Earth Return & Sample Science Options

[nprev]

Now that the 'boop' has been completed (with spectacular success!), we'll move the discussion to here for subsequent mission events.

[xflare]

oh, Now NASA saying main parachute deployed at 20,000 feet instead on 5,000.

At least it made a soft landing.

[PaulM]

oh, Now NASA saying main parachute deployed at 20,000 feet instead on 5,000.

At least it made a soft landing.

A NASA commentator on the livestream said that the capsule was autonomous and decided to open the parachute at a higher altitude because it had already had the required deceleration for this to succeed.

[rlorenz]

A NASA commentator on the livestream said that the capsule was autonomous and decided to open the parachute at a higher altitude because it had already had the required deceleration for this to succeed.

Hmm. Not sure I understand this. Neither the press kit, nor the papers I've seen on the mission go into the chute deployment logic, but I can speculate given my experience of this problem on previous missions.

Usually the simple logic (to deploy a drogue chute in the Mach Number/Dynamic Pressure parameters for which it is qualified - for blunt capsules like this, usually this is at about Mach 1.4 to avoid the transonic regime where the capsule itself is unstable) is to observe the acceleration, and trigger a fixed time after a downgoing g-threshold. Sometimes there is an arm/fire combination of two thresholds, or an adjustable time delay depending on the g-history.

Drogue deploys, inflates, stabilizes capsule and slows it down. Then (for Earth returns, Dragonfly descent) there will be a prolonged drogue descent phase, at a high-ish terminal velocity, but nice and stable.

Then (either based on a timer, or sometimes a pressure trigger) the bridle to the drogue is severed and the drogue pulls off a bag allowing the main to deploy. Main deploys, slowing capsule down to a lower terminal velocity for landing.

Most capsules then just hit the deck (Soyuz triggers retrorockets just prior to contact). Usually parachutes remain attached - interestingly O-REx has a parachute release triggered at impact to avoid being dragged across the desert.

Typically there is a back-up timer to trigger drogue deployment, to defend against a failed accelerometer or g-switch (Not that these things fail often as far as I can tell, although the Galileo g-switches were miswired so that the timing of the two threshold-crossings was messed up and so the chute was deployed late. I guess Genesis did not have a backup timer - since its g-switches were installed upside down, the threshold trigger signal was never generated).

Depending on the entry condition, aerodynamic and atmospheric uncertainties, drogue deployment via timer will likely be outside the qualified envelope (e.g. too low q or Mach, and/or after the capsule has encountered conditions of poor stability). So the drogue is being used out of warranty, but better to try deploying than not.

What I am curious about is the logic for the main deploy on O-REx. Maybe that was set on a timer to go basically at the same time as the drogue backup timer (i.e. things have all gone to crap anyway, this may result in deploying the main early, but we dont know what the conditions really are anyway so might as well).

The commentator remark suggests the capsule was smarter than that, i.e. had a velocity trigger where the deceleration is integrated with time. Perhaps there is an instantaneous threshold for drogue deploy (that was somehow never triggered) and this velocity trigger fired both drogue deploy and release (main deploy) near together (late for drogue, early for main). Or perhaps the drogue deploy conditions were detected, and the drogue mortar fired, but somehow the drogue did not inflate (or shredded upon inflation, as e.g. Pioneer Venus' original parachutes did in Earth testing). But I am struggling to think if this was the case, what would lead the capsule to deploy the main *early*.......

(There is a logic on the Soyuz capsules that looks at a pressure sensor, and if the pressure changes too rapidly after the drogue deploy event, it decides the drogue didnt work right and deploys the backup drogue, but that's not quite the same).

Anyway, I dont have any inside knowledge on this one, and details clearly will take some time to emerge, but I think there was an interesting chain of events here and thought it useful to post some background. Here something apparently didnt work right, but thankfully things either just miraculously worked out, or carefully-thought-out backup strategies came into play successfully. Doubtless will be the trigger for lots of review board questions on DAVINCI and Dragonfly moving forward.....

Ralph

[StargazeInWonder]

We've had entries into Mars in the past where the atmospheric conditions were not as expected, but it seems there would be little excuse for inadequate knowledge of atmospheric conditions on Earth!

interestingly O-REx has a parachute release triggered at impact to avoid being dragged across the desert.

I sorely wish that that had been true for me the one time that I tried skydiving! (It was windier than expected. Ouch!)