Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

SpaceX launches first U.S. national security space mission

reuters.com

Post reply on HN
Print view View original

11–20 of 69 posts

Re: SpaceX launches first U.S. national security space mission

#11

Post by 5436436347 »

_0w8t wrote: post #9

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.

Re: SpaceX launches first U.S. national security space mission

#12

Post by 5436436347 »

wefarrell wrote: post #10
_0w8t wrote: post #9

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

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.

Re: SpaceX launches first U.S. national security space mission

#16

Post by _0w8t »

5436436347 wrote: post #11
_0w8t wrote: post #9

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 necess…

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.

Re: SpaceX launches first U.S. national security space mission

#17

Post by walrus01 »

bisby wrote: post #14
_0w8t wrote: post #9

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

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-...

Re: SpaceX launches first U.S. national security space mission

#18

Post by _0w8t »

walrus01 wrote: post #17
bisby wrote: post #14

Earlier quoted context omitted.

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.

Re: SpaceX launches first U.S. national security space mission

#19

Post by marvin »

_0w8t wrote: post #16
5436436347 wrote: post #11

Earlier quoted context omitted.

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 necess…

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.

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

Re: SpaceX launches first U.S. national security space mission

#20

Post by baq »

Ericson2314 wrote: post #15
_0w8t wrote: post #9

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

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

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