[AR] Re: What makes a useful launch vehicle?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: arocket <arocket@xxxxxxxxxxxxx>
- Date: Fri, 8 Mar 2019 18:41:15 -0500 (EST)
On Fri, 8 Mar 2019, David Summers wrote:
Would a launch of a 4 kg, 3U satellite to 600 km between 20 degrees and sun
synchronous be a useful capability?
*Somewhat* useful. Fitting real payloads into 3U is difficult, although
some groups have done it, and the elongated shape also makes life tough
for power and thermal control. 12U is a much more useful size.
(Note that although there's no written standard for 12U yet, it isn't
going to say 20x20x30cm and 12kg, in the same way that 3U is now usually
longer than 30cm and heavier than 3kg.)
It's also not uncommon for cubesats of all sizes to rely on structural
support from the P-POD dispensers they usually fly in; they aren't
necessarily able to take launch loads bigsat-style, cantilevered off a
lightweight mounting ring. They could be built to, but nobody does it
right now, and you'll be more likely to attract customers if you don't ask
them to custom-build for your launcher. The dispensers unfortunately are
heavy, often not a lot lighter than their contents; a 3U in a dispenser is
probably going to be 7-8kg. (The traditional P-POD carries a pair of 3Us,
but you can find people who will sell you a single-3U version.)
Fun facts from my market research: There were 1058 3U micro-satellites
launched as of the beginning of the year - almost half of all satellites!
However, before getting too intoxicated with that number :-), you should
subtract all the student/university/volunteer projects that don't have any
money, don't greatly care what orbit they end up in, have no particular
objection to flying as secondary payloads, and (in the US) often get
launched free courtesy of NASA.
There are two separate markets here, not one: the student (etc.)
projects, and the commercial satellites built for paying customers (by the
customers themselves, or by third-party satellite builders). Shouldn't be
hard to guess who's mostly going to be interested in your services. :-)
The commercial satellites see very strong practical pressure to move up to
6U or 12U or beyond.
What would make or break this vehicle? (Obviously, until we fly we have
nothing, etc.)
Even the commercial satellites often are able to fly as secondaries, given
the lack of affordable dedicated launches right now -- it's rather easier
to make the business case close if it's not relying on untried new launch
providers! To compete with that, you need to offer something that they
can't get that way.
+ Control of schedule is big, but requires dispatch reliability -- being
able to almost always meet a promised schedule. Establishing this may be
difficult until you've racked up a good number of launches.
+ Precise choice of orbit matters to some people. 600km is probably high
enough for a lot of people, although being able to go a bit higher (say
800km) might be helpful. High inclinations will be a very frequent
requirement, partly for Earth imaging (etc.) coverage, and partly because
low-cost satellite projects don't have worldwide ground-station networks
and hence the birds have to pass over the project HQ where the ground
station is!
+ Note, however, a subtlety: the word "precise". You need to be able to
call your shots, to go exactly where you've promised. (Can be a problem
if you try to economize by leaving the guidance system behind on an
earlier stage, e.g. by making the final stage a spin-stabilized solid.)
People who want sun-synch orbits can be particularly fussy about this --
cramming a real payload into a 3U probably doesn't leave room for any
propulsion, so they don't have any real ability to trim their orbit after
separation, and you have to hit the target pretty precisely for the bird
to stay sun-synch for its nominal operating life (even if that's only a
year or two).
+ Some payloads don't meet secondary-payload design constraints, e.g. if
they've got chemical propulsion on board they may have a hard time finding
a piggyback ride. If you're willing to fly them, they don't have many
alternatives. However, there is a potential problem if something bad
happens on the way up -- whose fault was it?
+ Speaking of which, the big reason for the heavy cubesat dispensers is to
absolutely guarantee that things like satellite structural failure cannot
endanger the rocket. Even without a primary payload on board, you need to
worry about the impact of an unexplained failure on your reputation. If
launching the payload "bare" means you're going to demand extra analysis
etc. to be sure of structural integrity, that will turn customers off
quickly -- the lavish application of engineering man-hours on such things
is one reason why traditional satellites are so expensive.
Gotta go; possibly more later.
Henry
Other related posts: