NASA Tech Breathes Life into Potentially Game-Changing Antenna Design | Hacker News Reader

NASA Tech Breathes Life into Potentially Game-Changing Antenna Design

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77 pointsBrajeshwar39 comments

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

Thanks! Macroexpanded:

Large Balloon Reflector: a potentially game-changing antenna design - https://news.ycombinator.com/item?id=38043955 - Oct 2023 (77 comments)

The thing that nobody has illustrated properly is that the balloon is just a way to hold the outside edge of the parabola taunt, there's a flat metalized sheet inside, that basically cuts the interior volume in half, that gets stretched into a parabola by differential pressure, forming the reflector.

You then put your feed horn on the outside of the balloon at the right place, aimed at the parabola inside, and you have a high gain antenna.

I found an image, note the 2 hoses in the lower left, one for each half, to allow regulating the pressure differential.

https://www.researchgate.net/profile/Stella-Parisi/publicati...

Why not put the horn inside the balloon such that it will be in the correct position once inflated?

It's simpler on the outside, less holes to make in the envelope of the balloon.

Every wire/tube/whatever you need to be inside the balloon is another place for a seal to fail. If the material of the balloon is transparent to the signal you're looking for (as it must be: the reflector is inside and the the universe you want to look at is outside), then there's no benefit to be had.

Non-blog-spam link: https://www.nasa.gov/directorates/stmd/nasa-tech-breathes-li...

Do you have a reference for this? In other channels there have been discussions about this and how it might work. A lot of questions about "hold the outside edge" and "parabola". Specifically if the outside edge is being held by the inflation the interior shape will become a sphere, if the outside edge is rigid like the balloon in the image shows, does it really save weight?

A couple of discussions about if you made the "balloon" part sort of like a tube balloon animal where you created interconnected tubes of the backing frame as one balloon that when you inflated formed the shape of a parabola. But neither the press release nor that image suggests anything like that, and positioning the horn would be a challenge.

Definitely curious, waiting for the paper.

From the previous HN post[1] about it, I was linked to the company's own page[2] which includes a more precise description of the technology. The reflector is indeed spherical and there is an array of transceivers along the "focal line".

[1] https://news.ycombinator.com/item?id=38043955

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

Thanks, the picture instantly explains it. The previous reporting was confusing and clearly missed the whole principle by mixing up spherical and parabolical.

I think you are conflating two separate designs. The company developing this concept explicitly uses the phrase "spherical reflector" and contrasts it against previous designs based on parabolas.

https://www.freefallaerospace.com/nasa-balloon/

I think you're right. Looking through their antennas page shows a lot of similar spherical designs, including the AllSky antenna that's being tested on the same balloon as the inflatable one. They look remarkably similar in design: spherical with a moveable transceiver to be able to steer the beam without moving the antenna.

https://www.freefallaerospace.com/antennas/

Ok... I stand corrected... interesting that there are two different approaches at play here. So, the moving feed inside a balloon is an interesting twist.

Hey dang can you replace the link with this? https://www.nasa.gov/directorates/stmd/nasa-tech-breathes-li...

Spy satellites are rumoured to use inflatable stealth shields to deflect radar and optical sensors so that they can hide from adversaries: https://patents.google.com/patent/US5345238

"An inflatable shield for suppressing the characteristic radiation signature of a satellite is described. The shield is conical-shaped and made from a thin synthetic polymer film material coated with a radiation reflecting material, such as gold or aluminum. At least one subliming agent is contained within the shield to inflate the shield when exposed to heat. An ultraviolet curable slurry coats the inner walls of the shield and permanently hardens the shield upon exposure to ultraviolet radiation from a self-contained source. The shield optionally may include absorbing and desiccant agents to absorb unwanted gas and water and prevent interference with the primary mission of the satellite. Additional means may be included for moving and positioning the shield with respect to the satellite."

This particular design inflates a shield after positioning in orbit and hardens the interior with UV light.

There's more info at the NASA link: https://www.nasa.gov/directorates/stmd/nasa-tech-breathes-li...

> 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 stories like this where the perfect experience matched with a person with specific knowledge collide. If this happened to a normal person, this revelation wouldn't have clicked. So many great inventions come from unique experiences/accidents like this.

Oooh maybe inflating ball is a plausible alternative to the proposal for the Lunar Crater Radio Telescope

https://www.nasa.gov/general/lunar-crater-radio-telescope-lc...

https://www.nasa.gov/solar-system/lunar-crater-radio-telesco...

It's mechanically complex so they are proposing automated robotics pulling a foil mesh over cables that are fired out over the crater edges.

But what if they just land a huge inflatable ball inside a crater and it only takes a little gas inside to expand it? It would have to expand 1km but the material could be ultra thin? Hmm maybe not.

PBS Space Time episode on the subject with artist visuals

https://www.pbs.org/video/what-new-science-would-we-discover...

https://www.youtube.com/watch?v=9F3WRMlZEy4

I imagine the moon regolith and the micro meteroids (no atmosphere to stop them) would be pretty harmful to a balloon.

> huge inflatable ball inside a crater and it only takes a little gas

I would think a huge inflatable ball needs a huge amount of gas, which is hard to find on Moon?

Well the idea is that it takes far less gas by mass to inflate a balloon against the vacuum of space and support the weight of the envelope in lunar gravity as it does to inflate a balloon against 1 atmosphere and support the envelope in Earth gravity.

It's also worth noting that the pressure vessel need not be spherical either; it only needs to produce the correct shape for the reflective part when pressurized.

For the specific application of the LCRT though it's probably not going to win. For one, a reflector at the HF frequencies they are targeting doesn't even need to be continuous; it can be a large open wire grid and will still be radio opaque at the required frequencies.

Makes me wonder if a giant, clear beach ball could be converted into a parabolic dish simply by spraying a ferrous-based paint on a section/hemisphere of it (with the receiver injected via the inflation tube).

(They didn't specify how 1/3 of the balloon's surface was "alluminized"... spraying would be a pretty easy way to do it, if it worked.)

First you have to cut it in half, and add the metalized interior partition that becomes the parabola. Then plastic weld all the seams, putting it back together into a bisected sphere.

If you pressurize both halves correctly, with the right differential, Bob's your uncle.

If you have the means to weld seams in Mylar, it would likely be easier to build one from scratch.

Why cut it in half? Spray (or Mylar) on the outside.

A parabola isn't the same shape as a hemisphere. (Although, I'm not convinced the antenna under discussion actually is a parabola and not a catenary, or something in between.)

Some other comments posted a diagram-the actual reflector is inside the balloon. the balloon is there to keep the reflector in the right shape.

Obligatory GATR link: https://www.cubic.com/inflatable-satellite-antenna#

These are used in disaster response and other scenarios that demand rapid deployment of satellite terminals.

Does this mean there is already a commercial version? If so, what is the advantage of the NASA funded project?

GATR works on the ground in atmosphere. Having seen them up close, I don't think there is a lot that translates to an in-space application other than the concept itself. You're talking about a completely separate ground-up engineering effort.

More hype for NASA.

Congress should cut NASA's PR budget.

I have spoken with the guy behind cubic/GATR (actually had a conversation with him and Buzz Aldrin at Space Symposium quite some years ago.) It's surely the first thing that came to my mind, and a fairly mature technology at this point in use by lots of people in the field. I came to the comments specifically to note this system, and thankfully found your reference. I have wanted to try to DIY one for a moonbounce station ever since.

The top half of the sphere is not the reflector.

The reflector is a diaphragm membrane inside the sphere that forms an efficient parabolic shape when pressurized correctly from above and below.

Until its first encounter with space junk or a micrometeorite, compromising the pressure vessel...

Having an exoskeletal structure supporting your dish is not without its advantages. But maybe you don't care enough for shorter missions?

I wonder if there are materials of one-way reflectance so that all of the sphere would be used instead of a particular face.

Why would this be used over a phased array where the paraboloid shape is created in software post-processing?

It's not the sphere that does the work, it's the parabola inside it, cutting it in half.

One-way reflectance isn't a really real thing in physics, the closest you get is the illusion from having a bright room and a dark room on opposite sides of a mostly-reflective boundary.

https://en.wikipedia.org/wiki/One-way_mirror

If you give me a polarized wave, I can give you a surface that reflects in one direction, and passes in the other, with awful efficiency. ;)

It wouldn't help. Focusing requires a concave surface, and the inside of the sphere is the only concave bit. Coating the outside would create an omnidirectional reflector, but it'd be super low-gain. We actually tried exactly this with Project Echo.

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

Also, "one-way reflectance" is largely a myth. One-way mirrors in police stations and stores are actually just partially reflective mirrors with a dark room on one side. A true one-way reflector would involve an optical circulator, which tends to be a small, sensitive, expensive component, not something you can coat over a flexible surface.

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

Lastly, the main advantage is cost and simplicity. Phased arrays use fancy semiconductors. This gives you a lot of features and benefits, but comes at significant cost. This antenna is more or less a Mylar party balloon, it would not surprised me if it's orders of magnitudes cheaper.

A good match for the Venus Ballooning project? (NASA/JPL):

https://www.youtube.com/watch?v=Ug6B2hnNk3I

A nice story but is there any actual evidence this happened in the way described.

> 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.

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