reentry behavior to think "oh, it's dangerous and scary and hot, so I'll just carefully ease slowly into the atmosphere with a really shallow trajectory so I'll spend a lot of time way up high in the atmosphere".  That's a reasonable way to think... but it's also dead wrong.  The fact is, reentry does two things:  1. Slows you down.  2. Heats you up.  But those two don't go hand in hand-- they're not proportional to each other.  A really shallow reentry generates lots of heat but doesn't slow you down much-- which means, paradoxically, that you have much worse reentry heating problems!  You're slow-roasted over a long period of time until everything heats up and kaboom.  But if you dive more steeply, then yes, the heat gets intense... but the slowing-down part increases a lot faster and it ends up being less thermally stressful.  I don't mean you should be diving straight down or anything, but don't go too shallow.  When you do your de-orbit burn, lower your Pe down well under 60 km.

Possibility #4 is that you're not aerodynamically stable with the heat shield in front.  To be aerodynamically stable, you want your CoM to be as close to the heat shield as possible.  Science Jr. is notoriously unwieldy in this regard, because it's big and "fluffy"-- large, but quite low mass.  Aerodynamically speaking it kinda "wants" to be in the trailing position, not out in front.  So you could be in a place where you start out holding perfectly :retrograde:  and are fine for a while, but then as the aerodynamic G-force of reentry builds up, at some point it starts to overwhelm your reaction wheels and you either flip, or else you turn enough away from :retrograde: that the Science Jr. gets exposed to the airstream and then kablooie.

This one can be a bit challenging to solve, it mainly depends on the fine details of how you have your components arranged.  Some tips:

  • Make sure you have a full load of ablator on the heat shield.  You probably don't actually need it all for heat control purposes... but more ablator = more mass = moves your CoM forward, which improves your aero stability.
  • Any components that are denser (in terms of kilograms per meter of height), you want to be towards the front of the craft.

One technique people have used successfully is, use a large-diameter heat shield (more drag), and also put one on both ends of the landing vehicle (front and back).  That way, the drag from them cancel each other out, and it's easy to maintain an orientation that protects the vulnerable Science Jr.  For added bonus, twice the shields = twice the drag = slowing down quicker.  Note that only the one in front needs to have ablator on it-- for the shield that's intended to be at the trailing end, give it zero ablator, to minimize its mass and help it stay in the back.

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Thanks  @Snark I read over that and it all makes sense!  I  feel  smarter  now.

When  I  get  time  I'm  going  to  post  pics  of  my  duna  probes  that  I  think  will  land  on  duna, and  hopefully  someone  here  can  give  me  some  pointers.

Im  pretty  confident  in  the  duna  probe, this  is  my  third  redesign  of  the  things

 

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