Space Access Update is Space Access Society's when-there's-news publication. Space Access Society's goal is to promote affordable access to space for all, period. We believe in concentrating our limited resources at whatever point looks like yielding maximum progress toward this goal.
For the moment, our main focus is on supporting the government's "SSRT" (Single Stage Rocket Technology) program, DC-X and its recently funded followon, SX-2. Space Access Update is thus for the moment largely about the technology and politics of DC-X and SX-2, though we also cover the subject of reusable SSTO (Single Stage To Orbit) policy in general.
We anticipate a change of focus soon if all goes well. Once SX-2 startup is (with your help!) assured, we plan to begin working on establishment of a healthy second X-rocket development track at NASA, and on getting development of suitable engines started for the fully reusable orbital transports that should come after SX-2 and NASA's X-rocket.
With luck and hard work, we should see fully reusable SSTO testbeds flying to orbit toward the end of this decade, with production ships a-building shortly thereafter. Join us and help us make this happen.
Henry Vanderbilt, Editor, Space Access Update
[For more info on Space Access Society or on the DC-X/SSTO video we have for sale, write us at 4855 E Warner Rd #24-150, Phoenix AZ 85044, or email hvanderbilt@bix.com. Please forgive any delay in our reply; we're a couple weeks behind in answering non life-or-death email right now.]
We've been running into a problem lately -- things have been happening so fast that we can't keep up; by the time we get an Update ready, two or three of the stories are already out of date, and we go back and rewrite them, and by then something else has changed... We're going to put stories out as we get them written until we get caught up again; look for a lot of shorter Updates over the next few weeks.
In this issue, the latest on DC-X.
In the next few days, weirdness at ARPA over the $35 million in SX-2 money they've been sitting on, the latest legislative SSTO funding news, NASA's rapidly evolving SSTO plans, and a possible compromise solution brewing to all the conflicting SSTO agendas floating around -- maybe even a solution we can support.
Fingers crossed on that last. And lots of phone calls and faxes and late- night policy-thrashing sessions. Oh well, we did volunteer!
Well, there's finally been a press release on what caused the problems with DC-X's June 27th flight. It's fairly sparse, but there's enough new info to fill in the picture reasonably well. We also have some information on the possibilities for repair and reflight of DC-X, possibly as soon as this fall. More on that in a bit.
Monday June 27th was of course the second flight of the new series and fifth flight overall for DC-X. It was also the flight where a pre-liftoff explosion tore up the graphite fiber/epoxy vehicle skin, the "aeroshell", on one side of the vehicle, causing the flight to be aborted to a safe landing in the open desert, about a hundred-eighty yards west of the paved landing pad. DC-X is currently grounded until further notice.
(We feel compelled to point out that a safe intact landing after launch explosions and significant vehicle damage is somewhat unusual for rocket vehicles, and is a strong indication that there's something to the DC-X program's emphasis on "savability", the aircraft-style ability to abort a mission and land intact if things go wrong.)
First, on the actual sequence of events leading to the DC-X damage. You may recall that in SAU #39, we concluded from evidence on our videotape of the flight that there were two explosions about one twentieth of a second apart right after engine ignition, and that both explosions were external to the vehicle, apparently in the ground-hugging plume of engine-precool hydrogen drifting southwards from the vehicle in the ~5 knot breeze.
The tape indicates that the precool plume ignited at ground level about fifty feet south of the vehicle, the flame travelling up the plume of vented hydrogen along the ground to the base of the vehicle, where it then apparently touched off vented hydrogen that had collected around the downwind side of of the vehicle in a second explosion. This second explosion seems to be the one that did the damage to DC-X.
The question this left open was, how was the vent plume ignited fifty feet south of the vehicle a fraction of a second after engine start? Today's press release provided the missing clue.
(Background: Hydrogen in moderate concentrations in open air will generally just burn without significant explosive effect, as we've seen in previous DC-X launches. Higher concentration and/or higher pressure increases the burn rate; if hydrogen burns in a confined space pressure can build up rapidly. The result can range over a spectrum from very fast burning with some overpressure to an explosion with a significant subsonic blast wave to detonation with a very sharp supersonic shock wave, depending on the concentration of hydrogen and the degree of confinement.)
There is an underground air duct leading to the DC-X launch stand, used to blow air in under the vehicle during engine precool, ironically to prevent explosions by keeping hydrogen from building up in the confined space under the vehicle. The duct comes up from the south; what we believe is the air intake for the duct blower is about fifty feet south of the pad. (The press release says the duct tubes are only twenty-five feet long, but that there are two of them -- possibly two sections of one fifty-foot duct?)
Apparently the ground-hugging plume of vented hydrogen drifting downwind from DC-X during engine precool impinged on the duct air intake. Enough hydrogen was drawn into the duct so when the engines lit, the hydrogen inside the duct ignited and then due to pressure buildup in the confined duct detonated. This was the first explosion heard on the tape.
The shock wave and flame front emerged from the duct intake into the open air portion of the plume, igniting it; the plume then burned back up toward the vehicle at high subsonic speed, as seen on our tape.
At that point, one possibility is that the slow, very steady breeze had left a region of stagnant (relatively still and undisturbed) air on the downwind side of the vehicle, and that hydrogen vapor had built up to a relatively high level there. Examination of our tape is inconclusive; the precool plume occasionally seems to creep upward along the downwind side of the vehicle, clinging to the aeroshell.
Another possibility is that the shock wave from the air duct detonation pushed vent plume hydrogen up against the aeroshell and compresed it just in time for the flame front to arrive.
Possibly a combination of these mechanisms was present. The result according to the press release was somewhere on the borderline between fast burning and a slow explosion. This produced a pressure wave strong enough to compress the aeroshell inwards on the south side of the vehicle. When the wave passed, leaving a partial vacuum in its wake, air pressure inside the aeroshell bowed it outwards and split it open.
With the advance warning that we are amateurs, and working with sketchy data, our overall conclusion is that this was a matter of a very specific wind direction, velocity, and steadiness having an apparently unforseen effect on the vented precool hydrogen vapor. In previous flights, the precool vapor has tended to rise from around the base of the vehicle almost immediately, rather than cling to the ground for a considerable distance as seen June 27th.
The airduct hydrogen ingestion and detonation might in 20-20 hindsight have been preventable (and then again might not have been; we don't have enough data) but the subsequent open-air explosion that did the actual damage even in hindsight seems a bit of a freak occurrence. To sum it up, this sort of thing is why we do flight tests.
We would have been very surprised if there was not some damage to the vehicle systems in addition to the shredded aeroshell, though obviously there wasn't enough damage to cause an immediate vehicle failure. The press release mentions one specific item, a small crack in the liquid hydrogen tank, caused when the actuator rod for the body flap on the south side of the vehicle was jammed against the tank by the pressure wave. They also mention unspecified damage to various ground equipment -- we know the TV camera immediately south of the pad went dead one frame after engine ignition, and it seems likely that anything else near the south end of the air duct that wasn't armor plated is also toast.
We expect that the internal plumbing under that section of aeroshell, hydraulic and electrical lines and such, took a beating; some will likely be replaced as part of any repairs.
We have heard that it should be possible to repair DC-X in a couple of months and finish the test program this fall, for a total additional cost on the order of five million dollars. This would not interfere with the handover to NASA, as the parts for the DC-XA upgrade won't be ready for installation until next spring sometime. We hear NASA in fact supports repair and reflight, as they quite sensibly would rather take over a proven flyable vehicle with as much flight test data available as possible.
Our opinion is that it's time to stop feeding DC-X flight-test funds through an eyedropper; it's a waste of all the time and money that's gone into getting this far. We say pay what it takes and get the job done.
The obvious source for the DC-X repair/reflight funds is the $35 million of SSTO money still unspent at ARPA, of course. Right? Uh, well... More on that tomorrow. The short version is that someone at ARPA seems to have their own ideas on what do do with that money, and it isn't DC-X/SX-2.
Henry Vanderbilt "Reach low orbit and you're halfway to anywhere Space Access Society in the Solar System." 4855 E Warner Rd #24-150 - Robert A. Heinlein Phoenix, AZ 85044 602 431-9283 voice/fax "You can't get there from here." (hvanderbilt@bix.com) - 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, intact crossposting is strongly encouraged. --