Hello Bruce,
Thanks for the positive response. Batting ideas around sometimes produces a concept that is a winner, so the more ideas voiced the better. Interleaving a response into your mail will be difficult so I'll do this list style.
1) Cubesats and amateur radio. Yes, Delfi C3 is the best but sadly one of the very few to carry a usable communications package. It's been a pleasure meeting the Delft students at the annual AMSAT-UK colloquium at Surrey University for the last few years. They started by asking us hundreds of questions, but by last summer it was the AMSAT members who were sitting back during the D -C3 presentation asking questions about their satellite and the innovative Op-amp (not MMIC) based transponder design. I'm certainly looking forward to hearing it in a couple of months time.
2) Batteries or not? An interesting topic. Certainly DO-64 and AO-7 have made the point that batteries are not essential and have in the past caused the demise of many AMSAT spacecraft. I have suggested placing the equivalent of a solid state relay in series with the battery a few times before. Basic idea is that when the battery fails, usually short circuit, it can be isolated allowing the spacecraft to be run in sunlight directly from the panels. Taking the topic forward a little.....How about Ultra capacitors as an alternative storage medium? I haven't checked the following, but if it's incorrect I know I can expect a reply from someone on the BB. Ultracapacitors are available at e.g. 470 Farads at 2.5V If we take 3 in series we have an energy store with useful voltage. But what can it power?
470F x 3 in series = 156F with a max voltage of 7.5V So total energy stored in Joules is
E = 0.5 x CV ^2 or E = 0.5 x 156x (7.5 ^2) or 4387 Joules
Now, and this where my memory gets a bit blurred.... Isn't the Joule a "Watt second" ? So, does this mean this small array of capacitors could supply 1 Watt to a load for about 1 Hour or 3600 seconds? - I included a bit of loss there for a buck-boost converter and not taking the voltage down to zero.
The downside could be what happens when a highly ionised particle impacts at high velocity with a charged Ultracapacitor. But that could be tested.
3) 29MHz antennas. Yes that would be a challenge. On receive it's not a problem as the natural noise level is very high but path loss is very low indeed. A loaded whip or even a loop or ferrite antenna could work. On transmit the higher the efficiency the better, but again low pass loss works in our favour.
Regards
David
In a message dated 01/06/2008 02:04:27 GMT Standard Time, [email protected] writes:
On Sat, May 31, 2008 at 5:40 AM, [email protected] wrote:
Hi Edward / group.
Most current construction or feasility study is centered on P3E / Eagle / Intelsat /HEO where launch opportunities are rare and costs are high.
Perhaps we should look at this problem from another viewpoint. Start with what launch opportunities AMSAT can afford and then retake the technology initative and investigate what minaturised payloads can we launch for
that price?
For example. Imagine a 2 or 3U cubesat type structure, or even one half
the
size of AO-51 on last weeks Russian launch to 1500km. With payloads
reduced
to transponders and a basic onboard computer and an Electrical Power
System,
it would be feasable to put RF comms equipment into a decent orbit on
29MHz
145MHz 435MHz with an RX on 1269. For bands higher than 13cm doppler is a problem and path loss is quite
high.
It may not be possible to provide the necessary DC power for transmitters in a small structure.
David:
I asked the same question at the end of a thread decrying the lack of HEO satellites, hoping to start a conversation on the topic. Let's have one here. The best of this group is when we bat around ideas such as this.
First, very soon we will have a great basis for future discussion in Delfi C3. Already, though, it shows that a 3x cube can hold transponder and deploy enough solar for a good transponder U/V (judging by its fantastic downlink now). Could we replace the science on board D C3 with a battery of chargeable cells and still stay within the cubesat weight requirements? Or, even more radically, can we do without batteries, given how we can live with AO-7 and D C3?
My guess is that deploying a 29MHz antenna could be hard with that limited space. But it would represent a fascinating challenge.
Perhaps the hardest part would be finding a ride to high LEO without propulsion. My sense is that Cubesats work economically because there are lots of them. Won't university groups interested in remote sensing see this as something worse than what they have with low LEO, for a greater cost? Maybe we could get a bunch of projects interested in high LEO together for one launch. Or, perhaps we could help one of the national groups that still have access to their countries' launch facilities, such as Japan or India or China. These might include a p-pod or two in a high LEO launch as a charitable act: much cheaper than a micro-sat.
Talking of DC power, the number of cubesats that fail due to power problems is huge. The answer is to get inventive with deployable solar arrays. With the engineering excellence AMSAT possesses it should not be impossible to arrange a structure where the entire outer layer contains extra solar
cells that
are deployed after seperation from the launcher. Imagine a 3U cube which
in
orbit becomes a 3U box of electronics covered in cells, with an extended
outer
3U shell that deploys forming a 6U structure producing nearly double the
DC
power. The 6U structure also makes antenna design easier
A neat idea. What about film solar panels that would be unfurled once in orbit? I think these probably have such a lower efficiency that they wouldn't be worth the extra area they might cover. In any case, Delfi C3 gives a pretty great worst-case scenario.
For a slightly more risky idea.....small satellite propulsion. Again, perhaps 3U cube, with the last section comprising a small motor. A single
burn
unit could provide a really nice elliptical LEO orbit, perhaps 680km to
2000km.
Wouldn't that be interesting. I notice that there is an Austrian
university
team who have developed a cubesat sized ion propulsion system asking if anyone would like to try it. So, while this may initially seem a 'wild
idea' it
is based on technology that is very nearly a reality.
Regarding the ion propulsion, what if we were willing to wait a year or two before use, so that all solar energy went into boosting the orbit for that time?
And then there's the transponder. How efficient could a SDX be? William, PE1RAH, is working on cubesat format UV transponders, I think.
Worth investigating?
I really think we should look into S-band downlink, too. The stats on s-band in HEO might be scary, but I think in LEO it would still be high enough signal to be great fun. And we'd be colonizing an important band.
David G0MRF