Originally posted by druid1
TextTextreply to post by
What a great thread- this theory is excellent and with all the flak you are getting I do believe you are on to something. It seems logical that if
there isn't much atmosphere then a parachute will not function correctly and will need to be much bigger. No one in my opinion has disproved you.
What we need here are facts and unfortunately the only ones with the info are the guys calling the shots. I am sure only time will tell or does anyone
have that Snowden chap's email
He actually hasn't provided any evidence that the chute is too small except his "feeling" that it is.
First of all, the chute IS bigger than chutes on Earth. MSL's chute was about 20 meters (about 65 feet) in diameter. That's a lot bigger than most
chutes used on Earth.
Secondly, the point of the chute is not to slow down the craft enough for a gentle landing. It only needs to slow it down enough for the retro
rockets to fire.
Most of the speed of the craft was NOT reduced by the parachute, but instead before chute deployment during aerobraking. Plus, the shape of the craft
(with the aeroshell cocoon still around it) was a lifting body
, meaning the falling craft created its own lift, which help guide it and slow it
down as it fell, chuteless.
The chute deployed once the craft was moving 450 m/s (1000 mph) and only needed to slow the craft down to
100 m/s (220 mph).
The chute was cut away from the craft while it was still moving at 220 mph. the chute was cut away about one mile above the surface, meaning there
was no parachute at all for the final 1 mile down.
A little after the chute was cut, the retro rockets then fired, which slowed the craft down enough for a soft landing.
So, yeah, the OP is right in a way because it is true the parachute would not be big enough to slow it down enough -- but it didn't have to. That's
what the retrorockets were for.
Someone (as the OP did) may ask if the parachute was even able to slow the craft down enough (from 1000 mph to 220 mph) before the retrorockets fired.
The OP's thought was that the air was 100 times thinner, therefore the chute would be 100 times less effective. Granted, the OP also took into
account the lower gravity (38% that of earth) so he said that would mean the chute would be 38 times less effective...
...using his logic, that would mean a chute would need to be 38x bigger.
However, his logic is wrong. Well, not his logic, but his belief that the chute would be 100 times less effective in 100 times thinner air. That
isn't true, because the drag force generated by the chute is NOT directly proportional to the density of the atmosphere. So 100x thinner air DOES
NOT mean 100x bigger parachute is required. The math doesn't work like that.
Here is a link to a similar question asked about the chutes used for the smaller mars pathfinder mission (but the physics apply to all Mars
...the drag FORCE a chute generates (therefore its deceleration), is proportional to the square of the velocity and only linearly proportional to
the atmospheric density; so even a thin atmosphere and a "small" chute will do much to slow our entry vehicle down once the heatshield'
And here is a link to calculations and simulations that were done for Curiosity's parachutes:
(link opens directly to a PDF file) ntrs.nasa.gov...