Parachutes for heavy lander on Eve

[Zosma Procyon]

How many parachutes are needed to safety land a 700+ ton rocket on Eve. My current design has 44 of the MK16-XL chutes, and 4 of the inflatable heat shields (which will be jettisoned when the main chutes open). Is 44 enough?

[Snark]

11 minutes ago, Zosma Procyon said:

How many parachutes are needed to safety land a 700+ ton rocket on Eve. My current design has 44 of the MK16-XL chutes, and 4 of the inflatable heat shields (which will be jettisoned when the main chutes open). Is 44 enough?

Whew, that's a big 'un!

Details below, but the short answer to your question is two fold:

• Maybe that's enough chute, but you might want to add some more.
• If you haven't landed a big ship on Eve before, "are there enough chutes" is only the start of your engineering challenges.  (And is probably the easiest one).

 

My gut feeling is that 44 might be a tad low, if you're planning to land purely on parachutes alone without any engine thrust to cushion the landing.  44 MK16-XL on a 700 ton ship would be like having one little Mk16 on a 5.3 ton ship.  Eve has 5x the atmosphere of Kerbin, so that would be like having one Mk16 on a 1-ton ship... which is fine in Kerbin gravity, but Eve's is higher.  Also, you might be landing at a higher elevation.  I'd suggest having somewhat more parachutes.  Perhaps double it.

However, the other thing to bear in mind is that landing a giant spacecraft on Eve is a big engineering challenge, and parachutes aren't the only thing to worry about.  For a ship that big, its own inertia can be its own worst enemy-- it's easy for a huge ship to end up ripping itself apart with the shock of landing, just breaking into chunks and collapsing like a pile of jackstraws.  Landing a multi-hundred-ton ship is hard.  Even if your speed on landing is lower than the impact tolerance of the parts, you can still end up snapping apart, especially if your landing zone isn't completely dead flat.  (And Eve is pretty darn bumpy.)

You're also going to run into some really heavy-duty heating on reentry, which may or may not be a problem depending on your ship design.

Also it can be tricky to have a design that stays aerodynamically stable during reentry (i.e. keeps the heat shields pointing forward), if you haven't carefully designed for it.

Also it can be tricky to jettison those big 10m heat shields without having them flip around and smash something on your ship.

My suggestion?  Test it.  Copy your ship to a sandbox save, launch to the pad, and then use the "Set Orbit" function on the Alt+F12 cheat menu to just put it straight into low Eve orbit, and go from there to see whether you can handle reentry and landing.

[Zosma Procyon]

29 minutes ago, Snark said:

Whew, that's a big 'un!

Details below, but the short answer to your question is two fold:

• Maybe that's enough chute, but you might want to add some more.
• If you haven't landed a big ship on Eve before, "are there enough chutes" is only the start of your engineering challenges.  (And is probably the easiest one).

 

My gut feeling is that 44 might be a tad low, if you're planning to land purely on parachutes alone without any engine thrust to cushion the landing.  44 MK16-XL on a 700 ton ship would be like having one little Mk16 on a 5.3 ton ship.  Eve has 5x the atmosphere of Kerbin, so that would be like having one Mk16 on a 1-ton ship... which is fine in Kerbin gravity, but Eve's is higher.  Also, you might be landing at a higher elevation.  I'd suggest having somewhat more parachutes.  Perhaps double it.

However, the other thing to bear in mind is that landing a giant spacecraft on Eve is a big engineering challenge, and parachutes aren't the only thing to worry about.  For a ship that big, its own inertia can be its own worst enemy-- it's easy for a huge ship to end up ripping itself apart with the shock of landing, just breaking into chunks and collapsing like a pile of jackstraws.  Landing a multi-hundred-ton ship is hard.  Even if your speed on landing is lower than the impact tolerance of the parts, you can still end up snapping apart, especially if your landing zone isn't completely dead flat.  (And Eve is pretty darn bumpy.)

You're also going to run into some really heavy-duty heating on reentry, which may or may not be a problem depending on your ship design.

Also it can be tricky to have a design that stays aerodynamically stable during reentry (i.e. keeps the heat shields pointing forward), if you haven't carefully designed for it.

Also it can be tricky to jettison those big 10m heat shields without having them flip around and smash something on your ship.

My suggestion?  Test it.  Copy your ship to a sandbox save, launch to the pad, and then use the "Set Orbit" function on the Alt+F12 cheat menu to just put it straight into low Eve orbit, and go from there to see whether you can handle reentry and landing.

Good call on testing it in the sandbox. It didn't even make it to the chute deploy phase before flipping around and breaking up. I think I may try landing with empty fuel tanks. But that means bringing the refining and mining equipment and a longer stay... work work work. That set orbit thing is trippy. One second it's on the pad the next in low orbit over Eve.

[Snark]

34 minutes ago, Zosma Procyon said:

It didn't even make it to the chute deploy phase before flipping around and breaking up.

A not uncommon problem when using the big 10m heat shields.

Some ways to mitigate that:

• Spam a bunch of airbrakes at the opposite end of the ship from the heat shield.  Airbrakes are pretty melty (they only can tolerate 1200 degrees), but as long as they're in the "shadow" of the heat shields, they will provide drag, but won't be exposed to reentry heat.  However, depending on the shape and the mass distribution of your ship, it could take a lot of them (or it may simply may not be practical).  If you do go this route, may be worth mounting all those airbrakes on radial decouplers, so you can ditch 'em once you're done with them.  No need to haul them back up off the surface.
• Or:  Consider also putting some of the 10m heat shields on the other end of your ship, i.e. facing .  The idea here is this:  those big heat shields on the side are super draggy, which is why you flip around.  So put some draggy ones on the side to balance them out.  Plus, if you have heat shields on both ends, it doesn't actually matter which way you're facing during reentry and you can just let it assume whichever orientation it wants. 

Here's an example of that latter approach in action:

Spoiler

It's a big reusable fuel tanker for bringing several hundred tons of fuel back to LKO from Minmus in one whack.  It's built around a single giant 5m fuel tank from the SpaceY mod, but you get the idea.  A heat shield on each end means it doesn't have to care which end faces forward during reentry.

It's steady as a rock.

Also, be sure to check where your CoM is.  During reentry, your CoM is going to end up pointing and there's very little you can do about that.  So make sure it's where you want it to be.

[Geonovast]

4 minutes ago, Snark said:

but as long as they're in the "shadow" of the heat shields, they will provide drag, but won't be exposed to reentry heat.

I just happened to have this handy, so ...

[Snark]

4 minutes ago, Geonovast said:

I just happened to have this handy, so ...

Nice! 

@Zosma Procyon, when you are looking at (and trying to understand) Geonovast's design, there are two really important aspects that make it work:

• First, as you can see, he's got some airbrakes hiding behind the heat shield, as I described.
• Second-- and really importantly-- the craft is really low and squat so that its CoM is going to be very close behind the heat shield.  That's really critical.

[Geonovast]

1 minute ago, Snark said:

Nice! 

@Zosma Procyon, when you are looking at (and trying to understand) Geonovast's design, there are two really important aspects that make it work:

• First, as you can see, he's got some airbrakes hiding behind the heat shield, as I described.
• Second-- and really importantly-- the craft is really low and squat so that its CoM is going to be very close behind the heat shield.  That's really critical.

This might help.

[Cpt Kerbalkrunch]

@Zosma Procyon, this lander is 500 tons (after dumping all the shields and whatnot), and it has 15 chutes. Six of those are drogues to slow it down enough for the regular chutes to open. I would think 44 is gonna be overkill. Don't think they'll hurt ya, though.

 

[Foxster]

I think you need to stand back and ask: "Do I really need to land a 500t craft?"

Most people's Eve problems are caused by over-engineering. You can get away with it just about anywhere else in the system but Eve expects skinny, light craft. 

[Cpt Kerbalkrunch]

32 minutes ago, Foxster said:

I think you need to stand back and ask: "Do I really need to land a 500t craft?"

Most people's Eve problems are caused by over-engineering. You can get away with it just about anywhere else in the system but Eve expects skinny, light craft. 

Not sure if you mean me or the OP but, since his lander is 700 and mine is 500, I'll assume you meant me. I didn't need a 500 ton lander necessarily, but I wanted to use the Mk1-2 and the 2.5m service module. The wants, needs, or expectations of Eve itself mean little to me. I like to play it straight without any kind of part clipping, command chairs, or mods of any sort. I also wanted a direct ascent home. No orbital rendezvous or refueling of any kind. Still comin' up about 100m/s short on that, but I haven't given up. A slightly more efficient launch profile or (if I absolutely have to) a landing zone that's a bit higher should do the trick. I can't speak for the OP, but I assume he also is sending a large ship simply because he wants to. I don't think anyone would send such a monstrosity because they thought it was the most efficient landing possible. Unless it's for a challenge or something, I don't pay a whole lot of attention to efficiency. I already know I could send a small ship to Eve (or anywhere else for that matter). To me, sending a behemoth (even when you don't need it, which you pretty much never do) is a lot of fun. Plus (and this might be most important of all), I just love the way the Mk1-2 looks. It's awesome. 

Edited January 3, 2018 by Cpt Kerbalkrunch

[OhioBob]

On ‎1‎/‎2‎/‎2018 at 8:14 PM, Zosma Procyon said:

 How many parachutes are needed to safety land a 700+ ton rocket on Eve. My current design has 44 of the MK16-XL chutes, and 4 of the inflatable heat shields (which will be jettisoned when the main chutes open). Is 44 enough?

You've already gotten a lot of good advice here, but I would like to add just one more thing on the parachute question.  To achieve the same speed of decent, a vehicle on Eve requires about 1/3 as much parachute area as an identical vehicle on Kerbin.

We can compute this using the equation for terminal velocity,

If we want the same speed of decent, i.e. the same terminal velocity, on both planets, then we just have to do this,

SQRT( 2 * m * g_K / ( ρ_K * A_K * C_d ) ) = SQRT( 2 * m * g_E / ( ρ_E * A_E * C_d ) )

where subscript K refers to Kerbin and subscript E refers to Eve.  The mass m and drag coefficient C_d are the same on both sides because it's assumed we have the same sized vehicle using the same type of parachutes.

We can now simplify by squaring both sides and canceling out the like terms,

g_K / ( ρ_K * A_K ) = g_E / ( ρ_E * A_E )

We can further rearrange like this,

A_E / A_K = ( g_E * ρ_K ) / ( g_K * ρ_E )

So we can now solve for the ratio of the parachute areas by plugging in the values for gravity and air density on Kerbin and Eve.  For air density we don't need to know the exact values, the relative value will do.  The air on Eve is about 5 times as dense as air on Kerbin*.

A_E / A_K = ( 1.7 * 1 ) / ( 1 * 5 ) = 0.34

Assuming we're using the same type and size of parachute, then we need only 0.34 times as many of them on Eve to fall at the same speed as we would on Kerbin.

* Note that the formula uses air density, not air pressure.  Density is a function of not only pressure but also of gas molecular weight and temperature.  So while I used a 5:1 ratio of Eve density to Kerbin density, that's not because Eve's air pressure is 5 times greater.  It is just a coincidence in this case that the ratio of the density is the same as the ratio of the pressure.  The molecular weight of Eve air is about 1.5 greater than Kerbin air, but Eve is also about 1.5 times hotter.  Those factors happen to almost exactly cancel out so we end up with Eve air near the surface being about 5 times denser than Kerbin air.

This same type of calculation can be performed to find the parachute ratio for any two planets.
 

Edited January 5, 2018 by OhioBob

[Snark]

On 1/2/2018 at 6:57 PM, Cpt Kerbalkrunch said:

Six of those are drogues to slow it down enough for the regular chutes to open.

Pretty sure you don't need drogue chutes on Eve, for any craft.

Or are you telling me you actually have a ship that's capable of reaching Eve's surface, unpowered, at speeds in excess of 200 m/s?

(In practice, the only place I've ever needed drogues is Duna.  Or, on rare occasions when I've built a craft with an exceptionally high ballistic coefficient, Kerbin.  But the latter case is pretty rare.)

[foamyesque]

The correct number of parachutes is 0. Use wings

[Cpt Kerbalkrunch]

1 hour ago, Snark said:

Pretty sure you don't need drogue chutes on Eve, for any craft.

Or are you telling me you actually have a ship that's capable of reaching Eve's surface, unpowered, at speeds in excess of 200 m/s?

(In practice, the only place I've ever needed drogues is Duna.  Or, on rare occasions when I've built a craft with an exceptionally high ballistic coefficient, Kerbin.  But the latter case is pretty rare.)

I guess you're referring to the fact that Eve has a very thick atmosphere, which helps to slow your craft considerably. However, I think you'll notice that it also has very high gravity, which tends to bring you to the surface rather quickly. Since seeing is believing, I made this video just for you. This is what happens to a 500 ton lander on Eve descent without drogue chutes. Enjoy.

 

 

[Snark]

1 hour ago, Cpt Kerbalkrunch said:

Since seeing is believing, I made this video just for you. This is what happens to a 500 ton lander on Eve descent without drogue chutes. Enjoy.

That's ... bizarre.  Quite aside from any expectations of "how slow should the terminal velocity eventually be" ... it gradually slowed down in thickening atmosphere (as one would expect) and then actually started accelerating slightly.  What the what?  I've never seen any craft do that ever under any circumstances.  Ever.

Its terminal velocity also seemed to make virtually no change, even when the atmosphere went from Kerbin-level pressure all the way down to Eve surface.

That's... seriously weird.  Do not understand.  Either it should have slowed down drastically as it fell, or else it should have accelerated a lot more at the start.  I do not understand how it can maintain a speed that varies only a couple of percent when the atmosphere changes by a large factor.  The heck?

[Cpt Kerbalkrunch]

17 minutes ago, Snark said:

That's ... bizarre.  Quite aside from any expectations of "how slow should the terminal velocity eventually be" ... it gradually slowed down in thickening atmosphere (as one would expect) and then actually started accelerating slightly.  What the what?  I've never seen any craft do that ever under any circumstances.  Ever.

Its terminal velocity also seemed to make virtually no change, even when the atmosphere went from Kerbin-level pressure all the way down to Eve surface.

That's... seriously weird.  Do not understand.  Either it should have slowed down drastically as it fell, or else it should have accelerated a lot more at the start.  I do not understand how it can maintain a speed that varies only a couple of percent when the atmosphere changes by a large factor.  The heck?

I attribute it to being huge, heavy, and about as aerodynamic as a cinder block. I don't think Eve knows what to do with it. The atmosphere says slow it down. The gravity says speed it up. A strange tug of war that no one seems to win. And believe it or not, that thing gets the Mk1-2 and 2.5m service module back to orbit with plenty to spare. Took an awful lot of (crashing) testing to do it, though. 

[NSEP]

Well, ALOT

It really depends on how much drag and surface area you have.

On 5-1-2018 at 1:01 AM, foamyesque said:

The correct number of parachutes is 0. Use wings

Wings and aerodynamics are hard.

[Magzimum]

I'm a fan of a squat rocket with a central taller structure, so that those Airbrakes can be mounted a bit higher up (and thus further away from the CoM).

This isn't on Eve, but on Tellumo (Galileo's Planet Pack).

Personally, I really enjoyed discovering all the new planets, so I will put specific numbers into a Spoiler. They're not too relevant for this thread...

Spoiler

It's pretty much the same problem on Tellumo: Slowing down from 4100 m/s in a thick atmosphere. I believe Tellumo at sea level is even thicker: 10 bars, but it's not as high (only 45 km until you're in space). But a lot of the land is quite high.

Will be posting the full report later, btw. My brave Kerbal made it back to orbit in this lander, so now I'm confident he will (together with the crew of the mothership) also make it home.

 

[foamyesque]

6 hours ago, NSEP said:

Wings and aerodynamics are hard.

 

Yes, but they're so rewarding when you get it right. Nothing quite compares to dropping several hundred tonnes of hardware on Eve like a bat outta hell

[Spricigo]

On 04/01/2018 at 10:01 PM, foamyesque said:

The correct number of parachutes is 0. Use wings

Ok. But wings equivalent to how many parachutes?

 

Edited January 8, 2018 by Spricigo