Could the entry of Mars with balloons work by inflating to equalize with the atmospheric air pressure, while establishing buoyancy before meeting the ground?

Would a balloon pop if dropped from space?

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As this article writes,

“Ballutes” – a cross between balloons and parachutes – may one day act as lightweight heat shields and braking mechanisms for cargo or even people landing on the Earth or Mars from space. The concept recently received a boost from NASA, which is funding a proposal to develop the idea.

Most spacecraft use solid heat shields to protect themselves when they slam into the atmosphere from space. Those used for a single landing have “ablative” heat shields that gradually erode in the atmosphere, while the space shuttle, which is used over and over again, boasts a reusable heat shield.

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Blimp on Earth, but a glider on Mars?

How high could a weather balloon be used on Mars without rupturing?

When the blimp achieves terminal velocity, the heat shield is dropped to increase buoyancy.

  • 1
    $\begingroup$ You would need helium gas for inflation. But high pressure tanks with helium would be too heavy for a balloon. $\endgroup$
    – Uwe
    Jan 8 '19 at 20:47
  • 2
    $\begingroup$ @Uwe After the balloon is active, the tanks can be dropped. I think parachute would be still needed in the subsonic part of the arrival, to decelerate the probe to a feasible speed to activate the balloon. Also the parachute can be dropped after the baloon is active, it might be non-trivial to do safely. The major problem I see, that the Martian athmospheric pressure is around 2% of the Earth, and so decreases also the lift the baloon can provide. $\endgroup$
    – peterh
    Jan 9 '19 at 4:30
  • 1
    $\begingroup$ @Uwe what about hydrogen? It could be liberated chemically so wouldn't require extreme pressurization. It only needs to stay inside the balloon for a short time so diffusion may not be a problem, and there's no oxygen for combustion. $\endgroup$
    – uhoh
    Jan 9 '19 at 4:56
  • 1
    $\begingroup$ Generating it chemically doesn't need a pressure vessel. Re-compressing it takes a huge amount of work and requires big, heavy tanks perhaps. $\endgroup$
    – uhoh
    Jan 9 '19 at 5:38
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    $\begingroup$ Tangentally related XKCD what-if.xkcd.com/62 . His summation at the bottom does have bearing on effectiveness of balloons as high drag devices though $\endgroup$ Jan 9 '19 at 10:02

Let's try to do a quick reality check:

Time to inflate:

  • It takes 7 minutes to get down from orbit to the ground
  • It takes weeks to fill a large blimp.

Blimps can't survive high forces

  • The quoted top speed for a blimp is 140km/h
  • For curiosity, the parachute was jettisoned at 360km/h, 1.8km high
  • Blimps can't support high Gs

Altho the lower air pressure is kind of helpful regarding the wind speed, you still have the supersonic/transonic issues, and irreconcilable G forces for the deceleration.

Blimps are very heavy

If you filled an Hindenburg sized blimp on earth with helium, the helium alone would have a 33498kg mass. That's 10 times curiosity !

And you will still need to add:

  • The mass of the magical blimp structure
  • The gaz canisters
  • The magical inflation system
  • The heat shield

If only we had a mass efficient way of decelerating quickly !

  • 1
    $\begingroup$ Taking weeks to fill a blimp, and a blimp's maximum speed of 140km/hr are highly constrained by economics of commercially blimps designed specifically to work near Earth's surface. These are not fundamental constraints. Your Hindenburg versus WaPo The blimp industry is changing, right over our noses and CNBC Giant blimp-like airships are making a comeback $\endgroup$
    – uhoh
    Mar 1 '19 at 1:55
  • $\begingroup$ @uhoh 360 km/h and Airbags are pretty fast. $\endgroup$
    – Muze
    Mar 1 '19 at 2:01
  • $\begingroup$ @Muze Airbags are NOT blimps. Airbags are NOT required to be lightweight, therefor they can be made very sturdy. Also Airbags are filled by gaz generator very close to a solid rockets engine. There is NO way to generate helium with such kind of system. $\endgroup$
    – Antzi
    Mar 1 '19 at 2:21
  • $\begingroup$ @uhoh Yes, 140km/h is not a hard constraint, especially on mars. But G loads are. Filling might be done faster, but we are talking about a system that needs to inflates in seconds. That's five order of magnitude faster. $\endgroup$
    – Antzi
    Mar 1 '19 at 2:25
  • 3
    $\begingroup$ Can confirm from personal experience that there are many ways to burst a terrestrial weather balloon trying to fill it in a hurry and/or crosswind. optimal balloon envelope material has minimal margin for contact abrasion or rapid flexing. partially filled balloons also tend to form spheres at the top of the envelope and spheres are dynamiclly unstable in a flow so tend to flail around by roughly the current sphere radius. Even if the materials can handle it the ride under a filling 500 meter envelope is going to be exciting. $\endgroup$ Mar 1 '19 at 8:56

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