|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
George William Herbert wrote:
But that way you end up with the parasitic weight of the rendezvous and docking gear on each of the modules, Or, a tug, or using one of the assembled modules as a tug. Or you can use whatever stage you were planning to use for your transfer orbit injection and deep space maneuvering, since that already has to have a fairly robust maneuvering capability. Rendezvous and docking requires one or more active spacecraft and two stabilized spacecraft. Stabilization is far easier and cheaper (and can be completely passive) than active, and the cost savings of passive modules is probably worth it for logistics flights. The Falcon V costs $16M to take 6000 kg to LEO. If you can make a propellant storage satellite that has a dry weight of 500kg and costs $1M, a propellant storage satellite with zero dry weight would have to cost less than $2.5M to be cost effective. I think this argues heavily in favor of passive propellant modules, perhaps with a nanosat style magnetometer/torque-rod attitude control system. Depending on your propellant selection, you could even fit *two* spherical tanks in the fairing, one strong enough to take launch loads and the other strong enough for the order of magnitude lower in-flight loads, transfer the propellant to the light tank on orbit, and once you've docked to the transfer stage, jettison the "heavy" tank. I'm trying to run the numbers on this, and while my (lack of) structural mechanics knowledge is making it slow going, I think you can end up with around half the cost per kilogram to a trans-mars orbit of a Delta IV Heavy, albeit with a smaller payload volume. -jake |
#22
|
|||
|
|||
|
#23
|
|||
|
|||
"Pat Flannery" wrote in message
... Jeff Findley wrote: I hope that Congress realizes this and puts a stop to it now. They certainly don't want a repeat of the shuttle/ISS experience (e.g. repeated redesigns due to mounting costs). Unfortunately, one can see that being exactly what happens based on recent NASA experience in regards to new spacecraft design. The project gets started, then around half way through, we decide that we don't need what we are designing or run into a technical snag, and start all over again from scratch. They should have a real sit-down discussion and figure out exactly what they want and what it's designed to specifically do before they go rushing ahead with the design, like they seem to be doing now. The Soviets carefully thought out Soyuz before they built it, and the fact that they made the right decisions gave them a quite versatile spacecraft that could be kept in use at a economical price for decades to come. If a more appropriate flexible developmental architecture was adopted at the onset, the final design would not have to spring fully formed from the heads of engineers with every conceivable technical contingency planned for and thoroughly tested in great detail ten years in advance. One of the reasons commercial ventures often require payback periods of only a few years is because attempting to forecast technical and market conditions further in advance of that was found to be uneconomic. It was better to adopt a far more responsive but less predictive management approach that could instead quickly adapt to real markets and technical developments as they happened. Lets face it, even the energy sector is now more responsive that the space industry... Pete. |
#24
|
|||
|
|||
"Scott Lowther" wrote in message ... George William Herbert wrote: One burnthrough and seven or so near burnthroughs. "Near burnthough" is a somewhat vague and essentially useless metric. Something like 840 GEM 40's have been flown, and only lost a few; but from all reports, the ones that have been recovered, alogn with the static test units, have spooked the engineers who've examined them. "Near burnthough" is what they were meant to do. Saves weight. No they're not. This is the same logic that Feynman slams when the engineers saw 2/3rds of O-rings burned through and still claimed it "had a safety margin" when in reality the O-ring was never supposed to even burn at all. So there was in reality no margin left. |
#25
|
|||
|
|||
On Mon, 23 May 2005 12:22:51 -0500, in a place far, far away, Pat
Flannery made the phosphor on my monitor glow in such a way as to indicate that: NASA may have taken a dim view of the proposed LockMart-Boeing space services merger and what it would mean in regard to pricing of commercially bought boosters. If you are required to buy commercially, and you have a sole-source supplier, then you have a situation where that sole-source supplier can charge whatever it wants. As opposed to the vibrant competition in SRMs? |
#26
|
|||
|
|||
|
#27
|
|||
|
|||
"Scott Lowther" wrote in message ... "Near burnthough" is a somewhat vague and essentially useless metric. Something like 840 GEM 40's have been flown, and only lost a few; but from all reports, the ones that have been recovered, alogn with the static test units, have spooked the engineers who've examined them. "Near burnthough" is what they were meant to do. Saves weight. Scott, this is the same sort of "engineering" that gave us the Challenger and Columbia disasters. It's this "It's never failed before, so it must be o.k." attitude that will come back to bite you in the ass, especially on vehicles with low flight rates. Even on airliners, problems like these crop up after thousands of successful flights. The SRB joints remain a possible failure point. They're one of the most critical parts of the design. The enormous gimballing nozzle is another. Jeff -- Remove icky phrase from email address to get a valid address. |
#28
|
|||
|
|||
George William Herbert wrote: Pat, please, that is a grossly simplistic analysis of the Shuttle SRBs. Such ridiculous handwaving is technically not credible, regardless of whether the SRB is a viable launcher baseline or not. Which do you think is a more complex and high tolerance piece of machinery- an SRB, or a Delta IV booster module? And the Delta IV booster module was specifically designed to be a simple low cost rocket stage. It's a gimbaling assembly on the nozzle, and hasn't screwed up yet Actually, I recall we very nearly lost a Shuttle due to one of the actuators coming close to failure. But it didn't fail, that's the big point; if we had only launched 20 or 30 SRB's then you could state that we have been lucky; but with 226 launched over twenty years with one failure you can see a statistical curve that indicates high reliability. It is now a mature and safe design. If you did want to simplify it even more you could do some sort of fluid injection like on the earlier solids for TVC. A simple concept, but requiring new development and qualification. Yes, and that's why I'd recommend keeping it just as it is now, a known quantity. We had one burn through in 113 flights, and we redesigned the SRB after that for better relibility. One burnthrough and seven or so near burnthroughs. Redesigned or not, joints are a failure point. The new ones are designed to give a back-up in case of the sort of leakage that led to the loss of Challenger were to occur again; and that loss probably wouldn't have happened if it had been launched in warm weather, rather than the cold conditions that the SRB was not originally designed to handle. One big safety difference with using the SRB as a first stage of a vertically stacked vehicle- no liftoff "twang" straining its joints such as occurs in the Shuttle. Pat |
#29
|
|||
|
|||
Ed Kyle wrote: It might work if you stretched it and added eight more RL10s! Or, if you can afford to pay for the development effort, use four RL60s - a combo that would provide a more impressive ISP than J-2S, but would weigh more. That would make one honking impressive second stage! Pat |
#30
|
|||
|
|||
Greg D. Moore (Strider) wrote: No they're not. This is the same logic that Feynman slams when the engineers saw 2/3rds of O-rings burned through and still claimed it "had a safety margin" when in reality the O-ring was never supposed to even burn at all. So there was in reality no margin left. Of course in this case we are talking about a motor for use on an unmanned vehicle. The SRB is a whole other ball of wax, and expected to have a far higher level of reliability, even if that means overbuilding it to gain reliability at the expense of performance. Pat |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Celestron Celestar C8 Dec Drive Motor / Hand Controller | dean | UK Astronomy | 3 | January 15th 05 12:27 AM |
Mars Exploration Rover Update - November 8, 2004 | Ron | Astronomy Misc | 0 | November 9th 04 05:13 PM |
Getting a Edmund 6 newt clock drive to work | robertebeary | Amateur Astronomy | 0 | June 23rd 04 05:07 AM |
Problems with Celestron 11" Ultima clock drive | Charles Burgess | Amateur Astronomy | 0 | June 20th 04 11:51 PM |
Meade #7502 R/A Clock drive - Speed ?? | mindy | Misc | 0 | September 5th 03 12:56 AM |