MBobrik

At the end of my post from the previous page is a picture of two possible shapes We have first to aim the panels to the sun to get maximum power to charge the batteries. Then,, we have to aim the antenna towards receiver on the ground to get reasonable bps. And then back to the sun. It's now just 44 RPM/min. And that's only when earth's magnetic field direction and strength is optimal. In reality, we will get maybe 1/3 of it most of the time. no longer necessary. the 44 RPM/min figure is with torquer coils directly around the craft.

Beryllium costs cca $748 per 100g. we need 13 g of it. If necessary we can make a wire from it by ourselves.

So it is really that simple as I initially suspected. I used the maths about rotating objects long ago ... LordQ got me confused a little. Then I don't see why we should not go for the megatorquer from the previous page total weight 13 g, 3 coils wound directly around each mid section of the sat surface, each 33 turns of 0.5 mm diameter beryllium wire, I = 2 A, U = 5 V will get us 44 RPM/min

I think I answered some of your questions in the long post on the previous page.

Wait a minute. that would just change the general direction of the rotation axis in space. Would it equally change the direction of the rotation axis relative to the satellite as LordQ pointed out ? If we can reorient the craft quickly enough, we can transmit directionally at any given point of the orbit. We will just to have to know in advance when. If we could send just twice per orbit, we would have to chase after the points around the world. If it can turn and communicate any time, each of us can connect with it twice per day without traveling too much ( some travel will still be involved to get the longest communication window ) EDIT : now I myself pushed my previous long and far more important post to previous page. @ LordQ, K^2 and anyone interested in the gory details of magnetotorquer theory, previous page, 174, the most bottom long post.

Seem I screwed up the constants last time, and LordQ got a completely different number. So I will have to re-derive it from scratch, and LordQ will check. let's define first a few parameters a maximum angular acceleration Lc diameter of the circular coil, side of the rectangular coil r radius of the coil defined simply as Lc / 2 for both shapes Sc = coil enclosing area Cc = coil circumreference L diameter of the satellite Mt mass of the torquer Ms = Mt / 3 mass of a single coil Mc mass of the rest of the satellite M mass of the satellite Ix x component of torquers inertia momentum Iy y component of torquers inertia momentum Iz z component of torquers inertia momentum It inertia momentum of the torquer Ic = 1/6 * Mc * L^2 is inertia momentum of the rest of the satellite f = Mt / Mc mass fraction of the torquer N number of turns A wire cross sectional area Sw = N * A total coil cross sectional area P max coil power I max coil current R coil resistance B magnetic field t torque ÃÆ’ resistivity of the coil àdensity of the coil q = It / Ic ratio Q torquer to sat match value ---- now, let's define a few custom fancy constants S1 = ratio of enclosing area of the basic shape to enclosing area of circular coil of the same dimensions S2 = ratio of circumreference of the basic shape to circumreference of circular coil of the same dimensions of course circular coil will have S1 = S2 = 1 but rectangle got area Sc=Lc^2 where Lc = 2*r, thus Sc=4*r^2 and relative to circle's À*r^2 we get S1 = 4/À similarly rectangle got circumreference Cc=4 Lc where Lc = 2*r, thus Cc=8*r and relative to circle's 2*À*r we get S2 = 4/À S3 = constant from the inertia momentum formula for circle we google standard values Ix = Iy = 1/2 Ms*r^2, Iz = Ms*r^2, now we consider 3 coils 1/3 of total mass each, oriented towards all 3 axes, and we thus get It = Ix + Iy + Iz = 1/2 * Mt/3*r^2 + 1/2 * Mt/3*r^2 + Mt/3*r^2 = 2/3 * Mt*r^2 and thus S3 = 2/3 for rectangle we google standard values Ix = Iy = Iz = Ms*r^2, now we consider 3 coils 1/3 of total mass each, oriented towards all 3 axes, and we thus get It = Ix + Iy + Iz = Mt*r^2 and thus S3 = 1 -------- Now, t = B * I * N * Sc P = I^2 * R Sc = S1 * À * r^2 because of how we defined S1 t = B * I * N * S1 * À * r^2 R = ÃÆ’ * N * Cc / A Cc = S2 * 2 * À * r because of how we defined S2 R = ÃÆ’ * N * S2 * 2 * À * r / A P = I^2 * R P = I^2 * ÃÆ’ * N * S2 * 2 * À * r / A I = √( P * A / (ÃÆ’ * N * S2 * 2 * À * r) ) we plug that into t formula t = B * √( P * A / (ÃÆ’ * N * S2 * 2 * À * r) ) * N * S1 * À * r^2 = B * S1 * √( P * A * N * À * r^3 / (ÃÆ’ * S2 * 2) ) = B * S1 * √( P * Sw * À * r^3 / (ÃÆ’ * S2 * 2) ) = B * √( P ) / √( ÃÆ’ )* S1 / √( 2 * S2 ) * √( Sw * À * r^3 ) It = S3 * Mt * r^2 because of how we defined S3 Mt = à* Sw * Cc * 3 because we have 3 coils = à* S2 * 6 * Sw * À * r Mt = It / ( S3 * r^2 ) It = q * Ic Mt = q * Ic / ( S3 * r^2 ) q * Ic / ( S3 * r^2 ) = à* S2 * 6 * Sw * À * r q * Ic / ( S3 * à* S2 * 6 ) = Sw * À * r^3 t = B * √( P ) / √( ÃÆ’ )* S1 / √( 2 * S2 ) * √( q * Ic / ( S3 * à* S2 * 6 ) ) = B * √( P ) / √( ÃÆ’ * à)* S1 / √( 12 * S2^2 * S3 ) * √( Ic ) * √( q ) = B * √( P ) / √( ÃÆ’ * à)* S1/S2 / √( 12 * S3 ) * √( Ic ) * √( q ) a = t / ( Ic+ It) = t / ( 1 + q ) / Ic a = B * √( P ) / √( ÃÆ’ * à)* S1/S2 / √( 12 * S3 ) * √( Ic ) * √( q )] / ( 1 + q ) / Ic = B * √( P ) / √( ÃÆ’ * à)* S1/S2 / √( 12 * S3 ) / √( Ic ) * √( q )] / ( 1 + q ) = B * √( P ) / √( ÃÆ’ * à)* S1/S2 / √( 48 * S3 ) / √( Ic ) * 2 * √( q )] / ( 1 + q ) we set Q = 2 * √( q )] / ( 1 + q ) so we get a = √( P ) * B / √( Ic ) / √( ÃÆ’ * à)* S1/S2 / √( 48 * S3 ) * Q which we can split into input parameters part √( P ) * B / √( Ic ) coil material part 1 / √( ÃÆ’ * à) and geometry part S1/S2 / √( 48 * S3 ) * Q which can be further split into shape part S1/S2 / √( 48 * S3 ) match part = Q -------- q = It / Ic = ( S3 Mt * r^2 ) / ( 1/6 * Mc * L^2 ) = 6 / 4 *S3 * Mt /Mc * Lc^2 / L^2 q = 2/3 *S3 * f / (1-f) * (Lc/L)^2 and optimal Lc/L where q = 1 will thus be Lc/L = √( 2/3 * ( 1 / f - 1 ) / S3 ) which is 3.559 even for S3 = 1 and f = 0.05 so we won't get Q = 1 under any reasonable circumstances now, let's take the opposite approach - fix Lc/L = 1, coil fits into the sat exactly and let's see what performance we can get out of this geometry and compute it for f = 1 %, beryllium coil and two coil shapes Now, let's put all the numbers together B = 4e-5 T Mc = 1.3 Kg (total mass will be thus 1.313 Kg ) L = 0.1 m P = 10 W ÃÆ’ = 3.6e-8 Ωm à= 1690 kg/m^3 Computing the geometry independent parts we thus get a = 0.348393 * S1/S2 / √( 48 * S3 ) * Q and computing the geometry factor we get for the circle 0.035178 and 0.035003 for the rectangle. So the circle is marginally better, but would take some place inside, so we take the rectangle. a = 0.0122558 rad/s^-2 = 44 RPM/min which is unexpectedly high, and thus either I made some mistake, or just fiddling with the parameters, we can get an order of magnitude more from the torquer. I spent three hours writing this together, my fingers hurt. It turns out, rocket science is hard, even when no rockets are directly involved.

Actually, it's just vector addition. changing spin axis is no different from spinning up and down.

This would require too much continuous free space for the camera to move around. I would suggest, we stick the sample capsules together like beads on a string, and then we would just move them around to the camera back via pulleys. Requires much less continuous free space.

unlocked GPS unit, magnetic field sensor, gyro, real time clock [if it counts as a sensor], thermometers, solar panels themselves can double as photodectors, and so can the media CPU's external camera function as a star (more like sun, earth and moon) tracker. I would guess, that cubesats are intentionally released always on the day side, but even if it were launched just after entering shadow, it would just run the 45 minutes on batteries like it will be doing on all next orbits. How it will be released ? they will be just pushed out at ~1m/s relative to the container on ISS.

Well, I did a few calculations myself. but not about power usage. I tried to compute the maximum angular acceleration we can squeeze out of the magnetotorquer. And came to some very interesting numbers and very interesting ideas. The BIG number first. If we use some very unorthodox solutions, we may get angular acceleration up to 50 RPM/min at 10 W power and the magnetotorquer coils weighting 5 % of the total mass of the spacecraft ( assuming that the sat got relative permeability ~1 ). Which means we would be able to change the spacecraft's axis of rotation within a few minutes, and thus be able to reorient the sat from its usual sun basking orientation, do high speed transmission via a a directional high gain antenna, and then turn back. Now comes unorthodox solutions part. It would require beryllium wire, and the magnetotorquer would have to have a very unconventional shape - three orthogonal circular coils with diameter 2.25 times larger than the cube's side. Which means it would have to be folded, and expand to its full shape after satellite release. Either by spring action, or something more exotic like shape memory polymers. Now the maths. I won't write the long tedious derivation here, just the resulting formulae. AngularAcceleration = B * √(P) /√(Ic) * 1 / √(20*ÃÆ’*ÃÂ) * Q where Ic is the inertia momentum of the rest of the satellite without the magnetotorquer Ic = 1/6 Mc * L^2, where Mc is mass of the cubesat without the torquer and L is size of the cubesat B is magnetic field P is input power ÃÆ’ is coil material electrical resistivity àcoil material density Q is a dimensionless constant that describes how well is he magnetotorquer matched with Ic of the rest of the satellite. It runs from 0 = useless to 1 maximum = perfect match. Of course we are aiming at perfect match Q = 1 Q = 2 * √(q) / (1+q) where the small q is the ratio of magnetotorquer's inertia momentum It to the inertia momentum Ic of the rest of the satellite q = It / Ic We can see that Q reaches maximum 1 when q is also 1. Of course we want not that the torquer weights half of the satellite. Therefore we will have to make up with its diameter. It turns out that the torquer coil diameter can be easily computed from the mass fraction of the torquer. d/L = √( 4/15 * (1/f -1) ) where d is torquer coil diameter L is cubesat's size, same as above f is the torquer mass ratio f = Mt / M we say f = 5 % so we get that d = 2.25 times cubesat's size, which means it will have to be stored folded and unfold around the sat, but it's not as dramatic as if we needed it to be say fifty times sat''s diameter. Now back to the original equation. We can see that in the 1 / √(20*ÃÆ’*ÃÂ) factor both resistivity and density of the coil play equal role, so we should not simply choose the least resistive material, but the one which has the least ÃÆ’*ÃÂ. After a short search, it turns out, that the best feasible material is beryllium, which beats copper with 157 % more acceleration, all other things being equal. Now, let's put all the numbers together B = 4e-5 T Mc = 1Kg L = 0.1 m P = 10 W ÃÆ’ = 3.6e-8 Ωm à= 1690 kg/m^3 Q = 1 we get AngularAcceleration = 0.014 rad/s^2 = 51 RPM/min . Now back to waiting for K^2 and his numbers. If someone is interested in verifying my numbers, I would appreciate that very much, because, well, it look too optimistic to me.

Not to be negativist myself, but what percentage of ESA's big projects went significantly over the budget ?

True. I myself did indeed declare someone's idea impractical, more than a few times. Maybe Skylon is impractical too, or maybe, as mentioned above, just the company designing it is not up to the task. But I was not talking not about someone calling just skylon, or a bunch of other projects feasibility into question. I was talking about people like Nibb the negativist who do always.

I think you misunderstood my post. the 'reasons' in the proverb were not like asking for proper justification, but like finding any excuse why to dismiss the entire idea.

We have a proverb here. Not sure about the exact equivalent in English, but it goes like "those who want (to do something) seek ways, those who don't, seek reasons." And people like this are all about finding reasons why that particular space travel technology will fail.

Yeah I see. the axis would just reset after the weight balance is restored. Conservation of angular momentum. Switching between 4 dipoles would give us ccca 1.46 db over omnidirectional. The best solution would be, if we could get a directional antena, and rotate it to keep it aimed. .

I've found some hardened MRAMs, the smallest around ~ 1 MBit ( 16 bit x 64k ), which would be ideal for our purposes, but found no price. I guess, it can't be that high because they are already selling 16 MBit and 64 MBit chips. Can gross rotation changes be sped up by deliberately shifting weight to cause precession ? Something like small rods extend diagonally, the craft start precessing because of uneven mass distribution, when it hits the desired axis of rotation, thre rods retract and thus the precession stops.

Basically, every ordinary star is a electron/proton/alpha particle mixture star. Separate proton star ? even if it somehow held together, instead of falling apart immediately, its electrical field intensity would be strong enough to cause dielectric breakdown of the vacuum itself, and it would discharge back to ordinary hydrogen and a spray of positrons and gamma rays.

Negativist Nibb building something ? Nah, he figured already out that everything is impossible and/or futile... On the other hand, this is just argument from authority. And not exactly a trustworthy one. Nothing personally against Alan Bond, it's just he is looking for investors. And in that situation, it is tempting to underestimate costs and exaggerate returns.

While we are writing our armchair guesses, other are busy actually computing the properties of such a world. http://arxiv.org/abs/1401.2392

Sure. We have the cost of certification (and fuel efficiency, and longevity) on the other side, and the cost of developing something much more demanding on the other side. But I am not sure that the added cost of certification is low enough ( or the added cost of developing a hypersonic instead of transonic plane is high enough ) to allow us to say right off the bat that the 12 G cost is too low to be real. It might be the case, but it is not obvious that it is.

What about the xenon produced by the reactor itself and other gaseous radioactive fission products ? Are they sufficiently immobilized inside the fuel ?

Is this even a disadvantage ? How much of the development costs is just the bureaucracy around building a passenger plane with all those certificates, tests and re tests, safety measures, supervision of production and stuff ? I believe that if someone started building an A330 minus all the stuff that is there to assure that something moving 5-6 times faster than a car, and 10 km above ground is actually safer than cars, he would end up with an order of magnitude less cost.

Well, any improvement would be appreciated because my knowledge about design of antennas and RF components is essentially zero. I just googled specs of an existing transceiver and tried to increase its range with other components I've googled. Do you already know the specs of the rad hard 8051 ? Because the one I googled is pretty much weak... no inbuilt RAM nor ROM, no USB, I2C and only 12 MHz, so to the rad hard CPU we would have to buy additional say rad hard 32 kB SRAM and flash which would multiply the cost. And one more question. How agile will the sat be in changing its axis of rotation ? Will it be capable of changing from its presumed sun oriented axis of rotation to one more suitable for data transmission, and back reasonably fast ?

Optimal planet for life. My personal subjective opinion... Well, first, farther from the star. Earth is already uncomfortably near the inner edge of solar habitable zone. Wild guess, outer third of the habitable zone. To avoid extensive snowball episodes, it would have to be larger and heavier than earth, so it can retain more atmosphere, and have stronger volcanism and plate tectonics, nice effect would be, that such a planet will retain its internal heat for much longer. And the star should be smaller, to add planet lifetime. Of course there is an upper limit on mass because too high gravity might place too strict limits on the evolution of complex lifeforms, and too thick atmosphere would prevent photosynthesis. land/ocean ratio should no be much different from earth. Similarly, there is a lower limit on the star mass to avoid tidal lock or stripping the planet from its moon by tidal forces. (and of course, red dwarf flares, and the issue with too little high energy light to drive photosynthesis ) The planet should have a large moon, preferably mars sized (the planet itself is bigger), to stabilize its axis, and, if a civilization is to develop there on its own, to tempt it to come over and terraform it.

So only the biggest, by orders of magnitude, showstopper remains ... encouraging...