Subject: Space-tech Digest #158 Contents: engine/tank failures (2 msgs) modulating thrust (7 msgs) tank costs... (3 msgs) ------------------------------------------------------------ From: henry@zoo.toronto.edu Date: Mon, 12 Jul 93 15:29:07 EDT Subject: engine failures To: Space Tech >In our hybrid rocket, if lose an engine, we lose a big chunk of delta V >since the solid part can't be used by the other engines. So we our out >of luck in an engine failure, and therefore don't have to prepare a >control strategy for it. You can't re-route the fuel, true... but would it be possible to jettison the failing engine immediately? This might still save the mission. (Some of the engine-pod concepts for a 1.5-stage NLS design had the nice feature that you could unload a failing engine immediately, reducing the dead-weight penalty in the engine-out case.) Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ From: henry@zoo.toronto.edu Date: Mon, 12 Jul 93 16:16:13 EDT Subject: possible tank failure To: Space Tech A piece in AW&ST just reminded me: the May 27 Proton failure *may* have been a tank failure. The symptom was loss of pressurization in the second stage; the exact cause is not yet known. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: Matthew Feulner Cc: Space Tech , gwh@soda.berkeley.edu Subject: Re: FWD>RE>modulating thrust Date: Tue, 13 Jul 1993 06:38:59 -0700 From: George William Herbert >From: George William Herbert >>With low altitude winds, you get a slightly different problem. >>Right off the pad, the rocket can get blown around a lot >>and bad things can happen if it gets blown too far over. >>You can avoid this by careful launch planning, but you have >>to design for occational gusts. In this case, motor ang >>guidance reaction time compared to the vehicles lateral >>ballistic quotient (mass divided by lateral surface area) >>is the design rule. > >Can you be more specific? I can't compare seconds with kg/m^2. >Thanks >Matt Ok, here's the basic procedure I've been using... Assume that a worst-case gust hits the rocket just as it's lifting off. the tower protects the bottom part of the rocket from the wind, but not the top part. I've been using a sudden 75 knot gust as my windspeed for these calculations. I modeled the forces by assuming that the wind had full effect above the tower, but not below it. For some small time interval, from T=0 through some time after rocket clears the pad, calculate the initial angle (0 at T=1), the force on the rocket due to wind, force due to any control actuation (including the time delays), and calculated any overall moment on the vehicle. Divide by angular momentum of the rocket to determine what its angular accelleration is, and integrate the angular accelleration to determine the total tilt angle at any given time. If the rocket passes some fixed value of angle, it's defined to be "a problem" due to range safety problems. I use 15 degrees as this angle right now. Basic dynamics 8-) -george william herbert [ps: sorry for multi-day delays recently; I got a really nasty flu] ------------------------------ Date: Sat, 10 Jul 93 11:48:45 EDT From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: modulating thrust >Date: 09 Jul 1993 13:28:54 -0500 >From: Matthew Feulner >Subject: FWD>RE>modulating thrust fo >To: Space Tech >From: John Roberts >> - With all the changes in thrust for steering, how hard will it be to >> make sure you end up where you wanted to go? >I had hoped you could increase the thrust of one side while decreasing >the thrust of the other for no net vertical effect. I guess this depends >on whether you can get higher than nominal thrust, and how much higher. Keeping total thrust the same while modulating the thrust of individual engines is a fairly complex control problem. It might be simpler to modify the overall thrust profile (i.e. by changing the total burn time). I presume that's what the Shuttle would do if an SSME were to fail. (I don't think they'd try to run the remaining engines at 150%.) >> - Unless the engines are spaced far apart, the torque supplied by >> differential thrust will be very low. At least three engines are >> needed to control pitch and yaw. If the engines are spaced far apart, >> then control of relative thrust of the engines has to be very precise, >> and failure of a single engine will badly unbalance the loading, making >> survival very unlikely unless there are a large number of engines. >I have no feel for what torques need to be appied during ascent. Failure >of a single engine would seem to make survival unlikely no matter what. (Torque probably isn't the correct technical term, but I think it describes what we're talking about.) The further the axes of thrust of the engines from the axis of the center of gravity of the spacecraft (in other words, the greater the spacing of the engines), the greater your ability to control pitch and yaw by differential thrust, but also the greater the imbalance caused by the failure of one engine. That doesn't necessarily imply that an engine failure would result in the loss of the spacecraft, but you'd have to have more redundancy (number of engines, and/or ability to compensate the thrust) than you would for a spacecraft with the engines spaced closer together. The Shuttle could not survive the failure of one of the SRBs (which are spaced fairly far apart) even with steerable nozzles, because the imbalance would be too great. However, for much of its flight it could survive the failure of one or two SSMEs, or even get into orbit, because the SSMEs are relatively close together (and also because they have steerable nozzles). >> - Differential thrust won't control roll (unless engines are directed >> off-axis). >True. Is roll control necessary on the first stage, though? It depends on the requirements of the mission. :-) (Also see Henry's note.) Control issues aside, I doubt you'd want to have a manned spacecraft rolling at 60 RPM. (Does anybody know whether the Shuttle has the ability to use its wing control surfaces to help control roll during the first part of the ascent? I'd imagine they'd prefer to steer with the nozzles, but if a sideways actuator were to stick, there could be some value in having the wing surfaces as a backup.) John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ From: davem@ee.ubc.ca (Dave Michelson) Subject: Re: modulating thrust To: roberts@cmr.ncsl.nist.gov (John Roberts) Date: Sat, 10 Jul 93 9:14:43 PDT Cc: space-tech@cs.cmu.edu John Roberts writes: > >Date: 09 Jul 1993 13:28:54 -0500 > >From: Matthew Feulner > > >I have no feel for what torques need to be appied during ascent. Failure > >of a single engine would seem to make survival unlikely no matter what. > > (Torque probably isn't the correct technical term, but I think it describes > what we're talking about.) "Moment" is probably a better term in this context... -- Dave Michelson University of British Columbia davem@ee.ubc.ca Antenna Laboratory ------------------------------ From: henry@zoo.toronto.edu Date: Sat, 10 Jul 93 19:36:54 EDT To: space-tech@cs.cmu.edu Subject: Re: modulating thrust >Keeping total thrust the same while modulating the thrust of individual >engines is a fairly complex control problem. It might be simpler to >modify the overall thrust profile (i.e. by changing the total burn time). >I presume that's what the Shuttle would do if an SSME were to fail. Correct; in fact it's normal for modern launcher guidance systems to try to recover in such a manner. In the last Atlas failure, the Centaur and the payload actually made it into the correct parking orbit despite a major loss of thrust in the Atlas booster engines, because the Centaur fired longer to compensate. The launch was a failure only because the Centaur didn't have enough fuel to fix up the Atlas's shortfall *and* boost the payload out of parking orbit into the correct transfer orbit; it ran out of fuel during the second burn. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: henry@zoo.toronto.edu Cc: space-tech@CS.CMU.EDU, gwh@soda.berkeley.edu Subject: Re: modulating thrust Date: Sat, 10 Jul 1993 19:34:49 -0700 From: George William Herbert Henry mentions that 10Hz response is good for dealing with stratospheric winds according to Mich Burnside-Clapp... I will in general agree, but want to go into more detail in the basis for _why_ this is so and when it isn't, which is the case with the vehicle I'm working on. Any given rocket will have a maximum angle of attack which it can survive; this is based on the strength of the airframe and aerodynamic effects. Since our atmosphere is far from uniform, you'll get variations in winds aloft which can significantly affect the angle of attack over a very short period of time (say if you fly into a 400 knot jet stream suddenly...). Since rocket velocities at this time of flight are typically 1500 to 6000 knots, 400 knots of difference applied in a fraction of a second will be a pretty significant change in alpha (angle of attack). There's an inverse relationship between the bending strength of the vehicle and the speed and force which such wind changes have to be compensated by vehicle guidance. With the Saturn V, and similar class vehicles (Shuttle, Energia) the vehicles are able to survive pretty significant angles of attack (five to ten degrees) and have actuator swing times (stop to stop in either axis) measured in the one to three second range. Lighter, more fragile vehicles need faster reactions. With low altitude winds, you get a slightly different problem. Right off the pad, the rocket can get blown around a lot and bad things can happen if it gets blown too far over. You can avoid this by careful launch planning, but you have to design for occational gusts. In this case, motor ang guidance reaction time compared to the vehicles lateral ballistic quotient (mass divided by lateral surface area) is the design rule. SSTO vehicles (DC-X), to a first order approxomation, have good alpha ranges but poor ballistic quotients, and are likely to run into low-altitude problems. Smaller vehicles of any type encounter more problems, because they have less mass per surface area. Solid rockets and pressure-fed liquids have relatively high structural strength, thus nominally have good alpha ranges and are OK at altitude. Detail design can affect this quite a bit. In particular, metal-tank pressure fed vehicles have huge bending strength margins due to the mechanics of pressure vessel loading (presuming cylindrical vessels...). Vehicles which are overdesigned to start with and have metal pressure-feed tanks are unlikely to have problems in high altitude regimes unless the payload fairing fails. My rocket, which is not only essentially a solid but a very dense one to boot, has a very high ballistic coeficient and has few problems at low altitude (my first-order work on this was that it could survive a fifty knot wind shear at 1.5 times the top of the pad height with actuator responses in the 1.5 to 2.0 second range. Thus, presuming my payload shroud will survive five or so degrees alpha at 30 km altitude, it's pretty resistant to wind problems. -george william herbert Retro Aerospace ------------------------------ From: henry@zoo.toronto.edu Date: Sun, 11 Jul 93 15:41:29 EDT To: space-tech@cs.cmu.edu, gwh@soda.berkeley.edu Subject: Re: modulating thrust >Henry mentions that 10Hz response is good for dealing with >stratospheric winds according to Mich Burnside-Clapp... Upon reflection, I believe Mitch was thinking of vertical landings rather than coping with windshear on the way up. Things do get easier when there isn't something large and hard and motionless nearby. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: John Roberts Cc: space-tech@cs.cmu.edu Subject: Re: modulating thrust Date: Mon, 12 Jul 93 07:30:55 -0400 From: Chris Jones From: John Roberts Date: Sat, 10 Jul 93 11:48:45 EDT (Does anybody know whether the Shuttle has the ability to use its wing control surfaces to help control roll during the first part of the ascent? I'd imagine they'd prefer to steer with the nozzles, but if a sideways actuator were to stick, there could be some value in having the wing surfaces as a backup.) The shuttle elevons are used during launch (this is one of the reasons the APUs are needed: to power the hydraulic systems which move the surfaces (and move the SSMEs as well)). Although I have a vague recollection of them being used for flight control, when I looked, the only references I could find were to using the elevons to lessen wing-loading during the passage through the thick portion of the atmosphere. ------------------------------ Date: Mon, 12 Jul 93 22:51:25 -0400 From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: tank costs... George said: > T-1 (690 MPa, 100ksi yield) steel tanks run about $1.25-1.50/lb ... > ... including full assembly, welding, inspection and test of the > tank. This cost number is very interesting. In contrast, the TRW study assumed a cost of $7-11/lb. (1993 dollars) for large tanks made of HY 140, designed to operate at 2/3 of the K_tu at 600 F. Where does the cost difference come from, I wonder? HY 140 is an obsolete alloy designation, but a similar steel, HY 130, is available today, and is "easily welded by inert gas and covered electrode processes". HY 130 has a higher fracture toughness than T-1, with K_Ic ~ 250 ksi (in)^1/2, and is resistant to crack propagation. Pressure vessels made from HY 130 tend to leak at cracks or exhibit gross plastic deformation before burst. This should make testing less catastrophic and less expensive. How do they weld and test T-1 (ASTM A678?) pressure vessels? Paul ------------------------------ To: dietz@cs.rochester.edu Cc: space-tech@cs.cmu.edu, gwh@soda.berkeley.edu Subject: Re: tank costs... Date: Tue, 13 Jul 1993 06:57:51 -0700 From: George William Herbert T-1 is A514 by the way... I'm not quite sure what A678 is 8-) A514 can be welded using any process that normal steel can be, though you have to use higher strength (110xx series) electrodes. Weld times are identical to lower strength steels. Some slight changes in procedure are needed to not overheat the material, but that's just procedure not technique. My impression is that industrial-grade welding of steels with yield strengths greater than T-1 is still a pretty risky proposition, especially in thicker sections. Even T-1 grade materials can be hard to work with in sufficient thicknesses (the Seawolf submarine, for example, had to have about 35% of its welds junked and redone after detail inspections found microcracks, and it's made of HY-100... Verrrry expensive). I base my cost estimates off the time needed to weld the thicknesses of materials I'm working with, large margins for welder effeciency and welding time, and $20/hr welders. I add in cost to bend the plates and labor time needed to assemble the sections together for welding. I do reality checks at odd intervals by talking to local fab shops and checking existing designs costs and comparing with mine. So far, indications are that $1.50/lb for T-1 tanks is valid as long as I don't have the work done in the San Fransisco Bay Area (where I live, one of the highest cost areas in the US). It's about 10% higher right here, but I'm not going to put my fab plant somewhere with these costs of doing busines... -george william herbert ------------------------------ Date: Tue, 13 Jul 93 11:23:03 -0400 From: dietz@cs.rochester.edu To: gwh@soda.berkeley.edu Subject: Re: tank costs... Cc: space-tech@cs.cmu.edu George said: > (the Seawolf submarine, for example, had to have about 35% of its > welds junked and redone after detail inspections found microcracks, > and it's made of HY-100... Verrrry expensive). HY 130 is similar to HY 100, I think,and the fabrication process Boeing advocated with it did not involve any detailed inspection of the welds. This could be because either they perhaps had higher margins than on the Seawolf, or because the microcracks in the Seawolf were of concern because they reduce the fatigue life of the material, especially when in seawater (obviously a concern in submarines, but not in expendable boosters). The following offers an interesting comment on the behavior of cracks in welds in HY 130: Experimental tanks [of HY 130] with 1-inch-thick walls, 35 in. in diameter with two longitudinal welds and hemispherical ends were tested by [ref. omitted]. A surface crack (about 2.25 in., 0.8 in. deep) was introduced into the longitudinal weld of two tanks. One tank was subjected to 225 cycles (the last 25 with 3 percent NaCl solution in the crack) at membrane stresses about 70 percent of the parent metal yield. The pressure was then increased to 8800 psig at which point thje tank leaked at the crack. The membrane stress at failure was 143 ksi as compared to a reported weld F_ty = 150 ksi and a parent metal F_ty = 137 ksi. The second tank was pressurized to burst at 8950 psig. This tank after failure showed considerable evidence of gross plastic deformation. (from Aerospace Metals Handbook, 1990) This reference also says that hydrogen embrittlement can cause weld cracking in HY 130, but this can be avoided by using very dry electrodes (baked and stored in containers with dessicant). BTW, what's the behavior of ASTM A514 like at low and high temperatures? HY 130 remains fairly ductile down to at least -80 F, and retains much of its strength at 600 F. Metals for use in solid rockets need to have good elevated temperature behavior. Paul ------------------------------ End of Space-tech Digest #158 *******************