11
$\begingroup$

Perhaps you've flown in a plane on an especially cloudy day when all you can see are clouds beneath and above you. It's a beautiful sight, strata'd white all around. So what if the plane were flying on Jupiter instead and enjoying the cloud strata there?

There's a couple of assumptions here:

  1. There are floating colonies for the planes to fly between.
  2. Getting metals to supply an aviation industry is economical.
  3. Fueling these airplanes is also economical.

Assume that the plane can be built or delivered into Jupiter's gravity well without difficulty.

At what altitude would this plane likely fly and how would it differ from modern airliners in terms of shape? If it's possible, try to minimize pressure differences between the cabin and surrounding atmosphere since more strength equals more weight. The propulsion systems are likely to be very different so I'm not worried about that part yet, though if you have thoughts to share on that then go ahead and include it.

$\endgroup$
3
$\begingroup$

It seems to me that the trick is to go deeper into Jupiter to get denser atmosphere, not to try to climb to reduce gravitational pull.

Requiring lift to equal gravitational pull, we are concerned with:

$${{Sv^2}\over m} \sim {g\over \rho}$$

Where $S$ is the wing surface area, $v$ the speed, $m$ the aircraft mass, $g$ gravity and $\rho$ density.

This basically states what you would expect: more gravity makes it harder to fly, and higher pressure makes it easier.

For the Jovian atmosphere, See for example Wikipedia.

Jupiter does not really have a surface, at least until you get deep down the the metallic hydrogen core.

"the pressure level of 10 bars, at an altitude of about 90 km below the 1 bar with a temperature of around 340 K, is commonly treated as the base of the troposphere"

Noting that the radius of Jupiter is over 70,000 km, this altitude will still effectively have the same gravity as the 1 bar level, i.e. around 2.5g.

With 10x the pressure and much the same temperature as Earth, but a hydrogen composition, overall atmospheric density will still be lower by around a third.

So at that altitude, you've got your work cut out. However, it seems to me do-able; not unlike flying at earth gravity at 0.25 atmospheres, which is around 50,000ft.

However, it is not the same as flying at 50,000 ft here: remember the pressure is 10x higher. This will have implications on the structural engineering.

Alternatively, if one keeps going deeper, behaviour may become more like that of a submarine in a liquid: build a pressure vessel that has neutral buoyancy.

Follow up remark.

The main problem with flying is that there is nowhere to land. The "aircraft" will need to sustain "flight" indefinitely or descend to a level where it is neutrally buoyant.

| improve this answer | |
$\endgroup$
  • 4
    $\begingroup$ XKCD on a Jupiter submarine (tl;dr: nowhere near possible: the density of Jupiter's atmosphere is 1 g/cm³ about half-way to the center) $\endgroup$ – Nick T May 30 '16 at 22:29
2
$\begingroup$

Usable aircraft on Jupiter are very, very unlikely.

Per this fact sheet from NASA, an atmospheric pressure of 1 bar occurs at an equatorial radius of about 71 km, but the local gravity is about 2.5 $g$s. At this altitude, the atmospheric density is about .13 that of Earth, and local temperature is about -108 °C.

In order to get the local gravity down to 1 g, it is necessary to rise to an altitude of $$h = 71 \text{ km} \times\sqrt{2.5} = 112 \text{ km}$$

As it happens, this occurs at an atmospheric pressure of 0.1 bar and a temperature of -161 °C.

Ignoring temperature effects, this suggests that the density of the Jovian atmosphere at 0.1 bar will be about .013 that of Earth at sea level. As a comparison,

shows a similar density occurring at about 90 km for terrestrial atmosphere.

Since the NASA boundary for outer space is 100 km, it's hard to see how any aircraft, be it lighter or heavier than air, could function. The atmosphere is just too thin at altitudes with reasonable gravity.

| improve this answer | |
$\endgroup$
  • $\begingroup$ What does gravity have to do with it? It affects the air and plane the same, so lift is unchanged. 71 km above what? $\endgroup$ – JDługosz Aug 30 '15 at 21:54
  • $\begingroup$ @JDługosz Lift comes from relative velocity between the plane and the medium you are flying trough. Why should increase in gravity increase lift? $\endgroup$ – Taemyr Aug 30 '15 at 22:15
  • $\begingroup$ @JDługosz - Force of gravity doesn't affect aircraft performance (to a first approximation, although force of gravity and atmospheric density do not track perfectly), but the ability of people to live, work and play does. I'm not at all convinced that humans can survive at 2.5 gs in the long term. I know that NASA has done some research on the subject, but my Google-fu has deserted me. $\endgroup$ – WhatRoughBeast Aug 30 '15 at 22:29
  • $\begingroup$ Who said the aircraft is for people? It is presupposed that it's neededvat Jupiter, not in near-jupiter-space. $\endgroup$ – JDługosz Aug 30 '15 at 23:52
  • $\begingroup$ @JDługosz - From the OP - "1.There are floating colonies for the planes to fly between." $\endgroup$ – WhatRoughBeast Aug 30 '15 at 23:54
2
$\begingroup$

Overall: I would say that powered flight is at the outside edge of feasible on Jupiter. How would you design a aircraft for Jupiter?

1) Dirigible using heated hydrogen for lift. While you could manage buoyancy, the end result would be not usable. Not sure how your floating cities are supposed to work, but even using the unobtanium for lift still has a problem. Landing these may be impossible due to high wind velocity (350 kph being common).

If the cities are allowed to move with the air currents you still have problems controlling flight in the intervening space to avoid being blown off course. Due to the size of Jupiter balloon flight will be very slow to cover significant fractions of the planetary circumference.

2) Airplane. You need a lift / air-frame mass ratio greater then 2.5 to keep aloft and carry cargo. A heavy lift plane C-5 Galaxy has a maximum take-off weight of 769,000 lbs and dry weight of 380,00 pounds a ratio of 2.02 : 1 -- s, A 747-400 is 2.22:1, Airbus A310-300F is also 2.21:1

So, given a little more attention to better materials, etc. a lift ratio over 2.5 is likely feasible (though a bit expensive), not that the net cargo capacity will likely be fairly poor.

You still have a pretty marginal design though. Is there anything you can do? Yes, assist the plane at take and landing just like they do on an aircraft carrier. It may expensive, but it is cheaper than building sky bridges.

What about fuel? For a hydrogen atmosphere, you need to carry an oxidizer. But you have a real problem, the weight is really working against you. Burning hydrocarbons on earth, you carry only a small mass fraction of the end products of combustion. Both C02 and H2O are mostly oxygen by weight, so you get most of the mass from the atmosphere. And on earth a 2.2 lift ratio allows you to carry a lot fuel (perhaps 30 tons or so) and still carry a reasonable amount of cargo. On Jupiter, the physics and chemistry are both working against you pretty hard.

Maybe it is time to make a nuclear air plane. The US air-force wanted some of these in the days before ICBM so that they keep planes in flight for weeks at a time. The nuclear plant that they liked used a molten salt reactor -- no heavy pressure containment vessel, and other advantages made this look at least feasible. A 747 requires about 90 MW for takeoff, about 45 MW for cruising -- this is small compared to a commercial nuclear plant.

Anyway, GE built a small (2.5 MW) molten salt reactor for testing and it was generally considered viable. Unfortunately I could not find any data re: the mass of this experimental reactor. They actually flew the reactor around, though they did not use it to operate the plane.

Also for those keeping their fingers crossed, Lockheed Martin claims to have a small fusion reactor available in a 100 MW prototype by 2025. The only size claim I've seen is mentioned in the article in that it would fit on a truck.

So, I would say that powered flight is at the outside edge of feasible on Jupiter. Larger designs in particular may be feasible as a nuclear plant does not necessarily scale up in a linear manner. Given the large size of Jupiter, airplanes would certainly be very desirable to reduce transportation times.

Finally, some things that would be desirable for flight on Jupiter-- speed of sound would be roughly 3 times that of earth normal atmosphere (at the same temperature) because the molecules are very light, and the viscosity. Hydrogen has less than half the viscosity of air though helium is slightly higher than air, the overall mix should be a little less than half of air. So you get to cruise a lot faster for the same energy consumption.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Regarding the 90 MW for the 747 to take off, this answer over on Aviation calculates that, for a Boeing 747-400, one particular type of engine provides 45.7 MW per engine, for an aircraft with four engines, so on Earth, a 747 needs more like 180 MW than 90 MW. Another answer to the same question gives (uncited) the figure 216 MW for the two engines on a 777, which is squarely in the same ballpark. $\endgroup$ – a CVn Nov 29 '17 at 15:16
  • $\begingroup$ @MichaelKjörling -- hmm, I think I used power of 747-8 from (aviation.stackexchange.com/questions/19569/…) originally which is the 90 MW I specified. Not a real airliner guy, could there be that much variation between models? $\endgroup$ – Gary Walker Nov 29 '17 at 17:11
  • $\begingroup$ That's the same question I linked to an answer to. :-) See the comment at aviation.stackexchange.com/questions/19569/… and the comments following it. I'll be fair on you, though, as Jan Hudec's answer was posted much later, and Kevin's does appear reasonable at first glance. Jan's analysis is not perfect either since it looks at maximum rated thrust, not take-off thrust, but take-off thrust is likely close enough to maximum rated thrust that this distinction is mostly academic. $\endgroup$ – a CVn Nov 29 '17 at 18:19
  • $\begingroup$ The simple way of seeing which alternative is more likely to be correct is to consider how an aircraft starts to move under its own power from a standstill. It does so by using its engines, which unlike in a ground vehicle are not connected to the wheels. The engine thrust therefore must be non-zero (and significant, because the aircraft has lots of mass and thus inertia) at a speed of zero (otherwise the aircraft would just remain where it was), so Kevin's simple use of P=Fv falls apart. Ergo, even without analyzing the math, Jan Hudec's analysis appears more likely to be correct. $\endgroup$ – a CVn Nov 29 '17 at 18:19
  • $\begingroup$ @MichaelKjörling -- thanks, have a degree in mech. engineering, just too busy to wade thru these right now. I seem to recall take-off thrust is usually about 90% of rated thrust too. $\endgroup$ – Gary Walker Nov 29 '17 at 19:16
2
$\begingroup$

Something other answers don't seem to have mentioned yet: Jupiter has some pretty wild winds. The red spot you see in pictures is a storm that is literally the size of the Earth. So, I feel like the important question here is not whether you could design a craft that can move about in Jupiter's atmosphere, but whether it can stay stable. From this article I found after typing in "Jupiter winds" on google:

Jupiter has bands of wind and jet streams that crisscross along the surface of the planet and rotate at different speeds — sometimes differing by up to 220 miles per hour (100 meters per second). Many thought these streams might just exist on the planet’s exterior, a bit like the winds on Earth. But now, researchers think the jet streams actually stretch deep within the planet

Link: https://www.theverge.com/2018/3/7/17085462/nasa-juno-spacecraft-jupiter-fluids-winds

| improve this answer | |
$\endgroup$
1
$\begingroup$

There is no reason why a craft can't move through a fluid by using that fluid as reaction mass and using fluid dynamics to have shapes that are harder or easier to pass through the fluid in various orientations.

That covers swimming as well as flying, and the appearance of whatever works for the chosen pressure, gravity, and other fluid properties might be something you would characterise as something else again.

A basic glider shape will have a gliding effect. Add enough power and anything can fly, regardless of aerodynamics. Between the two is a practical design.

For plausible SF ideas on the craft, you need to be more specific regarding the depth (outside pressure and composition) and range of operating environments. A previous answer assumed near-vacuum. I'd be more likely to envision high density, all, the way down to supercritical state.

| improve this answer | |
$\endgroup$
0
$\begingroup$

you could fly a specially designed airplane on jupiter, but it would not be like one that flies on earth. Altitude is a matter of design, does it float on some limit of density? or use principles of lift? A bladder of less dense gas would float, but would need to be strong enough to not be crushed. This is not really a question of 'can' but rather 'what parameters need to be determined for a plane to fly?'

As your question asks this, I would proffer:

  • A more rounded or spherical shape, possibly like an egg, to better distribute the forces on the hull.

  • Rigid delta wings, like a B2 Stealth, instead of protruding bird like wings like an airliner.

  • Propulsion would likely be rocket engines to gain the force needed to move in the atmospheric soup.

| improve this answer | |
$\endgroup$
  • 3
    $\begingroup$ There are a lot of question marks here for an answer, and a lot that isn't backed up. $\endgroup$ – HDE 226868 Aug 30 '15 at 16:02
  • 2
    $\begingroup$ Jupiter's atmosphere consists of primarily (91%) hydrogen and helium. I challenge you to find any composition of gases that is significantly lighter in weight than Jupiter's mixture of these two. Lighter-than-air craft are feasible (for some value of feasible) on Earth because nitrogen and oxygen (the two primary (99%) gases in Earth's atmosphere) are quite heavy. $\endgroup$ – a CVn Aug 31 '15 at 7:21
  • 1
    $\begingroup$ And continuing on what @HDE226868 wrote, we prefer answers that provide some citations or references for the claims made. You don't need a doctoral thesis for a [science-based] question, but you are more likely to receive upvotes if your answer clearly answers the question as asked within the stated constraints and preconditions, and supports the claims made by reasonable references, relevant math, or whatever happens to be relevant in the case of the specific question. (Not all questions benefit from math-heavy answers, but for some, like this one, at least IMO it doesn't hurt.) $\endgroup$ – a CVn Aug 31 '15 at 7:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.