Satellites To Go Flat With DiskSat

Whether they are giant communications satellites or 1U CubeSats, most spacecraft are boxy vehicles that have antennas, solar arrays, and other elements sticking out of them. The Aerospace Corporation wants to change that trend with DiskSat, a flat, compact spacecraft that resembles a giant CD-ROM. During last month’s Small Satellite Conference, Parabolic Arc sat down with Richard Welle, senior scientist at Aerospace’s Space Applications Laboratory, and Darren Rowen, director of the Small Satellite Department, to discuss the project and its upcoming demonstration mission. The interview has been edited for clarity and length.

Parabolic Arc: Describe DiskSat.

Richard Welle: DiskSat is a concept for essentially large-aspect ratio satellites that are designed to be stacked. So, a representative sample of what we’re building for this demonstration mission is a disk that is one meter in diameter and two and a half centimeters thick. Nominally, it has some protruding components; it will stack with a center-to-center distance of 55 mm in the dispenser. And the idea behind that is to have a lot of surface area in a low-mass satellite, to have a lot of area for solar cells, a large area for RF [radio frequency] apertures, if the mission calls for it.

And in a low-mass satellite that gives a very high power-to-mass ratio, which is suitable for electric propulsion missions requiring a large Delta-v. So, we will be flying with about 100 [to] 120 watts of solar cells on this mission. We’re expecting to come in at about 12 kg [26.5 lb] per satellite, and we have electric propulsion on board that should be good for several hundred meters per second in Delta-v.

PA: Where [are] you guys with DiskSat?

Welle: The program is well underway. We are up to CDR [critical design review] for both the bus and dispenser. Next week we’ll have a CDR on the bus and two weeks later a CDR on the dispenser. And then we’re on track to deliver flight hardware a year from now, [on] the first of August of 2024. We are still working to identify a launch. We don’t have a launch yet, but there’s a lot of enthusiasm, and we hope to find a launch fairly soon thereafter. So, we’ll see on that.

Parabolic Arc: And what missions are you focusing on?

Welle: The demonstration mission is a proof of concept on the satellite itself and the dispenser. There are a couple of kinds of payloads of opportunity, but not really what the mission is designed for. The intention is to provide a vehicle that can do missions that require a lot of power and large apertures. So, for example, radar missions, communications missions that might require high power transmitters and large antennas, and also missions that need a lot of orbital agility using electric propulsion for high Delta-v.

And finally, missions in very low altitudes where drag might become an issue. The idea is, we haven’t demonstrated this, but this is the intention that a DiskSat flown edge on has a fairly low cross-sectional area. So, the drag is low, but we have plenty of power and thrust for drag makeup. We expect to be able to operate down to 300 km [186.4 miles] altitude on a continuous basis with this satellite.

Rowen: One of the key features is that deployables for antennas, especially solar arrays, can be challenging. And there’s a packaging challenge to try to stow them and unfurl them and keep them in the right shape, especially for certain types of antennas. And so, with the DiskSat, no deployment is required. The solar panel and whatever type of aperture you have for your RF system are already installed, and very flat. So, there’s no need to do calibration or shape it in any way.

Parabolic Arc: Right, very easy to deploy as well.

Rowen: Yeah, you deploy the disk and then there’s no further deployments required unless there’s a need for yet more power or something like that. But the first mission will have no deployables; it’ll just be the disk itself.

Parabolic Arc: You mentioned [the] ability to maneuver. Is that particularly useful for the military?

Rowen: There’s a lot of interest in maneuverability in orbit. [There are] many military applications for lightweight satellites that can maneuver around.

Parabolic Arc: What about the commercial aspects of this?

Rowen: I think when you get down to lower altitude, there are new mission areas available there. If you can do sustained flight at 300 or 250 km [186.4 or 155.3 miles], there are different types of collection you can do with those altitudes that are not currently serviced. Aperture radar is a great example. I think it’s that the resolution is a function of the range cubed, is that right?

Welle: Well, the signal strength is a function of the cube of the range.

Rowen: So, as you get closer you get much better resolution with synthetic aperture radar.

Welle: Another application might be, for example, the space-based Internet of Things, where they have small sensors, and components on the ground that want to communicate with something in orbit. They don’t have a lot of range.

Rowen: Internet of Things with the base being the receiver; bringing the receiver closer, you get more signal strength for receiving the signal.

Parabolic Arc: What are the biggest challenges of launching and operating these things?

Welle: The dispensing system, and we knew going in that the dispensing system was going to be the hardest part of this. The satellite itself is a flat structure. It actually should be, in principle, easier to build than a cubical satellite just because of access to components distributed on a flat surface, basically. It’s not completely two-dimensional, but certainly, access is much easier.

Parabolic Arc: So you’re going to be launching four of these?

Welle: Four. That’s correct.

Parabolic Arc: Theoretically, how many could you launch if you wanted a constellation?

Welle: That’s an interesting feature of the DiskSat. If you look at Cubesats, CubeSats are volume-limited, but the DiskSat is intended to be a mass-limited satellite. It has a very large volume, but it’s not intended that the volume of the satellite be filled. You put in as much as you need to do your mission and the rest of it is excess volume that can be used to support surface area for power and aperture. And then in terms of launching them, it again will be mass-constrained rather than volume-constrained.

If you think, for example, what we’re building right now is two and a half centimeters thickness and 12 kg, 1-meter diameter. If you stack 20 of those, then you’re up to 250 kg [551.2 lb], which now starts to exceed the mass capacity of small launch vehicles. But, yet the volume will still fit in the [launch vehicle] volume. So, the constraint on the number that you can launch at the time is going to be a mass constraint. How heavy is your satellite rather than how many fit in a volume?

Parabolic Arc: And what kind of payload shroud is this designed to fit into?

Welle: The demo mission is configured to fit in anything that’s 1-meter class or larger. So, it could go on an Electron, it could go on an ESPA-port, it could go on a larger launch vehicle. So, we’re fairly flexible on that.

Parabolic Arc: You have a lot of options.

Welle: I said that the version that we’re flying is in what we call the 1-meter class, because it fits in a 1-meter class launch vehicle. If you know you’re planning to launch on something else, the concept is intended to be extensible in the same way the CubeSats are extensible 1U, 2U, 4U, and the perimeter dimensions of the disk can be extended to whatever the capability of your launch vehicle is.

And we’ve looked at potential emissions for a five-meter diameter DiskSat that would fill up the payload fairing on a Falcon 9. I would not do it at two and a half centimeters. It would probably be thicker. I haven’t done the numbers, at least ten [centimeters], I would guess, probably more. But the nice thing about the structure [is] it’s a graphite epoxy with face sheets with an aluminum honeycomb core. It’s a very low-density core, so increasing the thickness doesn’t add a lot of mass to the satellite. Most of the mass of the satellite structure is in the face sheets.

Parabolic Arc: Is there anything you think is important?

Welle: I think that from my perspective, we’ve talked about it at this conference, and there’s been a lot of interest in it from a number of sources. And we’re very hopeful that we’ll be able to get follow-on missions beyond the demonstration mission. We don’t have anything definite planned yet, but we’re looking at a number of possibilities of things that could be flown on DiskSat. And I think I’m very encouraged with the reception that we’ve had here at this conference.

Rowen: I think we’re seeing it as kind of a new form factor that there’s the CubeSat form factor. And then there’ll be the DiskSat form factor, which may be better suited for certain missions that need large aperture for antennas or radar.

Parabolic Arc: It can do communications. We discussed the Internet of Things, reconnaissance, Earth observation, and RF frequency collection. Anything else?

Welle: Actually, the one area that is important and our NASA customer is interested in is deep space or cis-lunar applications. Because of the high Delta-v capabilities, there are a lot of options for maneuvering in space beyond GEO [geostationary Earth orbit], including lunar orbital missions and other kinds of applications in that environment.

Rowen: I know NASA talked about a lunar comm network or relay system where you could put small constellations in lunar orbit and use them for comm purposes.

Welle: In principle, it would also be possible to fly it to the Moon from Earth orbit, probably GEO orbit. It takes a few thousand meters per second of Delta-v to get to lunar orbit from GEP. But there are propulsion systems that can do that, so it may be possible.

Once you’re in lunar orbit, there are various missions that you can do: surface observation, communication, relay, and various applications. We haven’t taken that analysis very far yet.

Rowen: I know a lot of NASA science instruments would benefit from the large RF aperture opportunity without having to unfurl something, a lot of instruments based on radar or other similar microwave-type applications.

Parabolic Arc: Do you see this going on beyond that? Would it be difficult to send all the way to Mars or Venus?

Welle: They sent CubeSats to Mars. In principle, you can do the same with the DiskSat. We have not really explored that in detail.

The thermal environment around the Moon will be a challenge. That’s something, again, we haven’t put a lot of time into that yet. Radiation shielding perhaps, or radiation-tolerant components would be appropriate.

Rowen: We’ve already been doing some work on another mission that is going to go into a GTO [geostationary transfer orbit] and so already looking at upgraded components for radiation tolerance. Similar components would be useful for a lunar mission. But, you also have to add other capabilities, like a radio that is designed for that kind of range and a suitable ground network to communicate with the system.

Welle: Navigation, attitude control, I mean, you get a whole new set of problems when you leave LEO.

Parabolic Arc: Do you guys have anything else to add at this point?

Welle: I always like to acknowledge our sponsors on this. We have NASA STMD [Space Technology Mission Directorate] supporting mission development here and Space Force looking for a ride for us. We do like to acknowledge their support. It’s been very helpful.