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SpaceX had to use a fully expendable Falcon for this launch. What makes a GPS satellite so heavy?
by fpoling 8y ago
SpaceX had to use a fully expendable Falcon for this launch. What makes a GPS satellite so heavy?
- wefarrell 8y agoThey’re in geostationary orbit, which is much higher.
- 5436436347 8y agoNo GPS sat's are in GEO - they're all in MEO, which is about half as high, though comparative in terms of dV requirement.
- 5436436347 8y agoTwo 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.
- fpoling 8y agoStill 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.
- marvin 8y agoThis sattelite was some 8000kgs, vs. 4000-ish kgs for the previous-generation GPS satellites, if I remember correctly.
- Roritharr 8y agoAny idea why their weight increased so much?
- nradov 8y agoThe latest generation of GPS satellites has more antennas and more powerful transmitters. This improves signal reception and reduces the impact of jamming.
- walrus01 8y agoGPS 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.
- tgsovlerkhgsel 8y agoI wouldn't be surprised if the satellites had additional, classified capabilities.
- inamberclad 8y agoLooks 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 https://www.youtube.com/watch?v=QoUtgWQk-Y0
- the8472 8y ago> 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 https://en.wikipedia.org/wiki/Lockheed_Martin_A2100 [1] https://space.skyrocket.de/doc_sdat/ses-12.htm https://space.skyrocket.de/doc_sdat/ses-12.htm
- pilsetnieks 8y agoIon engines still need propellant, you might be thinking of the so far unproven EM drive.
- elihu 8y agoIon engines need propellant, but they need a lot less of it to achieve a similar result as a traditional rocket engine.
- InclinedPlane 8y ago3. USAF requires the upper stage do a deorbit burn to avoid long lived high altitude space debris.
- deleted 8y ago[deleted]
- bisby 8y agoAccording to the SpaceX stream, it was a combination of satellite weight, high orbit, and inclination change (tilting the orbit)
- walrus01 8y agoInclination changes are very "expensive" in terms of delta-V: https://www.google.com/search?q=plane+change+delta+v&ie=utf-8&oe=utf-8 https://www.google.com/search?q=plane+change+delta+v&ie=utf-...
- fpoling 8y agoInteresting. 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.
- njarboe 8y agoI 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.
- walrus01 8y agoVandenberg 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.
- njarboe 8y ago
- Ericson2314 8y agoDid they originally intend to use a bigger rocket? It seems unlike them to throw one away at any price.
- baq 8y agoThis rocket was worth billions in opportunity dollars. I’m sure not a single tear was shed.
- Ericson2314 8y agoHmm? Is the government paying billions for the launch? I doubt it.
- gsnedders 8y agoSpaceX 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.
- Ericson2314 8y agoYeah 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.
- gsnedders 8y agoI 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).
- elihu 8y agoPerhaps they used one of their earlier rockets that are only designed to be reused a couple of times.