Beagle 2 in HiRISE, Possible Targets Options

[cbcnasa]

It does look so thin and pourous how much was it able to slow the craft?

[Shaka]

Spinnaker ripstop nylon has a coating which renders it non-porous.
At least, at sailing yacht windspeeds.

[djellison]

The Beagle 2 book I have is out in the Shed, so I'll double check tomorrow - BUT - ballpark figures...

The parachute, at 10m across, and the lander, at something more like 60kg, combine to give a MUCH lower terminal velocity than MER. More like 40-50mph, than the 150+ that MER would have had without RAD's.

I'll pull out proper numbers tomorrow.

[rlorenz]

Spinnaker ripstop nylon has a coating which renders it non-porous.
At least, at sailing yacht windspeeds.

A parachute has porosity on a macro-scale (e.g. in a ribbon parachute, or
the gap in a disk-gap-band chute) and on the microscale (the permeability of the
fabric itself). Both are important in inflation, drag and stability performance :
zero porosity is typically rather bad.

Note that at the low Reynolds numbers characteristic of Mars flight, what looks
like a porous fabric does not, in fact, behave as such since the viscous nature of
the flow reduces the effective permeability.

[nprev]

Very cogent explanation, Ralph, thanks! I thought something like that might have been at work.

Somebody on UMSF once described parachute design as a "black art"; I believe it. This is just one more in a long list of often very subtle interdependent variables.

LATE EDIT: This also probably explains why Mars parachutes aren't made in eye-searing high-contrast neon colors for easy orbital identification. Since fabric porosity affects performance, introducing fancy colors by whatever means would doubtless complicate control of that parameter. Never add complexities when they aren't truly needed.

[Shaka]

zero porosity is typically rather bad.

Would you care to elaborate on your concerns?
Stuart H says the material was tested on earth. Can we assume that means appropriate wind-tunnel or drop tests?

[djellison]

I wish I could just copy and paste the whole section out of 'The Guide to Beagle 2' - I'll paraphrase the parachute section...

The design was changed after establishing that the airbags would have to be going at less than 60kph. So they shifted to this 10m diameter ring-sail. The finished 'chute had 9 rings of 28 panels making 252 in total. The sails are sewn together in the vertical direction, but only at the corners to the next rows.

Tests seemed to include drop tests in Nevada, drop tests from a Hot Air Balloon near Oswestry, England, A pulley-fed extraction test, and a lorry-pulled test at one of the old airship hangers in Bedfordshire.

Tests predicted the terminal velocity with a lander of 57kg would be 16m/sec (35.8 mph)

Direct quote

"The material which was originally intended for spinnakers, used by racing yachts, is nylon, uncoated by any plasticiser. Instead has been calendared (rolled) to flatten the fibres thus reducing porosity. The fabric weights only 22.8g/m^2"

[mchan]

Like it would get ripped to shreds depending on the drag at some density and deployment velocity and strength of material.

The ribbon parachutes used to decelerate nuclear bombs from supersonic speeds appear to be very porous.

[Shaka]

This is unquestionably a fascinating case history of engineering trade-offs - and in something as prosaic as a parachute!
(Though, presumably, we could just about choose a spacecraft system at random, and construct a similar saga.)
- COST vs Effectiveness vs Risk of Failure.
- Commercial Off-The-Shelf (COTS) technology vs Specialized Parachute Black Magic (SPBM).
- COTS costs less than SPBM, (OK?), but:
-- How like a yacht spinnaker is a Mars re-entry parachute?
-- How does the non-permeable coating of spinnaker ripstop nylon (SRN) hamper its function in a parachute?
-- How does SRN without its coating improve function, and what is the effect on its strength and durability?
-- How does flattening the uncoated SRN improve its porosity and function, and what is the effect on its strength?

No doubt the list of ponderables could be continued, with reference to weight, bulk, construction, and, yes, even color etc.
It's a game only the Anointed can play with gusto.

[gwiz]

Would you care to elaborate on your concerns?

Zero-porosity means all the air entering the parachute has to leave again at the edge. It is difficult to impossible to get this to happen uniformly, and non-uniform edge venting leads to the parachute swinging from side to side.

[djellison]

Essentially a macro-scale equivalent to the vent hole in the top of the MER design?

[nprev]

Disk-gap chutes make LOTS of sense now, thanks, Gwiz! Venting's gotta happen no matter how porous the material is, so better to let most of it exit parallel to the center of mass & opposite local vertical (at low speeds) or parallel & opposite to the angle of attack (at high speeds) to minimize undesirable & uncontrollable lateral venting.

[gwiz]

Essentially a macro-scale equivalent to the vent hole in the top of the MER design?

A vent-hole at the top is a solution to the edge-venting problem that goes back a couple of centuries.

[Phil Stooke]

Every time I see a new post in this thread, I hope someone is going to say "Is this dark spot in HiRISE image ---- the Beagle 2 impact site?" Is anyone looking?

Phil

[djellison]

Mark Sims told me that about half the ellipse has been covered by HiRISE - and that people are looking.