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Comment by _0w8t | Hacker News Reader

SpaceX had to use a fully expendable Falcon for this launch. What makes a GPS satellite so heavy?

Two things:

1. A GPS satellite is in a very high orbit compared to LEO, and so all stages were requested to burn to completion, to ensure the satellite has enough margin to reach that orbit.

2. The second stage ends at a ~800 km orbit, while the satellite itself needs to increase this to ~12,000 km (I'm rounding these numbers). The sat has to carry enough on-board propellant to raise it's orbit that much which necessarily increases it's wet-weight due to the rocket equation.

> while the satellite itself needs to increase this to ~12,000 km (I'm rounding these numbers). The sat has to carry enough on-board propellant to raise it's orbit that much which necessarily increases it's wet-weight due to the rocket equation.

Looks like they use a liquid fuel thruster lift it to its final orbit[0]. Some other missions have used ion engines for orbit raising[1]. I wonder why that wasn't incorporated into the GPS design.

[0] https://en.wikipedia.org/wiki/Lockheed_Martin_A2100 [1] https://space.skyrocket.de/doc_sdat/ses-12.htm

Ion engines still need propellant, you might be thinking of the so far unproven EM drive.

Ion engines need propellant, but they need a lot less of it to achieve a similar result as a traditional rocket engine.

Still SpaceX routinely recovers the first stage after launching geostationary communication satellites. I thought GPS should be lighter as they are closer to Earth and needs to send much less information in their signal, so the power requirements and size of Solar panels should be lower.

Looks like GPS-III 3,880 kg at launch. That's fairly hefty, but Falcon 9 has launched other, more massive satellites all the way to GTO (Geostationary Transfer Orbit).

The only information I can find on why this one was so energy intensive has to do with the high inclination the GPS satellite has to be in (55 degrees), which is pretty far off from inclination of the launch pad in Florida.

Wikipedia says this launch was originally planned on a Delta IV. That would make sense because it has higher specific impulse hydrogen engines. The Falcon does well in LEO, but improved Isp really helps as you go farther out.

Here's a video comparing ULA's rockets to SpaceX: https://www.youtube.com/watch?v=QoUtgWQk-Y0

This sattelite was some 8000kgs, vs. 4000-ish kgs for the previous-generation GPS satellites, if I remember correctly.

Any idea why their weight increased so much?

The latest generation of GPS satellites has more antennas and more powerful transmitters. This improves signal reception and reduces the impact of jamming.

GPS as received by a small hockey-puck sized antenna is a VERY weak signal, like -140 to -150dB. It only works because the channel size is so narrow and the modulation is very rudimentary. Having a satellite with much larger photovoltaic arrays and really powerful transmitters is one method to overcome regional jamming.

I wouldn't be surprised if the satellites had additional, classified capabilities.

3. USAF requires the upper stage do a deorbit burn to avoid long lived high altitude space debris.

According to the SpaceX stream, it was a combination of satellite weight, high orbit, and inclination change (tilting the orbit)

Inclination changes are very "expensive" in terms of delta-V:

https://www.google.com/search?q=plane+change+delta+v&ie=utf-...

Interesting. It makes sense then to launch such satellites closer to poles. That explains why there are plans in Norway to build a space launching site at around 70 latitude near Tromsø.

I think it is mostly about the path the rocket is allowed to travel during launch. In the US, for equatorial orbits, one launches from Cape Kennedy to the east, and for polar from Vandenburg to the south. Maybe off angle launches aren't allowed so a lot of correction is needed after the satellite reaches orbit to get to an inclined orbit? I am not an orbital mechanics specialist or space launch planner. Corrective comments are encouraged.

Vandenberg is mostly used for polar orbit launches because it's a path over the ocean. Not possible to launch eastbound from there because first stages would fall on populated areas. This is also the reason why the Israeli space program launches into rare retrograde orbits (opposite direction of earth's rotation), westbound over the ocean, because they don't want to drop 1st stages on their eastern neighbours.

Correct. But the question is, if one wants an inclined orbit, why can't you launch out of Kennedy to the northeast or out of Vandenberg to the southwest. Wouldn't this save all the delta v correction you need to do after getting into an equatorial or polar orbit and then transitioning to an inclined orbit?

You can do both of those. NRO missions have launched out of Kennedy to the northeast (e.g. NROL-76) and Vandenberg to the southwest (e.g. NROL-25).

According to bisby at the head of this thread, one of the reasons that SpaceX did not recover the booster is that the rocket had to enter an inclined orbit. Maybe it did launch in the direction for an inclined orbit and used a little extra fuel, like a polar orbit does, due to the fact that you don't have all the speed of the spinning earth adding to your orbital velocity. I didn't know that the spinning Earth speed was a significant factor on payload to orbit of rockets.

Tromso is also a hub for satellite communications and comms companies, which helps.

KSAT/Kongsberg satellite services punches well above its weight in terms of how important it is to global LEO and polar/inclined orbit data relay services.

Did they originally intend to use a bigger rocket? It seems unlike them to throw one away at any price.

SpaceX will happily sell you a single-use Falcon 9 (or Falcon Heavy) if you need the mass for your launch. Of course, one should expect to pay for the privilege.

Yeah I'm curious at what price. My guess is it massively contradicts their own PR on expected number of runs and thus their opportunity cost.

I expect price-wise they're much closer to the competition there, but given they're still getting launches they can't be completely uncompetitive.

That said, remember they could potentially be using a rocket that's flown twice before (and AFAIK three launches is the most they've done so far), and as such may view the rocket as largely expendable anyway (they lose the ability to use it for re-entry experiments and to pull it down after the launch, but nothing else).

This rocket was worth billions in opportunity dollars. I’m sure not a single tear was shed.

Hmm? Is the government paying billions for the launch? I doubt it.

Perhaps they used one of their earlier rockets that are only designed to be reused a couple of times.

They’re in geostationary orbit, which is much higher.

No GPS sat's are in GEO - they're all in MEO, which is about half as high, though comparative in terms of dV requirement.

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