Tom Spilker

@jpa It is indeed possible to leave Earth in a direction that takes you well out of the ecliptic plane, yet encounters Jupiter. However, 1) this requires a huge ∆V (change in velocity, both direction and speed) upon leaving Earth, and thus a huge quantity of propellant or multiple inner-solar-system gravity assists, and 2) the Jupiter approach V∞ (the direction and speed of approach to Jupiter, before Jupiter's gravity significantly affects the spacecraft's trajectory) is very different, possibly making the desired gravity assist at Jupiter infeasible.

I need to get some dinner, but this has been fun!

Unfortunately I arrived after the primary investigations had already been done. They'd quit using the 300 kW transmitter. Reduced staff made the room for my lab in the transmitter building. Fortunately, with access to a Fully Equipped machine shop!

Yes indeed! It was really great that the whole team, both engineers and scientists, were part of the Electrical Engineering department at Stanford. They pulled off some very difficult implementations. Not all of them worked quite as anticipated, such as the balanced line.

Yep, I was too. Although both of my parents grew up on farms, I was always a city kid.

Yes, these are radar observations, not the Mariner 5 radio occultation.

Oops, the should be "sturdy metal pipes".

The cattle guard was a flat array of study metal pipes, perpendicular to the road's axis, spaced maybe 9-10" apart and as wide as the road. The spur went uphill to the gate, so I wasn't about to try to jump the guard going uphill!

Signal received at the same dish. Originally they would transmit until the reflected signal was about to arrive, and they would shut down the transmitter to receive. But they saw that the Doppler shifts were large enough that they could simultaneously transmit and receive, and that allowed longer integrations.

The main road up there went about 100 m to the SE of the antenna. There was a short spur that went from the main road up to the gate to the antenna, with a cattle guard about 10-15 m from the main road, maybe 4 m wide (distance along the road). When I would run the dish I would run up to that gate, then on the way down jump over the cattle guard!

Yep! Hence the need for extreme purity. And yep! Hundreds of MHz. If my memory serves me well, ~300 MHz was their upper limit.

The water served as the dielectric.

You can imagine the electric power that went into this device. A hundreds or so amps on tens of thousands of volts! Every now and then they popped the circuit breakers, which at those power levels sounds like a cannon. The engineering manager in the early days was Lon Raley, a tall and big man. He was once right in front of the breaker cage when they fired, and he spilled hot coffee all down the front of his nice white shirt.

Yep, they had to apply for a license from the FCC, and get some waivers.

The coaxial system had an outer conductor diameter of about 30-40 cm, so the internal fields were pretty strong. Because it was enclosed, they needed to circulate water between the inner and outer conductors as a coolant. Because the E fields were so strong, they had to super-distill all the water they used.

Yep, just like the behavior of a Jacob's Ladder.

Back then there were fewer regulations. Since the facility was NASA- and NSF-funded they probably had whatever layers there were at the time. I'm sure that now the balanced-line approach would be quickly nixed. After a couple of months of such mess they redesigned the feed, using a coaxial system, all E field contained internally.

When that happened, the arcing made rough spots on the conductors, often with sharp points of melted conductor material (copper). If you tried to start up again without sanding down the lines, the E field concentration around those sharp points made it start arcing again, immediately.

Somewhere I have a photo Len gave me, showing such an arc. The lines were about 3' apart, and the arc must have extended 6' above them, a big horseshoe.

When first built, the line from the transmitter building to the antenna was an above-ground "balanced line", so two conductors (hefty ones) held a fixed distance apart. But when they started using it at full power, when anything like a bird or a really big bug would fly between the conductors it would trigger arcing between the conductors.

Early 1970's, I think. Von Eshleman and Len Tyler told me about it. Both were private pilots.

That happened every morning for a week, before someone figured out that Hayward was in one of the dish's side lobes! Once they (Stanford) stopped transmitting so close to the horizon, Hayward's problem disappeard.

They decided to start earlier in the morning, with Venus lower on the horizon, to give a longer integration. The morning of the first run, the receivers at the control tower at Hayward airport blew out!

Great! OK, stories about the Stanford Radar Telescope. The transmitter on that telescope couple put out 300 kW as designed, and with some modification could put out 450 kW for short periods. For one experiment, after all the data was recorded (after tracking Venus for nearly two hours, the data reduction revealed the SNR was still too low.

@uhoh I'm on, let me know when you are there.

Buona notte!

OK, I'll take a look. I should exit now, before I do tackle that question. It's well after my usual dinner time here. My stomach is going after my liver, and I don't even like liver! But it's been a pleasure, uhoh, thanks for setting up the chat!

Eventually I told JPL management that "I never want to earn my living in a job with the word 'manager' in its title."

Solid iron: it depends on the temperature profile. For any profile halfway physically reasonable, there'll probably be a (possibly very thin!) layer of liquid iron. For odd profiles, you might get multiple transitions from solid to liquid and back.

I know people who get paid to solve these kinds of modeling problems!

Oh yes, phase transitions would be important. Fortunately at terrestrial planets the pressures don't get high enough that quantum-mechanical transitions, like molecular hydrogen to metallic hydrogen, don't occur.

Yeah, I don't do it nearly as much as I used to. But they're still more fun than management tasks, which JPL kept trying to get me to do. Eventually I put a sign on my office door: "I solve problems with units like km/s or W/(cm^2 - sr); if your problem involves units like $ or full-time-equivalent-employees, I can recommend someone else to help with it."

I'll have to look up the equations of state for things like iron/nickel mixtures, and silicate minerals, and see how closely rho(P) = rho_0 + CpP models that.

I think that rho(P) = rho_0 + CpP makes the integral fairly easy. That'll be fun!

Not actually bored; I'm in the middle of writing up my sections of reports for a couple of studies I've been involved in, one about aerial platforms at Venus, one about small (~30 kg) atmospheric entry probes at Uranus or Neptune. I love doing calculations! I loathe writing reports!

Yes, the rho(P) = rho_0 + CpP might work, good idea.

Hmm. Tonight this'll probably displace some work I'm supposed to do! But my wife (Project Scientist of the Cassini mission) is in Paris, so...freedom!

I'm thinking about the analytical solution problem and how to set it up. Specifically, how to model the density as a function of pressure. You might have to model that via interatomic spacing, assuming that spacing goes as something like 1/p^n : the higher the pressure, the smaller the spacing, with zero spacing being forbidden.

I wonder how much stellar physicists have had to modify their models on the basis of the data we've gotten from helioseismology. That has identified stratification in the sun's interior, notably levels at which the primary heat transfer mechanism (apparently) goes from convective to radiative and then back again.

Hmm, there probably is an analytical solution, I'm not sure how simple it would be. I'm pretty sure there'll be an exponential in there somewhere.

The net result is that, with a density profile that is everywhere higher than in the original case, the average density increases. To keep average density constant as size increases, you have to have a higher ratio of lighter constituents.

Hmm, good question. I haven't worked that out, mathematically. It would probably be an interesting exercise!

If you increase the radius of the planet with the same materials and the same relative profile (core radius, mantle thickness, etc. are all at the same ration as surface radius) the surface acceleration increases, so the pressure profile steepens. So at the same depth, the density is higher than in the original case. The de4nsity at the center is higher also.

Yep! You see it flattens out as you get close to the center of Earth. That's because the gravitational acceleration is going to zero as you approach the center, so the pressure isn't increasing much any more.

Oh, OK. Can you pull up things from the web while we chat? If so, take a look at en.wikipedia.org/wiki/Structure_of_the_Earth#/media/…

How do I activate the audio comm channel on my end?

OK, great! I've never done anything like this before so there might be a learning curve!

Do I just type something into the box here?

@uhoh , "the same density profile (say 1.5 at the surface to 15 in the center)," Yes, that's what I assumed you meant, and it's a perfectly good postulate. It's just not what actually happens. If we want to continue this we should probably move to chat. I've never done that before, have you?

@uhoh , yep, it's beyond what a lot of people would be willing to tackle. That's why I say it's a "nit". But I find that phenomenon very interesting in the way it must be included when doing interior models.