
NASA just launched 8 satellites from a rocket dropped from a plane at 40,000ft - andrewflnr
http://www.theregister.co.uk/2016/12/16/nasa_orbital_atk_cygnss_launch/
======
ucaetano
Lifting a rocket that high before firing it carries another benefit besides
the (small) altitude gain and lower fuel requirements due to drag: Max Q, the
maximum dynamic pressure that has to be sustained by the rocket usually
happens at a lower altitude than that (depending on the mission).

This means your rocket has to sustain a far lower structural load, making it
safer and lighter.

~~~
valarauca1
Max Q for the Peagus and Saturn V are actually at the exact same altitude.
~14-16km. And nearly identical pressures.

The thing is because all rockets follow the same trajectory to orbit, The
gravity curve. So you hit similar speeds at similar altitudes. If you don't,
you either aren't making it to orbit, or you're wasting fuel.

This is because the gravity curve is a straight line if you ask Einstein, just
earth's gravity makes space-time look wibbly-wobbly.

Newton calls this the derivative of the balastic arc.

There isn't a magical trajectory to orbit that is cheaper.

~~~
PhasmaFelis
I was disappointed to realize that "balastic arc" was just a typo. I thought
I'd learned a new word. :)

------
sargun
Orbital ATK has been building and launching the Pegasus XL
([https://www.orbitalatk.com/flight-systems/space-launch-
vehic...](https://www.orbitalatk.com/flight-systems/space-launch-
vehicles/pegasus/)) for a bit now. It is an extremely cheap platform that is
launched from an airplane.

What's neat is the User Manual is available online:
[https://www.orbitalatk.com/flight-systems/space-launch-
vehic...](https://www.orbitalatk.com/flight-systems/space-launch-
vehicles/pegasus/docs/Pegasus_UsersGuide.pdf) [pdf]

~~~
greglindahl
Extremely cheap? Initially it was $6mm, now it costs almost as much as a
Falcon 9 launch.

Part of that is due to its extremely low launch rate -- this was the first
launch since 2013 -- but still.

~~~
sargun
if Orbital launched as much as SpaceX, the Pegasus cost before margin could be
20% of what it is today.

~~~
greglindahl
With Stratolaunch buying a bunch of Pegasus XL rockets, we'll know in a couple
of years.

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

~~~
comboy
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?

~~~
wrigby
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

~~~
planteen
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.

~~~
phamilton
For those of us just watching, what is attitude? At first I though spellcheck
might have corrected it from altitude, but 3 times among 2 differently people
I see attitude.

~~~
andrewflnr
It's orientation, AFAIK, what direction it's facing.

~~~
planteen
Yep. Think of it like right ascension, declination, and roll from astronomy.
Given attitude, your position, and the time, you can know where you are
pointing on the Earth (lat/lon).

------
jbuild
Just an FYI, NASA live streams mosts of these launches here:
[https://www.nasa.gov/multimedia/nasatv/#public](https://www.nasa.gov/multimedia/nasatv/#public)

------
Animats
The Pegasus launch system has been around since 1990. First private orbital
launch system that worked.

------
velox_io
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?

~~~
karkisuni
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.

~~~
velox_io
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...](http://www.mide.com/pages/air-pressure-at-altitude-calculator)

~~~
karkisuni
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.

------
astrodust
Given how SpaceX has landed things on a drone ship, presumably they're
considering launching from a rocket that's been lifted above the bulk of the
atmosphere using a balloon of some sort.

You'd probably save a ton of fuel if you could launch from 40 or even 100K
feet.

~~~
RelaxBox
Eh, not really. It's not getting to altitude that's expensive, it's speed.
Orbit is blisteringly fast.

~~~
astrodust
It's a lot easier to get up to speed if you don't have to deal with cutting
through a heavy chunk of atmosphere on the way up. Plus, rocket designs could
vary, be less concerned about aerodynamics, if they were starting from a
launch platform already in very thin air.

A wider, shorter rocket might work better in thin air, for example, but that
would be impractical launching from sea-level as is tradition.

~~~
msandford
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 (2000mph+, 15% of escape velocity
([http://www.sr-71.org/blackbird/sr-71/)](http://www.sr-71.org/blackbird/sr-71/\)))
it seems like it could make the rocket equation for a plane-launched rocket
much, much more forgiving.

~~~
Scramblejams
One big reason it'll never happen: The square-cube law.
[https://en.wikipedia.org/wiki/Square-
cube_law](https://en.wikipedia.org/wiki/Square-cube_law)

~~~
msandford
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.

------
peteretep
This is probably a tremendously dumb question, but what are the major
impediments to literally sticking a big freaking balloon to the top of a
rocket and floating it up much of the way?

~~~
blater
Why bother with the rocket? It sounds mad, but you can actually just send the
balloon. to orbit...
[http://www.jpaerospace.com/atohandout.pdf](http://www.jpaerospace.com/atohandout.pdf)

~~~
mrfusion
Why do they need the space station part?

~~~
blater
Because the orbital ship is structurally unable to cope with higher
atmospheric density so must be assembled and maintained at altitude and never
go below the height of the station. There's a few challenges there...

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

~~~
greglindahl
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.

~~~
partycoder
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.

