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NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

theregister.co.uk

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Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#61

Post by jenamety »

wrigby wrote: post #30
comboy wrote: post #24

Earlier quoted context omitted.

I can't get my head around how this thing works. Apart from the direct GPS signal it's able to also receive the part of it that was reflected by the ocean? How does the reflected signal tell anything about the wind speed? And btw what are star trackers used for? As a fallback for GPS?

IIRC, star trackers are used as both a position and an attitude reference. GPS gives you only position (though more sophisticated systems can be used to determine attitude). While a GPS outage is a minor inconvenience for most of us, it could be pretty catastrophic for spacecraft if they didn't have another position reference. EDIT: See below - I was off; they're just attitude, not position

Interesting, thanks. I wonder at what altitude GPS becomes ineffective? What then? We'll likely need some positioning satellites further out at some point? maybe at L4, L5 Lagrangian points?

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#62

Post by msandford »

Scramblejams wrote: post #33
msandford wrote: post #11

Earlier quoted context omitted.

I have this dream that someday we'll build a SR-710 that's 10x the size and carry a BIG payload. And then putting stuff into orbit will basically be strapping your thing to a "big" upper stage (but small relative to a whole rocket stack), loading it into the cargo bay and then some fancy flying. I know it'll never happen for a variety of reasons. But it's such a cool plane and when you think about cruising at mach 3…

One big reason it'll never happen: The square-cube law. https://en.wikipedia.org/wiki/Square-cube_law

Well 10x doesn't necessarily mean 10x longer, it might mean something with 10x the total wing area and thus 10x the lifting capacity. Which might only be say 3x the length and 3x the width.

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#63

Post by badminton1 »

greglindahl wrote: post #31
badminton1 wrote: post #29

Well, kudos to NASA but also kudos to ISRO for taking the initiative in making satellite launches more cost effective.

Pegasus XL is a private launcher (NASA is the customer), and it's not cost-effective. It costs nearly as much as a Falcon 9 for a much smaller payload. I'm not sure where the India reference came from? The EU and Japan both operate smaller launchers. A couple of startups are getting close to their first launch, too.

I see. I made the wrong assumption then that sending 8 satellites from a rocket that gets some help from a plane (requiring less propellant/size) was more cost-effective. Thanks for clarifying.

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#64

Post by velox_io »

karkisuni wrote: post #60
velox_io wrote: post #59

A little off topic.. I've always wondered why NASA lunched rockets from Cape Canaveral (only 3 meters above sea level). Granted, it's a little tricky to transport rockets, but surely it would have made more sense to move the operations to a mountain near the equator? How much fuel would be saved by lunching 3000 meters above sea level instead?

Not much at all. Low earth orbit is ~200km so the mountain is only helping by 1.5% of the needed altitude. Altitude is not even the most difficult part of getting to orbit. Getting to the necessary horizontal speed to stay in space takes a hell of a lot more fuel than going straight up.

So say a 4000m launchpad is feasible, that's 2% of the needed altitude. While gravity will be roughly the same (yeah it's higher altitute, but there's the mass from the mountain). There is much less atmosphere* (~50% less pressure & 1/3 less of it to get through).

Given that payload to fuel ratio is roughly 1:10, this does seem to add up to a significant saving.

*Air pressure numbers http://www.mide.com/pages/air-pressure-at-altitude-calculato...

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#65

Post by Merad »

velox_io wrote: post #59

A little off topic.. I've always wondered why NASA lunched rockets from Cape Canaveral (only 3 meters above sea level). Granted, it's a little tricky to transport rockets, but surely it would have made more sense to move the operations to a mountain near the equator? How much fuel would be saved by lunching 3000 meters above sea level instead?

Obligatory xkcd: https://what-if.xkcd.com/58/

In short, gaining altitude, even 3000m, isn't that hard for a rocket. The problem is that they have to go fast.

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#66

Post by karkisuni »

velox_io wrote: post #64
karkisuni wrote: post #60

Earlier quoted context omitted.

Not much at all. Low earth orbit is ~200km so the mountain is only helping by 1.5% of the needed altitude. Altitude is not even the most difficult part of getting to orbit. Getting to the necessary horizontal speed to stay in space takes a hell of a lot more fuel than going straight up.

So say a 4000m launchpad is feasible, that's 2% of the needed altitude. While gravity will be roughly the same (yeah it's higher altitute, but there's the mass from the mountain). There is much less atmosphere* (~50% less pressure & 1/3 less of it to get through). Given that payload to fuel ratio is roughly 1:10, this does seem to add up to a significant saving. *Air pressure numbers http://www.mide.com/pages/air-pre…

you're missing the point. altitude and drag losses are nothing compared to horizontal velocity. you need to be going 7.8km/s. Per second! That takes a lot more energy than gaining any kind of altitude or overcoming atmospheric drag. It's not significant savings.

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#67

Post by gozur88 »

velox_io wrote: post #59

A little off topic.. I've always wondered why NASA lunched rockets from Cape Canaveral (only 3 meters above sea level). Granted, it's a little tricky to transport rockets, but surely it would have made more sense to move the operations to a mountain near the equator? How much fuel would be saved by lunching 3000 meters above sea level instead?

The short answer is being as close as possible to the equator and able to launch over an unpopulated area (like an ocean) is more important than altitude.

There might be some benefit to launching off of a really tall mountain, since it would make your first stage engine more efficient, but not enough to justify the extra logistics.

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#68

Post by toomuchtodo »

planteen wrote: post #37
wrigby wrote: post #30

Earlier quoted context omitted.

IIRC, star trackers are used as both a position and an attitude reference. GPS gives you only position (though more sophisticated systems can be used to determine attitude). While a GPS outage is a minor inconvenience for most of us, it could be pretty catastrophic for spacecraft if they didn't have another position reference. EDIT: See below - I was off; they're just attitude, not position

Star trackers are usually just used for attitude, not position. I've heard of research to use them in conjunction with a camera pointed at Earth to resolve coastlines to get position, but it's very theoretical at this point AFAIK.

What's the difficulty level of writing star tracker software for consumer grade CCD devices? I've always had writing and open sourcing a star tracker on my bucket list, similar to how this person did the same for a from-scratch GPS receiver [1]. Same for determining your position in space using pulsars as beacons [2].

[1] http://www.aholme.co.uk/GPS/Main.htm

[2] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201500...

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#69

Post by trailbits »

comboy wrote: post #24
planteen wrote: post #5

I worked on the software for the star trackers in these. Glad to hear they have already made contact. :-)

I can't get my head around how this thing works. Apart from the direct GPS signal it's able to also receive the part of it that was reflected by the ocean? How does the reflected signal tell anything about the wind speed? And btw what are star trackers used for? As a fallback for GPS?

The earth is bathed in radiation from GPS transmitters and these clever satellites are using doppler radar techniques to see how those signals are being reflected from ocean waves. From the doppler shift you can infer wind fields. Consumer GPS receivers only compute position from these signals, but there is a lot of interesting science you can do from the raw signals with a hacked GPS receiver.

Re: NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft

#70

Post by [deleted] »

jenamety wrote: post #61
wrigby wrote: post #30

Earlier quoted context omitted.

IIRC, star trackers are used as both a position and an attitude reference. GPS gives you only position (though more sophisticated systems can be used to determine attitude). While a GPS outage is a minor inconvenience for most of us, it could be pretty catastrophic for spacecraft if they didn't have another position reference. EDIT: See below - I was off; they're just attitude, not position

Interesting, thanks. I wonder at what altitude GPS becomes ineffective? What then? We'll likely need some positioning satellites further out at some point? maybe at L4, L5 Lagrangian points?

[deleted]
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