Large Balloon Reflector: a potentially game-changing antenna design

mannykannot · 2023-10-28T01:51:50

I am pretty sure that something like this was conceived of by David E. H. Jones, under the pen name Daedalus, for one of his tongue-in-cheek columns in either New Scientist or Nature.

IIRC, the scheme, for creating a large space telescope in situ, went something like this: 1) create a flat film of some uncured polymer resin, polymer solution or heat-softened plastic on a large ring; 2) inflate it into a hemispherical dome by applying moderate gas pressure on one side; 3) allow the material to harden; 4) release the pressure, which will cause the bubble to adopt a parabolic shape; 5) use ion beam deposition to apply a reflective surface. I can't say whether my recollection of the geometrical claim on which this proposal is based is accurate, or, if so, whether it is correct.

Most of Jones's proposals were wildly yet entertainingly infeasible, but not always, and he is somewhat well known for conceiving of a form of 3-D printing (using a laser-polymerized monomer) before any serious proposals for the concept. There are a couple of collections of his Daedalus columns.

https://en.wikipedia.org/wiki/David_E._H._Jones

Wingman4l7 · 2023-10-28T16:06:27

There's also the old concept of spun liquid mirrors, (1994, Amateur Scientist column of Scientific American): https://optica.machorro.net/Optica/SciAm/LiquidMirror/1994-0...

asimpleusecase · 2023-10-28T04:28:34

When I was a child , so a long time ago, at the Huntsville ( huntsville alabama )Space Museum they had a demo that was something that looked like a pie pan that was 10 feet across with Mylar stretched across it. And a tripod that was attached at the edge and stood over it. At the top there was a cone of metal and inside the cone was a copper tube that ran up one of the tripod legs and spiralled around the inside of the cone then ran down one of the other legs. They had a household vacuum cleaner attached to a tube at the bottom of the pie pan. When the turned on the vacuum the Mylar got sucked down and it made a parabolic surface. The sunshine was then focused into the cone and the turned on the water and it ran through the copper tube . It came out a stream of water for a few minutes but then turned to steam. Always loved that contraption.

Intralexical · 2023-10-28T15:39:39

Reminds me of liquid-mirrored telescopes:

https://en.wikipedia.org/wiki/Large_Zenith_Telescope

myself248 · 2023-10-28T03:07:09

This was in my browser history from earlier this year:

https://gizmodo.com/upcoming-cubesat-feature-inflatable-beac...

The military has used them for years on the ground, evidently this is also a nasa spinoff:

https://spinoff.nasa.gov/Spinoff2010/ps_5.html

I see the used ones for sale at ham radio flea markets sometimes.

smeej · 2023-10-27T23:37:51

I'm oversimplifying, but this is how I read it as a layperson, with a fair degree of joy!

NASA, for decades: Launching antennae into space is a difficult, costly procedure that requires billions of dollars.

Also NASA: Hey, what if we just...made the inside of one side of a clear balloon shiny??

foota · 2023-10-28T00:27:55

Who would win, the space industrial complex or one shiney balloon boy?

Given the other comment about this below (https://news.ycombinator.com/item?id=38044638), I'm surprised this hasn't been done sooner though, I wonder if there's some missing downside?

s1artibartfast · 2023-10-28T06:00:41

There are tons of downsides and tradeoffs.

Top concers I would have is working lifetime, stability, and percision.

You don't want a dish that is continually changing shape. Gas and materials shrink and expand as temperature changes. The balloon will also leak.

Intralexical · 2023-10-28T15:51:57

> The ballon will also leak.

The US solved this problem in the early Cold War for passive balloon communications satellites by putting a chunk of solid volatile material inside the balloon, so sublimation kept a constant internal gas pressure. I suppose you'd stick a cheap servo on a door to control the sublimation rate if you need a precise mirror.

Besides, so long as you've already got the expensive sensors, navigation, propulsion etc. up there, why not just keep chucking up backpacks crammed full of new balloon modules for when your old one pops?

s1artibartfast · 2023-10-28T16:57:35

How many decades or years does that last?

Intralexical · 2023-10-29T04:01:58

I'm sure you could try to look it up— IIRC I think satellite lifespan in Project Echo was limited by drag on the balloon from a relatively low orbit, not depletion of sublimator.

But if you're just talking about the sublimator, why wouldn't it last however long you want it to, depending on how much sublimator you put in it? Plastic's pretty airtight even if it's not perfect, and it's not like you need a ton of pressure to hold shape in a zero-g vacuum.

jimkoen · 2023-10-28T07:12:48

These sound like manageable problems in space though.

Overall the temperature of the balloon will stay pretty constant if it stays on a fixed orbit around the sun.

The argument about precision I don't understand - the article talks about creating telescopes with this concept but NASA wants to use this for radio communication - the comparison with JWST doesn't apply, as it's mirrors had to be manufactured to a much higher tolerance.

> lifetime

I mean we had bigelow Aerospace with their module and it seems to be pretty durable. [0]

[0] https://de.wikipedia.org/wiki/Bigelow_Expandable_Activity_Mo...

s1artibartfast · 2023-10-28T07:32:32

>Overall the temperature of the balloon will stay pretty constant if it stays on a fixed orbit around the sun.

OK, so you just ruled out 99% of use cases. The world only has a handful of satellites in heliocentric orbits.

Intralexical · 2023-10-28T15:43:34

> You don't want a dish that is continually changing shape.

https://en.wikipedia.org/wiki/Adaptive_optics

Decent ground-based telecopes have to constantly change shape anyway. Perhaps it would in fact be easier to manage with a balloon.

JWST also unfolded from panels, so presumably has a way to calibrate itself.

squarefoot · 2023-10-28T09:24:16

Space is cold; I wonder if they could find a material that is inflatable at room temp but would quickly harden in space, so they would inflate it using warm gas, then wait for it to cool down.

s1artibartfast · 2023-10-28T11:41:32

Space is also hot.

NASA temperature conditions for flight hardware is as follows: LEO -65 ºC to +125 ºC, with 6,000 cycles/yr depending on orbit height. GEO is -196 ºC to +128 °C, with 90 cycles/yr.

That is some serious Thermal stress. Never mind the fact that it is also being blasted with UV radiation.

squarefoot · 2023-10-28T17:27:31

> Space is also hot.

Yes, of course. I was wondering if there are materials that once cooled down would harden but then remain as such also when temperature rises again. Assuming it's doable, it would likely suffer from dilation and contraction anyway.

Intralexical · 2023-10-28T15:46:46

> That is some serious Thermal stress. Never mind the fact that it is also being blasted with UV radiation.

This sounds like a problem that is easily solved by simply adding another, bigger balloon that sits between it and the sun.

jdironman · 2023-10-28T03:06:09

One thing I am wondering about is SAT to SAT communication. I imagine the receiver / transmitter is facing "down", or is it maneuverable and does SAT to SAT support something like a BGP router network or something to ensure optimal comms.

dmbche · 2023-10-27T21:54:10

I can't see a picture of the concave shape, I was under the impression that the design is that one third of the sphere is collapsed inside itself to make a nice concave antenna that's cheap and light - is this what's happenning?

Edit0: Nope! Part of the sphere is transparent, and the inside is reflective. Nothing collapsed.

And it's working very similarily to a normal parabolic antennae, but being inflatable it's way lighter/smaller, leaving more weight and room for power and instruments. Massive win!

smoyer · 2023-10-27T23:03:35

Transparent to some portion of the electromagnetic spectrum ... We might not be able to see through it though.

denton-scratch · 2023-10-28T08:05:22

> it inflates like a beachball, providing a stable parabolic-dish shape

Except the surface of a beachball is spherical, not parabolic. A parabolic surface is not closed, so you can't inflate it like a beachball.

So I'd like to know either: how they control the shape of the surface, so the mirrored portion is parabolic; or whether the antenna surface is actually spherical, and not parabolic at all.

The portion of a parabolic mirror nearest the focus approximates to a sphere. If the deviation is less than 1/4 wavelength of the signal of interest, a spherical mirror will focus the signal perfectly, as if it were a paraboloid.

A paraboloid is what you need for distortion-free imaging; this antenna is apparently used only for signalling, so perhaps accuracy doesn't matter, because only the gain is important. But this is a NASA publication, so I wouldn't expect them to say it's parabolic if it's really spherical.

guenthert · 2023-10-28T12:55:31

Seems like an error in the article. [1] states: "FreeFall’s antenna technology is unique because it is using a spherical reflector. In the past, antennas always used a parabolic antenna. A parabolic antenna focuses energy to a “single point” – while a spherical antenna focuses energy to a “focal line”. Parabolic antennas are symmetric about only one axis which severely limits field of view. It also requires precise pointing for high gain and has more complex packaging, deployment and on-orbit operations. The spherical antenna provides a wider field of view for antennas and high gain without re-pointing of an antenna. Combining a spherical antennas with inflatables is the key to achieving a large aperture in a simple lightweight system."

[1] https://www.freefallaerospace.com/nasa-balloon/

denton-scratch · 2023-10-28T16:55:06

Thank you.

I'm not up to date on telescope design (by at least 40 years), but the Cassegrain design used a spherical mirror, I think. It was used for wide-field astrophotography, and (again, I think) it has a curved focal field, so it needs a curved sensor.

I'm certain that Cassegrain designs have been obsoleted. I'm just bragging about what an anachronism I am.

[Edit] The Cassegrain design also requires a refracting lens in front of the mirror; and the lens is a peculiar shape.

[Edit again] I may be thinking of a Schmitt. I've never played with anything more interesting than a Newtonian; I was just an amateur stargazer. All these fancy designs were made for professional astronomers. For me, they were strictly theoretical.

Intralexical · 2023-10-28T15:57:02

Arecibo is/was spherical Ftr, presumably because a paraboloid could only sweep a single path through space. They aimed it by moving the receiver.

sandworm101 · 2023-10-28T12:39:38

A few internal wires/cables might allow the final inflated structure to be roughly parabolic. All sorts of wacky shapes can be made with internal stays. Also, at low pressures the outer shape of the balloon could be made parabolic. Not every balloon need be perfectly round. When inflated a balance occures between internal pressure and outer structure. Just look at any birthday baloon. They arent spheres. And perfection isnt needed for a radio antenna. It just needs to be better than one of the same mass.

denton-scratch · 2023-10-28T17:01:59

> Just look at any birthday baloon. They arent spheres.

Yeah, I wondered about that. I guess a birthday balloon is a pretty complicated curve; but I doubt it comes closer to a paraboloid than it does to a sphere or a plane, anywhere on its surface.

But this thing isn't a telescope, it doesn't have to focus an image; it doesn't need to be a parabola, a sphere is fine for achieving highly-directional gain.

fgeiger · 2023-10-28T08:19:13

Maybe I misunderstand your point of spherical vs parabolic, but doesn't this paragraph explain it?

> The concept turns part of the inside surface of an inflated sphere into a parabolic antenna. A section comprising about a third of the balloon’s interior surface is aluminized, giving it reflective properties.

The balloon is approximately spherical but probably not exactly (actually, when looking closely, hardly any beachballs are spherical either). What is parabolic is the part of the balloon which is aluminized.

Edit: fixed a typo

persnickety · 2023-10-28T09:19:29

This references the problem, but doesn't explain the big question: how do they turn a section of a sphere into a section of a parabola?

Beach balls might not be spherical, but my bet would be on ovoid rather than parabolic.

denton-scratch · 2023-10-28T08:43:39

Yeah; I was wondering how they get the mirrored part of the balloon to take on a parabolic shape, rather than spherical; is that done by manipulating the balloon material somehow? Or are they settling for spherical as a good-enough approximation?

ortusdux · 2023-10-27T22:08:57

Reminds me of Project Echo

https://en.wikipedia.org/wiki/Project_Echo

Remastered documentary: https://www.youtube.com/watch?v=19kAuAVAnDc

Scott Manley: https://www.youtube.com/watch?v=19kAuAVAnDc

itishappy · 2023-10-27T22:17:51

Also NASA's Inflatable Antenna Experiment, which looks like it spun off a successful commercial product.

https://en.wikipedia.org/wiki/Inflatable_Antenna_Experiment

https://spinoff.nasa.gov/Spinoff2010/ps_5.html

https://www.cubic.com/inflatable-satellite-antenna

uoaei · 2023-10-27T22:26:25

I'm a little confused because the article uses 'spherical' and 'parabolic' seemingly interchangeably. I'm sure the scientists know what they're doing but just found the imprecision a bit odd for a nasa.gov article.

istjohn · 2023-10-27T23:06:29

This page states that it is spherical. https://www.freefallaerospace.com/nasa-balloon/

uoaei · 2023-10-28T00:09:15

Nice, thanks for finding that.

Excerpt:

> FreeFall’s antenna technology is unique because it is using a spherical reflector. In the past, antennas always used a parabolic antenna. A parabolic antenna focuses energy to a “single point” – while a spherical antenna focuses energy to a “focal line”. Parabolic antennas are symmetric about only one axis which severely limits field of view. It also requires precise pointing for high gain and has more complex packaging, deployment and on-orbit operations. The spherical antenna provides a wider field of view for antennas and high gain without re-pointing of an antenna. Combining a spherical antennas with inflatables is the key to achieving a large aperture in a simple lightweight system.

sbierwagen · 2023-10-28T01:42:22

Kinda funny. Antenna gain is beam width. Antennas with large effective apertures have narrow beam widths.

I can easily buy that it could be cheaper to launch a 50m spherical inflatable antenna than a rigid 25m parabolic antenna. But it's never going to be true that a spherical antenna with the same gain as a parabolic will have a wider beam width.

uoaei · 2023-10-28T03:41:48

I assume some part of the "focal line" aspect changes that calculation.

andreareina · 2023-10-27T22:48:58

I understood it as, the structure is spherical, with one parabolic surface that acts as the antenna.

teo_zero · 2023-10-28T05:56:17

No, it's spherical. Instead of a transceiver placed in the focal point, they use array of min. 3 transceivers. It will definitely not have the same gain of a parabolic of similar size, but it will be more flexible about where the target it aims at must be. They are advertising it not for communication from fixed point to fixed point, but rather between vehicles that are continously moving, where a less directional solution will be an advantage.

smoyer · 2023-10-27T23:07:14

You can also tune the secondary a bit through the design of the feed horn. Even better with a phased-array and some electronics!

Daub · 2023-10-28T11:22:26

From TFA:

> Some 30 years ago, a young engineer named Christopher Walker was home in the evening making chocolate pudding when he got what turned out to be a very serendipitous call from his mother.

> Taking the call, he shut off the stove and stretched plastic wrap over the pot to keep the pudding fresh. By the time he returned, the cooling air in the pot had drawn the wrap into a concave shape, and in that warped plastic, he saw something – the magnified reflection of an overhead lightbulb – that gave him an idea that could revolutionize space-based sensing and communications.

I love genesis stories like this. Reminds me of Feynman's plate: https://demonstrations.wolfram.com/FeynmansWobblingPlate/

pmorici · 2023-10-27T23:58:43

Is the idea here that this is new for outer space? There are ground based inflatable antennas that you can buy that have been around for a few years.

I’ve heard a trade off with these is they aren’t very ridged so in windy conditions they don’t work that great. Maybe that is less of a problem in outer scpace.

https://www.cubic.com/inflatable-satellite-antenna https://spinoff.nasa.gov/Spinoff2010/ps_5.html

MagicMoonlight · 2023-10-28T07:54:02

That makes way more sense than using it somewhere fragile like space.

You could have an emergency antenna on life rafts that suddenly inflates to a full size beacon.

eternauta3k · 2023-10-28T07:15:45

Re: wind, at what frequencies would it be ok to put them in a box / tent?

kunwon1 · 2023-10-27T21:29:24

Makes me wonder about an inflatable balloon antenna for amateur radio. I wonder how hard it would be to do this sort of fabrication (aluminizing the inside of a balloon) as an amateur

Better pictures here https://www.freefallaerospace.com/nasa-balloon/

ianburrell · 2023-10-28T04:55:50

I don't think it would be useful for most amateur radio which doesn't need the gain and balloon would be huge. It would be useful for people who do higher frequencies and use satellite dishes. Portable link to 2.4GHz mesh network would be example. It might also be useful for cheaply making big dishes for Moon bounce.

itishappy · 2023-10-27T22:28:34

buy space blanket (aluminized mylar)

tape it over garbage can

suck

???

profit

cwkoss · 2023-10-27T22:46:35

greenpowerscience is a fun channel who did a neat version of this https://www.youtube.com/watch?v=-1GV3FIOR6E

aqfamnzc · 2023-10-28T04:07:20

Just need to find a parabolic garbage can!?

eternauta3k · 2023-10-28T06:28:50

No, the garbage can just supports the edges.

brucethemoose2 · 2023-10-28T01:14:59

Balloons In Space are a good idea, and an antenna is not the only concept they've been proposed for:

https://www.projectrho.com/public_html/rocket/infrastructure...

https://www.projectrho.com/public_html/rocket/enginelist.php...

Even at large volumes, you get a ton of surface area for the mass, as well as a good spherical or parabolic shape for reflectors.

tda · 2023-10-28T05:53:04

The article really could use a picture. If I understand correctly you can get a parabolic shape if you have a stretchy film over ring and apply a pressure differential. The spherical balloon functions as the support ring for an internal reflective third layer. If both halves of the balloon are inflated to equal pressure the reflective layer is flat, but by changing the pressure in both halves the layer takes on a parabolic shape.

eternauta3k · 2023-10-28T06:26:07

As far as I understood, it's just a balloon with a reflective coating from the north pole down to some latitude.

tigershark · 2023-10-28T00:18:52

I don’t think that the explanation is entirely correct. Inflating a balloon will produce a spherical antenna not a parabolic one. Now I don’t really remember the implication of using a spherical receiver for radio waves, but in case of light waves it’s necessary to use a parabolic reflector instead of a spherical one to avoid spherical aberration.

satori99 · 2023-10-28T00:46:51

In this case, the spherical shape is an advantage. See uoaei's post (above);

https://news.ycombinator.com/item?id=38043955#38045556

aimor · 2023-10-27T22:01:15

A good idea. I wonder how robust it is to small debris, I assume it won't pop but will the thin material fold up on itself if there's a hole?

morcheeba · 2023-10-27T22:40:51

That was my first thought, too. My ideas was to make the plastic UV-curable so it would become rigid and not require inflation. I looked it up, and that's what they're doing - UV-curable ribs. It's also not a sphere; it has an inflatable ring. Pictures & paper here: https://asteroid.arizona.edu/KABAND_Inflatable_v3_public.pdf

Terr_ · 2023-10-27T22:07:22

I guess it depends on the odds (impacts per cross-section per time) for different debris size/composition/speeds in the planned orbit. I'm sure somebody's studying that, but I'm also sure it must involve a lot of complicated tables.

The particular mini-satellite is slated for a 6-month mission, so presumably they think it'll survive at least that long.

greesil · 2023-10-28T01:12:05

Inflatables are nothing new, but still awesome.

https://en.m.wikipedia.org/wiki/Inflatable_Antenna_Experimen...

blackhaz · 2023-10-28T12:43:05

By the way, inflatable parabolic SATCOM antennas have been sold by a company called GATR for at least 10 years:

https://www.militaryaerospace.com/rf-analog/article/16715763...

mlsu · 2023-10-28T00:46:20

The idea is that these would point to space, right?

I wonder if you could create a ground antenna as well, with a pair of these -- one acting as receiver and the other as transmitter, at a very high altitude, pointed at two different locations on the ground.

That would give you a very high gain antenna for two fixed points which might obviate the need for e.g. expensive undersea cable link.

eternauta3k · 2023-10-28T06:35:11

I don't understand why the article and everyone here are talking about spheres. Balloons come in all possible shapes, and in fact the most common birthday balloons are not spherical. Can't they engineer a balloon which has a parabolic surface when inflated?

dathos · 2023-10-28T06:45:31

I think that requires a lot more pieces and engineering in order to work properly

eternauta3k · 2023-10-28T07:08:38

What do you mean? We can make party balloons and long balloons for balloon animals without any additional pieces.

h2odragon · 2023-10-28T00:19:02

Looks to be something easy to experiment with at home. Heat welding vinyl isn't horribly difficult or risky. use copper tape to form your antenna elements. Complicated 3d shapes that can be collapsed and reinflated should be doable.

Durability might be a concern but there's a lot of prior art in marine applications to observe.

danboarder · 2023-10-28T01:03:48

I thought of using a similar technique to build low cost concentrated solar arrays with a pump that adjusts the air pressure within to automatically focus and optimize the incoming light onto the CSP hot end.

teo_zero · 2023-10-28T06:13:57

The keyword here is "focus": a spherical surface like this does not focus sun rays, you need a parabolic surface for that. While it would be possible to shape a film or another foldable material into a parabolic profile playing with air pressure and a specially crafted frame, I wonder what would be the advantage over a rigid parabolic mirror. The foldable/inflatable part is only an advantage for aerospace applications where size and weight are at premium.

eternauta3k · 2023-10-28T06:31:27

A balloon is not necessarily spherical, see e.g. the long balloons for making balloon animals.

prvc · 2023-10-28T04:12:58

I assume this is based on a spherical cap's being "close enough" to a paraboloid, for the purposes of this design.

MagicMoonlight · 2023-10-28T07:52:13

I’d be concerned that the balloon would pop the moment it comes across some rock or dust.

Using a huge balloon to make the shape and then covering it in a material in space could be an idea. Get the ISS crew to build a mega telescope in orbit.

incompatible · 2023-10-28T01:34:31

“At first, I was afraid to share the idea with colleagues because it sounded so crazy. You need a program within NASA that will actually look at the radical ideas, and NIAC is it.”

It doesn't seem like an even slightly crazy idea to me.

Scalene2 · 2023-10-28T02:59:27

The rocket crane to land a rover was pretty crazy, gonna be hard to top that.

ooterness · 2023-10-28T03:21:08

That still blows my mind. At some point, a room full of risk-averse engineers all agreed that "rocket crane" is the safest and most practical solution to a problem.

slowhadoken · 2023-10-27T23:36:36

Reminds me of the 320th Barrage Balloon Battalion.

peter_d_sherman · 2023-10-28T06:20:53

Interesting article!

>"Now, with an assist from NASA’s Innovative Advanced Concepts (NIAC) program, funded by the agency’s Space Technology Mission Directorate, which supports visionary innovations from diverse sources, Walker’s decades-old vision is coming to fruition."

Didn't know about NIAC before reading this. It sounds like a great group of people!

https://en.wikipedia.org/wiki/NASA_Institute_for_Advanced_Co...

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