Rocket Design: Egg-Safe Landing Plan

[greg_rack]

DISCLAIMER: I am not entirely sure whether I should open this thread here or in #general, but I will try! :')

Hi guys,

I am a freshman in aerospace engineering and i have taken part into a competition consisting in engineering a rocket so that it reacher 1km altitude and manages to touch back down without breaking a raw egg which will be stores inside its nose-cone section.

Without any solid engineering experience in my background, I am kind of lost for the design of a shock-absorbing system which might be able to prevent an egg from breaking when touching down at approx. 20m/s.
I will explain here the idea(and attach an hopefully eloquent design plan I have sketched) I'll most likely follow for the design, hoping to discuss it with you here.
In a nutshell, my idea was not to only absorb and dampen as much energy as possible, but also to leave the egg as much deceleration distance as possible... in order to prevent its rupture by cavitation, for expressive pressure developed in the fluids internal to the shell.
Given the super limited space(7cm∅ and 25cm length), I thought about using a soft (styro?)foam funnel in turn enclosed in a more rigid outer shell; the idea with the funnel is that of providing a calculable friction to the egg, so to control its deceleration when subjected to a certain force/impulse. The funnel could either be vertical or slightly sloped, to emulate the proportional force a spring might provide(but in a concentrated way, which would be fatal for the egg shell).

The egg will be itself covered in a slightly larger rigid "cover"(1.5mm distance between eggshell and cover) containing a thin layer of fluid(water, honey, oobleck?) submerged in a very non-dense sponge(just to provide stability, for the egg not to hurt against the rigid cover walls)with the aim of distributing the pressure equally over the surface of the shell.

I hope this gives a first insight on my idea... is this completely offset? Not really sure if it's science fiction or actual science
Hope you won't be too hilarious for what I've just presented!

Greg

 

[FactChecker]

20m/s seems like a very fast landing speed. Are you planning on using a parachute? Have you done a rough estimate on the deceleration g's required to stop from that speed in the small distance of the designed "crumple zone"? I suspect that it would break an egg no matter what you do.
Alternatively, if you encase the egg in an exact form-fitting mold that supports the shell, the shell might be able to remain intact. (It might get messed up internally even if the shell holds.)

 

[kuruman]

Yes, 20 m/s is 45 miles/h. Sounds too fast to me too. I would suggest trying Jello, any flavor. It will fill snuggly the space between the egg and its container leaving no room for relative motion between the two. If possible, orient the egg so that when it lands the velocity vector is lined up along the egg's long axis. Eggs are able to withstand much more compression along that axis than along an axis perpendicular to it. I would also investigate whether eggs from free ranging hens have stronger shells than those from caged hens.

 

[berkeman]

I would suggest trying Jello, any flavor.

 

[Baluncore]

Why not attach the wrapped egg, to the legs of, or harness on, a live pigeon. Release the bird at the 1 km apogee, to bring the egg down. It will probably not fly home with the load. A crow might land more aerodynamically, but it would then eat the egg.

 

[greg_rack]

Why not attach the wrapped egg, to the legs of, or harness on, a live pigeon. Release the bird at the 1 km apogee, to bring the egg down. It will probably not fly home with the load. A crow might land more aerodynamically, but it would then eat the egg.

That works! You might as well come here to the Netherlands that day and try to catch the egg on the fly right before touching down, that must help a lot

 

[greg_rack]

20m/s seems like a very fast landing speed. Are you planning on using a parachute? Have you done a rough estimate on the deceleration g's required to stop from that speed in the small distance of the designed "crumple zone"? I suspect that it would break an egg no matter what you do.
Alternatively, if you encase the egg in an exact form-fitting mold that supports the shell, the shell might be able to remain intact. (It might get messed up internally even if the shell holds.)

Actually, 20m/s is a very very high-order estimate. A drogue parachute will be deployed at apogee which will allow to reach such a terminal velocity, but at approx 200m altitude a main parachute will slow done the nosecone even more, in theory up to 5m/s.
How would you design the supporting mold? Would the sponge+liquid combination work? Jello as @kuruman suggested?? :)

 

[FactChecker]

How would you design the supporting mold? Would the sponge+liquid combination work? Jello as @kuruman suggested?? :)

I was thinking about making a cast of that exact egg and placing it in a hard enclosure that fits exactly. I don't know enough about a possible cast process to say more.

 

[Baluncore]

I was thinking about making a cast of that exact egg and placing it in a hard enclosure that fits exactly.

I think that is a recipe for an omelette.

The important thing about suspending the egg is that it be submerged in a fluid. The important thing about that fluid is that it needs to have the same density as the egg. The fluid equalises the pressure applied to the shell from all directions. That suggests the egg should be neutrally buoyant, ideally floating in a mix of egg white and yolk, (from another egg).

There are two problems remaining, one is the air bubble within the egg, the other is avoiding impacts where the speed of sound in the structure of the egg, and the suspension fluid, become important.

 

[bob012345]

Are there basic design guidelines you must follow? Also, small model rockets usually have very high acceleration. How do you know you won't crack the egg on the ride up to 1km?

 

[FactChecker]

I think that is a recipe for an omelette.

Internally? or cracked shell? I am talking only about the shell. It seems like it should be possible to support the shell so that it will not crack. Certainly, if the shell was glued to a hard encasement, it would be much harder to crack. Is glue essential? Suppose the egg was coated with grease and put into a mold made of clay or something harder. Wouldn't that make it much harder to crack?

 

[greg_rack]

That suggests the egg should be neutrally buoyant, ideally floating in a mix of egg white and yolk, (from another egg).

True! But wouldn't a thin layer of low-density foam soaked it in water(or whatever other liquid) provide the same "stabilizing" effect that submerging the egg in a liquid of its own density would, if not even better? Intuitively I'd say water would still be able to distribute force even if soaked into a sponge, and the latter will clearly provide static stability(instead of neutral)... omelette still?? I really have no clue.

the other is avoiding impacts where the speed of sound in the structure of the egg, and the suspension fluid, become important.

What do you exactly mean by this?

 

[greg_rack]

I was thinking about making a cast of that exact egg and placing it in a hard enclosure that fits exactly. I don't know enough about a possible cast process to say more.

You think failure by cavitation won't be a problem?

 

[greg_rack]

Are there basic design guidelines you must follow? Also, small model rockets usually have very high acceleration. How do you know you won't crack the egg on the ride up to 1km?

Good point. During launch an accel. of 100m/s^2 will be reached; that is what I'm trying to design the rigid fluid-housing "cover" against; seeing previous launches, I'm not sure why but it seemed like the crucial phase was always the strong shock at touch-down; could you have an explanation for this? Does this have to do with a higher impulse, concentrated force?

 

[bob012345]

Good point. During launch an accel. of 100m/s^2 will be reached; that is what I'm trying to design the rigid fluid-housing "cover" against; seeing previous launches, I'm not sure why but it seemed like the crucial phase was always the strong shock at touch-down; could you have an explanation for this? Does this have to do with a higher impulse, concentrated force?

Could be that cracking the egg might be more sensitive to the rate of change of acceleration (the jerk) than the acceleration itself especially if the acceleration changes relatively smoothly.

 

[greg_rack]

Could be that cracking the egg might be more sensitive to the rate of change of acceleration (the jerk) than the acceleration itself especially if the acceleration changes relatively smoothly.

I agree: looking online, eggs can withstand up to 600 "equally distributed" g's, keeping an acceptable jerk. But how to cope with the latter then?

 

[bob012345]

I agree: looking online, eggs can withstand up to 600 "equally distributed" g's, keeping an acceptable jerk.

Perhaps if the egg were within some kind of fluid pack with a pressure release valve to release the pressure during the impact so it doesn't build past the critical value?

 

[Baluncore]

Certainly, if the shell was glued to a hard encasement, it would be much harder to crack.

I agree the shell would be harder to crack if it was made thicker and stronger. But how much brute force concrete can you add.

Suppose the egg was coated with grease and put into a mold made of clay or something harder. Wouldn't that make it much harder to crack?

If the grease was an incompressible fluid and the volume was fixed, then it would be a fluid suspension as I suggested.

You think failure by cavitation won't be a problem?

The egg must be fully submerged in a fluid of the same density. That way, when subjected to acceleration, the hydrostatic pressures against the shell will be equal.

 

[greg_rack]

The egg must be fully submerged in a fluid of the same density. That way, when subjected to acceleration, the hydrostatic pressures against the shell will be equal.

Cool, but that takes me to another question: what about the air bubble into the egg? Isn't that going to be a very likely point of failure?
Should an air bubble be also left into the outer same-density liquid, approximately of the same size of the internal one?

 

[bob012345]

You could actually easily do experiments holding the egg in some kind of water balloon like structure and throwing it down at different speeds. The problem is can you afford to lift all that fluid mass with the rocket?

 

[Baluncore]

True! But wouldn't a thin layer of low-density foam soaked it in water(or whatever other liquid) provide the same "stabilizing" effect that submerging the egg in a liquid of its own density would, if not even better?

Fluid density is critical during acceleration. The sponge must not contain or trap any air bubbles, or hydrostatic pressure equilibration will be delayed by fluid flow delays.

the other is avoiding impacts where the speed of sound in the structure of the egg, and the suspension fluid, become important.

What do you exactly mean by this?

I mean the impact must be slow enough that speed of sound does not delay equilibration.

Could be that cracking the egg might be more sensitive to the rate of change of acceleration (the jerk) than the acceleration itself especially if the acceleration changes relatively smoothly.

That is where the speed of sound becomes important. A jerk may set up waves within the egg and suspension. Those waves will be a problem when the reinforce.

I agree: looking online, eggs can withstand up to 600 "equally distributed" g's.

The neutral density suspension fluid distributes the forces equally.

Cool, but that takes me to another question: what about the air bubble into the egg? Isn't that going to be a very likely point of failure?

Yes, that air bubble will be a problem. I see no easy solution that can be applied to reduce the bubble quickly before launch, so you may have to accept it as the weakest link. Vacuum packing eggs to remove the air would work if there was no shell and you could replace the air with a liquid.

Should an air bubble be also left into the outer same-density liquid, approximately of the same size of the internal one?

No. The problem with an external bubble is that it may move to the wrong end at the wrong time, or become a foam which will upset the neutral buoyancy of the suspension fluid.
A hot air balloon rises because the hydrostatic gradient inside the balloon is different from that outside. If the pressure inside the egg was greater than hydrostatic, because the external liquid contained a bubble or became a foam, then the egg would burst outwards, which is not it's strength. It is better to surround the egg with an equal greater pressure.

 

[greg_rack]

You could actually easily do experiments holding the egg in some kind of water balloon like structure and throwing it down at different speeds. The problem is can you afford to lift all that fluid mass with the rocket?

Absolutely not... weight-wise I have calculated a maximum thickness of 2mm surrounding the eggshell

 

[greg_rack]

If the pressure inside the egg was greater than hydrostatic, because the external liquid contained a bubble or became a foam, then the egg would burst outwards, which is not it's strength. It is better to surround the egg with an equal greater pressure.

So, lastly, what you would suggest is filling the rigid cover entirely with a similar-density fluid(better is slightly higher, or slightly lower?), submerging the egg in it, and speak words of faith :)

 

[FactChecker]

You think failure by cavitation won't be a problem?

I don't see how that could happen. If the egg is full of incompressible material how can cavitation occur? Maybe it has an air bubble in it that could compress and the cavitation can suck the shell into collapse it?

 

[bob012345]

This is already a thing..

https://www.nar.org/contest-flying/competition-guide/altitude-events/eggloft/

 

[Baluncore]

So, lastly, what you would suggest is filling the rigid cover entirely with a similar-density fluid(better is slightly higher, or slightly lower?), submerging the egg in it, and speak words of faith :)

If different then a very slightly higher density fluid.
Words of faith will have no effect unless you also cross your fingers tightly.