[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
SSRT: Space Access Update no. 69
------
Date: Sun, 4 Aug 1996 15:47:08 -0700 (MST)
From: Donald Doughty <doughtd@pr.erau.edu>
To: DC-X <delta-clipper@world.std.com>
Subject: Space Access Update #69 8/3/96 (fwd)
Reply-To: delta-clipper@europe.std.com
Subject: Space Access Update #69 8/3/96
Space Access Update #69 8/3/96
Copyright 1996 by Space Access Society
_______________________________________________________________________
The DC-XA "Clipper Graham" is, for all practical purposes, toast. Here's
a quick preliminary look at what happened on the experimental reusable
rocket's fourth current-series and twelfth overall flight, along with some
thoughts on where we go from here.
We've had a chance now to look closely at a tape from the NASA satellite
feed, two views of the Thursday July 31st flight and the post-landing
fallover/fire. We've also had time to talk to the usual suspects (aka
SAS's Advisory Board) about what all this means (this ain't a crisis, it's
an opportunity!)
The rumor mill meanwhile is pretty sparse, as it should be - there's an
accident investigation underway, to determine the causes and learn what
lessons there are to be learned, and nobody on the project has much to say
for the moment. Pretty much all that follows is from the tape and from
general knowledge of the vehicle and program.
Summary: DC-XA was apparently experiencing some sort of hydraulic/control
problem with the landing leg on the southeast corner of the vehicle
throughout the two-minute twenty-second flight. The leg then failed to
extend during final descent to the pad. Once engines cut off, the vehicle
toppled to the southeast. The liquid oxygen (LOX) tank ruptured massively
on side-impact, causing an immediate low-intensity explosion tearing apart
and setting fire to the forward section of the vehicle. On-pad fire-
suppression water sprays reduced but did not eliminate the fire; somewhat
over a minute later, the liquid hydrogen (LH2) tank ruptured, tearing
apart and burning the mid-aft section of the vehicle. Once fire-
suppression water supplies were exhausted, the fire was allowed to burn
itself out; range-safety rules precluded approaching the site for twenty-
four hours due to explosive devices associated with the vehicle's
emergency recovery parachute. As best we can tell, what's left of DC-XA is
a lot of scraps plus the rather scorched nose and tail sections.
Background: The DC-XA had last flown in June, the two-flights-in-two-days
fast turnaround demo. An attempt to fly in early July had been scrubbed
after first computer and then weather problems delayed things past the
available range time - White Sands Missile Range hosts dozens of different
test programs and time is tightly scheduled. After this, it was decided
to stand down until after a new hydrogen-burning auxiliary power unit
(APU) had been installed - this APU was to take over the job of providing
hydraulic pressure for the vehicle controls. (DC-XA engine gimballing,
body-flap actuation, and landing gear actuation are all via hydraulics.)
(A hydrogen-burning APU is a significant advance in that it can be run off
the ship's main fuel supply, reducing weight and simplifying support.)
DC-XA, it should be kept in mind, is by design a quick-and-cheap
experimental vehicle. In order to meet tight schedule and budget
constraints, many systems that in an operational vehicle would have
mechanical or procedural backups are single-string with no backups. This
is a frequent tradeoff in experimental vehicles; it is in general more
cost-effective to accept higher risk to get needed flight test data in a
fraction of the time and cost. In this case, DC-XA had neither a backup
landing-gear extension mechanism nor (apparently) a one-gear-up emergency
procedure in place.
The two available views of the flight are both from the west. One is
closeup, with the bottom of the vehicle often out-of-frame, and cuts off
~1/2 minute after landing. The other is a distant view and goes on for
nearly two minutes post landing.
In a previous flight of approximately the same duration, residual
propellants at landing were somewhere above 15% of the ~20,000 lbs full
load. This would place residual LH2 at something over 400 lbs and
residual LOX at something over 2400 lbs, assuming a 6:1 ratio and similar
propellant loading and consumption this flight. Vehicle dry-tanks weight
is ~20,000 lbs.
Sequence Of Events: Landing Gear. The closeup video shows that the landing
leg on the southeast corner of the vehicle (right-hand side away from the
camera) repeatedly partially extended then retracted again during the
flight, typically extending ~1/4 of full extension, typically extending
for ~1/10th second. This extension is clearly visible at least once
during the ascent and twice during the descent, and possibly visible other
times - uncertainty is due to changing vehicle angle and to the vehicle
base repeatedly going out of the bottom of the video frame. The extension-
retractions were quick, and were at no obvious fixed interval - at least
once during descent a very quick extend-retract-extend-retract sequence is
visible.
When time came to extend the gear during final descent, the two legs on
the west side of the vehicle came down normally - slightly out of sync,
but no more so than seen on previous flights. The leg on the northeast
corner came down ~1/2 second later. The leg on the southeast corner, the
same one that had been partially extending during flight, did not come
down at all.
We conclude from this that there was some sort of hydraulic/controls
problem with the landing gear, possibly associated with the hydraulic
system rework involved with installing the new APU, possibly associated
with the new APU itself. We note that there seemed no problem with engine
hydraulic gimballing or vehicle flight control. We don't know enough
about the vehicle hydraulic systems to reach any further conclusions.
Sequence Of Events: Post-Landing Fire: DC-XA took ~5 seconds from post-
landing engine cutoff to impact on its side. The vehicle is ~40 feet tall
and started toppling from a position with its center of gravity roughly in
between the SW and NE landing legs, pivoting to the SE on these two legs.
CG shift with residual propellant slosh likely accelerated the topple
somewhat once the ship was off-level.
Immediately on side-impact, the forward third of the vehicle aft of the
emergency parachute housing (the 'chute is in the straked nosecone
section) burst open with orange flame erupting and pieces flying short
distances. We conclude that the welded aluminum-lithium LOX tank broke
open on impact, releasing ~1 ton of residual liquid oxygen rapidly,
igniting everything ignitable in the forward part of the vehicle. We note
that this tank had serious fabrication problems involving welding the
seams (AlLi is notoriously difficult to weld); we suspect a seam may have
split open.
We suspect that if the LOX tank had not broken open, we'd still have a
slightly dented DC-XA, despite the landing gear failure.
The graphite-epoxy LH2 tank, meanwhile, apparently survived the side
impact intact, and stayed intact until the fire had been burning ~80
seconds. The LH2 tank then ruptured violently, sending pieces flying tens
of yards and emitting a large semi-transparent fireball that rose away
from the ship rapidly. The remaining hydrogen in the tank burned clear
and hot for a few seconds, then the remains of the ship went back to
smoldering.
Keeping in mind that the LH2 tank both was lower in the ship thus taking
less impact, and had several times less mass of residual propellant on
board, we still suspect we've seen a useful demonstration of the relative
damage-resistance of GrEp tanks versus AlLi.
We've also seen a demonstration that the WSMR three-mile radius safety
zone is probably considerably larger than needed - two hundred yards would
likely have been more than a safe distance from any of the events we saw
on this tape. This has implications for future operating sites, we think
- the noise radius is likely to be larger than the safety radius.
Meanwhile, we expect there's not much left of the DC-XA except rather
scorched nose and tail sections. We have hopes the engines may be
salvageable for use in a followon, but not a great deal of optimism - we
understand an initial evaluation of them was not good.
On the other hand, DC-X and then DC-XA already paid for themselves many
times over with the data and experience gained in the dozen flights they
made. We now know a lot of things are possible we only guessed at three
years ago. X-vehicles by their nature are ephemeral; many never survive to
make it into a museum. Though there is a case for transporting the
remains of DC-XA to a museum, if one exists with enough vision to display
the pieces as-is as a reminder that learning new things involves risk.
What Next?
It probably won't surprise anyone to hear that we at SAS think that this
changes nothing except the degree of availability of parts to build an
upgraded DC-X followon. We still think the case for a USAF/NASA
partnership building and flying a high-speed DC-X derivative over the next
couple of years is compelling: The technology complements X-33 in that it
explores numerous major alternate approaches - see SAU#67 for a detailed
list - and the project would also provide continued competition in the
reusable launch field at a bargain-basement price.
The immediate need, for you political activists out there, is to get
Congressional support for continued DOD funding for reusable rocket work.
The tactical situation is this: The Defense Authorization for FY'97
currently provides $25m for RLV work in DOD. The FY'97 Defense
Appropriation is on hold for now; work will likely get underway again in
mid-August during the month-long Congressional recess, and the actual
Defense Appropriations Conference will happen in early September.
What this means is, we have several weeks during the August recess to
contact members of the House and Senate Defense Appropriations
subcommittees back in their home states and districts, and persuade them
to support DOD RLV funding, preferably $50M targeted for a new DC-X
followon. Again, check SAU#67 for lists, though you might want to check
your phone book "blue pages" government section for local office numbers,
as they won't be in DC much this month.
It is also worth contacting members of the Defense Authorizing committees
(Senate Armed Services and House National Security) and selling them on
the advantages of DOD RLV work. This is not so immediately relevant, more
a matter of building for the future - but the future always arrives sooner
than we expect.
SAU#67 can be found at www.space-access.com. We'll be coming out with
more detailed info on how you can support reusable rocket development as
soon as we can, but meanwhile, if you're a self-starter, between this and
#67 you should have what you need. Ask for $50m for next year to get work
started on a DC-X followon. Go for it!
__________________________________________________________________________
Space Access Society "Reach low orbit and you're halfway to anywhere
4855 E Warner Rd #24-150 in the Solar System."
Phoenix AZ 85044 - Robert A. Heinlein
602 431-9283 voice/fax
www.space-access.org "You can't get there from here."
space.access@space-access.org - Anonymous
__________________________________________________________________________
- Permission granted to redistribute the full and unaltered text of this -
- piece, including the copyright and this notice. All other rights -
- reserved. In other words, crossposting, emailing, or printing this -
- whole and passing it on to interested parties is strongly encouraged. -