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What: A space probe in the shape of a narrow cylinder,
with a dish at one end, whose concave side faces the
cylinder. The whole thing looks like big mushroom, or
maybe an umbrella.
Why: The dish can be used for radio communication with
Earth, and for concentrating sunlight onto solar cells.
The dish can more easily be larger than ones used on
past probes. If the probe is going far out from the Sun,
where such a large dish is desirable, the Sun and Earth
will be in approximately the same direction anyway, so
it's convenient to use the same dish for both purposes.
Using concentrated solar power instead of larger solar
arrays can be expected to reduce launch mass and
material costthe dish can be thin metal or aluminized
plastic film. The cylinder provides the place to mount
the radio and the solar cells, as well as whatever other
hardware the spacecraft needs, without obstructing
much of the dish. (Typical on-axis dishes (like Cassini's)
require support legs for the secondary reflector, which
cause side lobes, reducing gain; this avoids that.) The
dish can fit conveniently within a heat shield, if the
probe is planned to enter an atmosphere. If convenient,
the dish can be deployed, expanded (if already partially
deployed), or uncovered (if hidden inside an aeroshell)
only upon arrival at the probe's destination, because its
power and communication requirements are probably
minimal until then.
*I came up with this concept previously, for my ice giants
mission, but I only realized today that it's worth posting
as its own idea, so what I'm saying is that I'm posting it
the same day I realized I should.
||In most cases the orientation of the vehicle is irrelevant to vehicle operations. Only when you have an engine on one side which is actively pushing the craft, or are in an atmosphere, does it matter.
||Hang on. I'm not sure of the geometry here, but if the dish is
focussing parallel solar rays to a point where the solar cells
are, doesn't that mean the solar cells would be in the way of
transmission? But maybe there's a way round that. [+]
||// In most cases the orientation of the vehicle is irrelevant to vehicle operations. Only when you have an engine on one side which is actively pushing the craft, or are in an atmosphere, does it matter. //
||As long as your spacecraft only performs orientation-insensitive experiments (like scalar magnetometry or zero-g organism growing), has an orientation-insensitive power source (like an RTG or solar arrays on all sides), isn't going close enough to the Sun that sunlight on the radiators is an issue (or has louvered radiators on all sides), and is close enough to Earth or a relay station to communicate using an omnidirectional antenna, sure. All other spacecraft (i.e. any going beyond Earth orbit, and most of those staying here) need to care about their orientations. Interplanetary spacecraft use either spin stabilization or three-axis stabilization (or both at different times) to aim their high-gain antennas at Earth.
||// Hang on. I'm not sure of the geometry here, but if the dish is focussing parallel solar rays to a point where the solar cells are, doesn't that mean the solar cells would be in the way of transmission? //
||Yes. There are probably a lot of potential solutions. Some I've thought of are:
* A wavelength-selective reflector to separate the beams at the focal point of the main reflector< br>
* A main reflector that somehow focuses the different wavelengths to widely separated foci
* Photovoltaic cells that are transparent to radio waves
||Regarding the first and last of those options, have you seen the silver blankets they put over the high-gain antennas on some spacecraft? I learned the other week that those are made of plastic coated in a thin layer of germanium. The germanium reflects light, keeping the dish cool, but allows radio waves through, because it's a thin enough semiconductor layer that the longer-wavelength radio waves don't excite its electrons.
||Due to the inexorable effect of inverse-square law, by the time you're at your system's asteroid belt the incident solar energy has dropped to the point where it's not viable to harvest it.
||From there on out, you have to deploy RTGs (so crude ! ) snd similar self-contained power sources.
||That's the point of concentrating it. Do you deny the ongoing solar-powered operation of Juno at Jupiter, or that Jupiter is further out than the asteroid belt?
||Yes. If you look at the original imaging, you can just see the extension cable in the top right corner of the frame.
||NASA photoshop it out on the published photos, of course. They do that on all sorts of stuff.
||I realized yesterday that there's a small inconvenience
with this. If you're traveling to
another planet, you're going to want to thrust in a
direction along your orbit, rather than
radially toward or away from the Sun, due to orbital
mechanics. On the other hand, you're
going to want your dish aimed at the Sun for collecting
light. This is fine if you're
using chemical propulsion with a big burn once in a while,
because you can just turn to
thrusting attitude for a few minutes and turn back.
||However, if you're using electric propulsion, you're going
to want to be thrusting
continuously, but you're also going to want lots of sunlight
collected at the same time to
power the thrusters. This means that the thrusters must
be aimed off of the axis of the
mushroom's stem to satisfy both requirements
simultaneously. That means that you can't
just have one thruster cluster at the 'bottom' of the
mushroom's stem, because the thrust
would then not be in line with the spacecraft's center of
mass, which would result in it
spinning. Therefore, you need to either put your primary
thrusters somewhere around the
middle of the stem, aiming sideways, or put some
thrusters at the 'bottom' of the stem and
some at the rim of the mushroom's cap. Not too difficult
either way, but a small
||I hadn't even considered electric propulsion for this kind
of spacecraft until yesterday, for
||Edit September 15: All of the above goes for solar thermal propulsion, too. I've been thinking STP would be good for a mission
doing an Oberth maneuver, also, and this kind of spacecraft could be used for that.