Subject: Space-tech Digest #149 Contents: George Herbert's "Zephyr" vehicle (17 msgs) ------------------------------------------------------------ To: space-tech@cs.cmu.edu Cc: gwh@retro.com, gwh@lurnix.com Subject: Request for Comments on this paper... (LONG) Date: Tue, 18 May 1993 02:57:05 -0700 From: George William Herbert The included paper is going to be presented at Case for Mars V in about a week, if nothing goes wrong before then. If anyone has any comments or suggestions about it, I'd gladly accept them. (sorry for the poor line-wrapping; it was translated from Mac WYSWIG into ascii in a not totally 80 column manner 8-( ) -george william herbert Retro Aerospace -----begin included paper Zephyr: A MicroSat Transfer Orbit Stage George William Herbert Retro Aerospace, Berkeley CA Abstract As microsatellites (sub-ton class satellites) have proved their merit in near-earth applications, the potential for light planetary explorers with similar sizing has become evident; small rovers, small science stations, and other applications have appeared in literature. This paper discusses the design of Zephyr, a transfer stage for microsat-class vehicles, capable of propelling 50kg class payloads on Mars Transfer Orbits from the Pegasus launch vehicle, for total projected launch costs of under $20 million. 0.0 Introduction The vehicle that was to become the Zephyr transfer vehicle/upper stage was concieved during a lecture on robotics in planetary exploration at International Space University's summer 1992 program. The concept began after a presentation on the JPL "Rocky" minirover; the author wondered if it was possible to send such payloads using small launch vehicles (and quickly and cheaply) instead of as a minor part of a larger mission. He had just come across the technical specifications on the engine that Zephyr now uses [1], put two and two together, and the first preliminary design was completed by that afternoon using basic design guidelines [2]. Since then, it has been refined and developed as a low-priority project at Retro Aerospace. 1.0 Requirements The first step undertaken in this project was identification of a complete, if not rigid, set of mission requirements. These are: % The transfer vehicle must be compatable with the existing Pegasus vehicle % The vehicle must be able to propel useful payloads onto a Hohman transfer orbit to Mars % The vehicle must be priced affordably for Microsat/Explorer class missions % The vehicle must use existing components as systems allow 1.1 Pegasus Compatability The Pegasus launch vehicle is currently the standard microsatellite launch vehicle. It can place a payload of approxomately 350 kg in a 150 km circular equatorial orbit, or lesser payloads to higher altitude and/or inclination. Its payload fairing has a useful envelope measuring 116 cm (46 inches) in diameter, with a maximum length of 182 cm (72 in). Pegasus has a relatively benign launch environment. 1.2 Useful Payload to Mars Transfer Orbit The author felt that the minimum acceptable payload for a mission would be on the order of 25 kg placed into MTO on an average alignment launch window. If a package of that mass were aerocaptured at Mars, the resulting payload to Mars surface would be on the order of 6-10 kg, enough for a basic minimal rover or immobile basic science station. 1.3 Affordable Pricing An initial guess at reasonable pricing for microexplorer payloads was that launch costs should not exceed twenty million dollars. As the Pegasus vehicle procurement costs are $12-15 million, this limits transfer stage costs to about $5 million. 1.4 Existing Components This final constraint was imposed to reduce development and certification costs and to reduce program risk. A lengthy, expensive development program for a cheap stage for cheap missions makes no sense. 2.0 Overall Configuration The vehicle that evolved within the stated constraints is named the Zephyr transfer stage. Its core is the SEP 8kn engine, around which are positioned 12 small spherical fuel tanks. Dry mass of the stage is approxomately 55 kg, and it can hold up to 290 kg of Nitrogen Tetroxide and Monomethyl Hydrazine propellants. The whole vehicle fits within the 116 cm diameter Pegasus shroud, though volume and shape constraints have forced an unusual aft-facing configuration during launch. [See Fig. 1] It is assumed that the Pegasus third stage's maneuver capability will be used to orient the Zephyr vehicle and payload in a forwards facing alignment before it seperates and the Zephyr fires. The overall configuration of the Zephyr vehicle was influenced by two tight constraints; the outer envelope of the Pegasus shroud, and the volume and shape of the relatively massive engine. Of the 182 cm length of useable volume within the shroud, the engine takes up 145 cm, and much of the diameter for that length. Around the front of the motor, where it is thinnest, are the 12 propellant tanks, each 34 cm (13 in) in diameter. Between the tanks and the engine are a sheet aluminum heat shield and some limited additional thermal insulation. The vehicle's systems are mounted on its front and between the rows of propellant tanks. [Fig. 2] 2.1 Overall Mass Budget Any transfer stage (indeed, any space vehicle [3]) is a highly mass-critical design. The following table summarizes the current mass budget for the Zephyr vehicle. Table 1: Mass Budget and Breakdown System Level Mass (kg) Totals Engine Mounts Detail 1.2 Propellant Tanks Detail 12.0 Motor Built 21.0 Battery Detail 4.0 Propellant Pressure Det. Est. 4.5 Other Structure Det. Est. 0.8 Margin & subtotal 5% 2.2 45.7 Insulation Est 3.0 Motor Actuators Est. 3.0 Guidance & Control Est. 3.0 Communications Est. 1.0 Margin & subtotal 15% 1.5 11.5 Total 57.2 Those systems which have been through at least one detail design examination are described as "designed". Detail estimates are those for which parametric modeling was used, based on other existing systems, in which the author has high confidence. Estimates are systems where parametric modeling had less precise background information and a correspondingly higher margin is used. The mass margin is about 10% if the motor, which has been built, is excluded from the margin calculation. This is reasonable for a design at this stage. Alternate payloads are listed in section 6.0 for a overweight condition (20% margin) should this prove inadequate. 3.0 Structural Design The structure of the Zephyr vehicle is broken up into a number of subsystems: the fuel tank structure, the engine mount structure, secondary structures, and launcher interface structure. 3.1 Fuel Tank Structure Fuel is contained in 12 spherical fuel tanks, pressurized to 330 PSI in operation. An early analysis of the vehicle indicated that using the tanks as their own support structure was the most effecient design solution: in addition to pressure loads they are strengthened to carry their own inertial loads. Each tank is a spherical carbon fiber / epoxy structure [4], 34 cm in diameter, with a stiffening cylinder about 4 cm diameter running through it along the Z axis. Minimum thickness, determined by the pressurization loading and a reasonable safety factor, is 1.5 mm [5]. The tanks are stiffened where other loads are significant. Detail design of the tanks is being delayed until a finite element stress analysis can be performed, but estimated mass per tank is 1.00 kg. The tanks are arranged in 2 rows of 6, hexagonally situated around the motor. The front row is the oxidizer tankage, the back row fuel. Each tank is attached to the two neighboring tanks in its own row and to the corresponding tank in the other row. The resulting interconnected structure of tanks is self-supporting. 3.2 Engine Mounts The engine is mounted at the axis of the vehicle. It is attached to the ring of oxidizer tanks (the front ring) by six stainless steel thin-wall tubes, with appropriate end hardware for the connections. The mounts mass 0.20 kg each. 3.3 Secondary Structures There are a number of secondary structures on the Zephyr vehicle; a thermal shield and insulation blanket seperates the engine from the fuel tanks; this is massed at 3.0 kg. The payload mass is supported directly from the top of the forward six propellant tanks, at the same positions that the motor mounts tie in. The vehicles other systems, such as control, propellant pressurization, are mounted on struts between existing primary structure nodes. 3.4 Launcher Interface Structure The Zephyr vehicle is connected to the Pegasus launch vehicle's third stage by three or more brackets. They are bolted to the Pegasus payload adapter ring and attached using pyro bolts to the Zephyr vehicle's structure at the top of the oxidizer tanks, at the same point the main motor is attached to them. This reduces the vehicle proper's dry mass. 4.0 Propulsion The Zephyr vehicle has a single motor, used for both propulsion and directional control. It is used to provide any needed velocity changes. 4.1 Motor Primary propulsion is provided by a SEP 8000 N (1750 lbt) storable bipropellant motor, which utilizes a ceramic matrix composite combustion chamber and nozzle. The engine burns 2.5 kg of nitrogen tetroxide and monomethyl hydrazine propellant per second, and develops a specific impulse of 320 sec. Its mass is 21kg, including its mounting bearing but not including actuator masses. This engine has been developed and test-fired extensively by SEP. This application will require no modification of the engine, and has burn times of 1/3 of the demonstrated life. 4.2 Propellant Feed Propellants are pressure-fed into the engine's combustion chamber, requiring a pressurized storage and feed system. The feed system is fairly simple, with pipes from the 12 tanks joining the two main feed lines and then connecting to the motor, where an internal cutoff valve controls operation. The motor and feed system all operate at 2.2 MPa (320 PSI). Pressure within the propellant tanks is maintained by a small hot-gas helium (Tridyne) generator system. The design of this system was scaled from existing testbed systems. 5.0 Systems This section describes other systems not yet covered. 5.1 Guidance Position and velocity vector information for the Zephyr vehicle are generated by a missile-type ring laser inertial system. Initial position and velocity are uplinked from the ground, and the guidance system merely has to provide a delta from those values over the roughly hundred-second burn time, for direction vector and burnout velocity control. An alternative GPS guidance solution is being investegated as promising, but is not yet baselined. Further flight data on GPS guidance is desirable. 5.2 Control The Zephyr vehicle is controlled by an on-board microcomputer of as yet undetermined type. Retro Aerospace is investegating using a ruggedized PC- type computer with existing realtime operating systems and appropriate input/output and control capabilities, at great cost savings compared to highly customized control computer design and programming techniques. 5.3 Directional Control Directional control of the Zephyr is provided by main engine pointing. Two electromechanical actuators (X, Y planes) provide control motions. Use of electromechanical acutators allows the vehicle to have no hydraulic or pneumatic systems, decreasing complexity. No secondary directional control is provided, though an addon system would be easy (at some loss of payload capacity) if a particular mission demanded it. 5.4 Electrical Power The Zephyr vehicle uses electrical power for three purposes: computer and guidance system power, main engine pointing actuation, and valve operation. By far the highest peak loads are from main engine pointing acutation, estimated at 1.4 kW averaged over the 100 second burn. Total power requirements include computer operation for three hours (90 minutes) after reaching orbit, in case checkout procedures or problems delay orbital insertion. Power is provided by a primary (non-rechargable) battery. 6.0 Performance The Zephyr vehicle as designed in detail exceeded its performance requirements to a large degree. The following table summarizes the vehicle's performance when launched by the Pegasus vehicle and if the Pegasus XL enhanced-performance vehicle is available. Table 2. Zephyr Mars Transfer Orbit Payload Summary Specified Mass Margin case Launch Vehicle Best Case Payload Worst Case Pegasus 51 kg 43 kg Pegasus XL 71 kg 57 kg Overweight stage case Launch Vehicle Best Case Payload Worst Case Pegasus 47 kg 39 kg Pegasus XL 67 kg 53 kg In each case, the best and worst case payloads were derived from constructive and destructive pairings of apehelion and perihelion for Mars and Earth [6]. Actual launch velocities for upcoming windows have not been calculated for more detailed payload in year/window figures. The payloads which Zephyr can accellerate to transfer orbits are quite reasonable for achiving scientific objectives on or around Mars. 7.0 Discussion The Zephyr vehicle as described in this paper has fulfilled the primary design requirements quite well. From the Pegasus vehicle, injection payloads of 45 kg are easily attainable, which should allow a on-the-ground payload of perhaps 15 kg at Mars if aerocapture decelleration is used. This is plenty enough for a miniature rover, science station, or for a single-experiment more detailed mission. As the total launch cost is low, and probe cost should be low due to very low mass, missions for under $30 million are quite possible. The political attractiveness of small, fast to develop and fly missions is also not to be discounted in a fluctuating budget situation. The Zephyr vehicle allows relatively good performance potential within this range, using conventional and low-risk technology. Additionally, potential near-earth applications may make developing Zephyr on a commercial basis feasible, unburdening those costs from scientific programs. 8.0 References [1] 8000-Newton Engine data sheet, SEP, 1992, Paris [2] Space Handbook, Doc. No. AU-18, ed. Cochran, C. D. et. al., 1985, Air University Press, Maxwell Air Force Base, Alabama [3] Multi-Role Capsule System Description, Hempsell, C. M and Hannigan, R. J., in JBIS (full cit. unknown) [4] Design with Advanced Composite Materials, ed. Phillips, Leslie N., 1989, Springer-Verlag, London [5] Introduction to mechanics of solids, Popov, Egor P., 1969, Prentice-Hall, Englewood Cliffs, NJ. [6] Hohmann Ellipse Transfer Data, Armento W. J., in Handbook of Chemistry and Physics 65th edition, ed. Weast, R. C., 1985, CRC Press, Boca Raton, Fla. 9.0 Authors Address George William Herbert Retro Aerospace 2240 Blake #101 Berkeley, CA 94704 USA Phone: (510) 849-4853 Internet Email: gwh@retro.com or gwh@soda.berkeley.edu (both guarantees fastest response) ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 18 May 93 16:54:44 EDT Cc: space-tech@cs.cmu.edu Subject: Re: Request for Comments on this paper... (LONG) To: George William Herbert Overall, looks reasonable. I missed a mention of what is done about roll control (even if the answer is "nothing"). I also missed some note on the specific merits of this engine. Given its bulk and the relatively short burn time, in an ideal world one would opt for lower thrust to reduce engine mass and bulk. (I conjecture that the answer is "there aren't many off-the-shelf small engines with good exhaust velocities".) Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: uunet!zoo.toronto.edu!henry@uunet.UU.NET Cc: George William Herbert , space-tech@cs.cmu.edu, gwh@lurnix.COM Subject: Re: Request for Comments on this paper... (LONG) Date: Tue, 18 May 93 14:36:02 -0700 From: gwh@lurnix.COM Henry writes: >Overall, looks reasonable. >I missed a mention of what is done about roll control (even if the answer >is "nothing"). The answer is indeed nothing. I'm playing with having it not roll and with low RPM rolling, and am tending towards using a low RPM roll to help spin stabilize it. But there is no onboard system to control roll rate. >I also missed some note on the specific merits of this engine. Given its >bulk and the relatively short burn time, in an ideal world one would opt >for lower thrust to reduce engine mass and bulk. (I conjecture that the >answer is "there aren't many off-the-shelf small engines with good exhaust >velocities".) That conjecture is correct. As far as I could find out, there are no other small engines with Isp over 295. Ideally, I think an engine about half as powerful would be a better balanced configuration for Zephyr, but one of my design requirements is that it use off-the-shelf hardware wherever possible. And designing and testing a new engine is beyond my means. Developing the stage properly is going to be expensive enough... Thanks for the comments. -george william herbert ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 18 May 93 20:01:14 EDT To: George William Herbert Cc: space-tech@cs.cmu.edu Subject: Re: Request for Comments on this paper... (LONG) >The answer is indeed nothing. I'm playing with having it not >roll and with low RPM rolling, and am tending towards using >a low RPM roll to help spin stabilize it. But there is no >onboard system to control roll rate. Hmm, any stray torque, from nozzle irregularities or what have you, is going to be a problem in that case. I suppose one could depend on the payload for a little bit of roll authority, since it's going to need something for cruise and course corrections. A semi-related topic (since it could be solved as part of the same system): how are you planning to separate propellants from pressurant for free-fall startup? Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: 18-MAY-1993 19:50:38.49 From: "Gordon D. Pusch (708)252-3843" Subject: RE: Request for Comments on this paper... (LONG) To: SPACE-TECH@BITNET.CC.CMU.EDU > 3.1 Fuel Tank Structure > ... Each tank is a spherical carbon fiber / epoxy structure [4], 34 cm > in diameter, with a stiffening cylinder about 4 cm diameter running > through it along the Z axis. ... The tanks are stiffened where other > loads are significant. Detail design of the tanks is being delayed > until a finite element stress analysis can be performed, ... I'm a "beam jockey" not a structural engineer, so I can only make guesses why you chose this design. Are "stiffening tubes" a common feature of "self-supporting" tanks? Is the function of the stiffening- tube to offset compressive loads during launch? (It seems reasonable that the tanks will be subjected to compressive inertial loads --- especially the oxidizer tanks. I reason this based on your statement that the Zephyr is launched "aft-facing" which I assume to mean "nozzle-forward;" also, that the tank-assembly connects to everything else at the (front of?) the oxidizer tanks. If I'm visualizing your vehicle's configuration correctly (is your paper available via FTP? I'd like to see the figures...), this would *seem* to imply the tanks are subjected to compression during launch... I can't remember the maximum "gee-load" Pegasus pulls on launch, and you don't appear to mention it in your paper; I'm guessing it's less than about 10g. Unless I'm making a gross error on the back of this envelope here ;-), it looks like the compressive load can't possibly be much more than 2e3 Newtons --- whereas the *tension* exerted by the 300-psi propellent appears to be about *2e5 Newtons*... I wonder if the internal stiffening-tubes are really necessary; is a one-percent *decrease* in the hoop-stress around the waist of the tank worth worrying about? Again assuming I've correctly guess your configuration, I reason that during the Zephyr's burn the tanks will experience a *tensile* load. The initial acceleration appears to be around 2.3g; I don't know enough about pressure-fed engines to estimate max-gees, but given your stated masses, it *can't* be more than about 10--12g... Again, the external tension looks to be small compared to the initial tank pressure... >... The tanks are arranged in 2 rows of 6, hexagonally situated around > the motor. The front row is the oxidizer tankage, the back row fuel. > Each tank is attached to the two neighboring tanks in its own row and > to the corresponding tank in the other row. The resulting interconnected > structure of tanks is self-supporting. You don't give details RE: *how* the fuel and oxidizer tanks are connected together. Perhaps you intend to use a cylindrical collar of 4cm diameter, and this is why you want a 4cm-diam stiffening tube in the tank. If this is correct, then from some sketches of forces and moments I've drawn, it looks like this will squash the tank during launch, and stretch it during burn... I vaguely remember that there is an optimum point somewhere farther out (1/2 or 2/3 the tank-radius?) where an external force may be applied to produce zero first-order deformation of the tank (is this why the legs of large LP-gas tanks are attached where they are?). Perhaps this is a better point to attach the tanks together --- although from the moments, it looks like the attachment point would then be under shear-stress, which I gather fiber-composites don't like much... :-( Alternatively, it occurs to me that rotating the fuel-ring relative to the oxidizer-ring by 30 degrees ("hexagonal-close-packed" configuration) would give you *two* support points instead of one, approximately halving the local stresses; it would also lead to a more rigid structure, and shorten the tank-assembly by about 5cm. Unfortunately, It would also produce additional shear-stresses which wouldn't occur in the axial- mount configuration you've described. Whether or not they'd be a problem would probably require a finite-element run... :-( Gordon D. Pusch ------------------------------ Date: Tue, 18 May 93 21:21:20 -0400 From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Re: Request for Comments on this paper... (LONG) Cc: gwh@lurnix.COM, gwh@soda.berkeley.edu > That conjecture is correct. As far as I could find out, > there are no other small engines with Isp over 295. Ideally, > I think an engine about half as powerful would be a better > balanced configuration for Zephyr, but one of my design requirements > is that it use off-the-shelf hardware wherever possible. How about the shuttle RCS engines? The primary thrusters have a thrust of 3870 N, and Sutton says the Isp ranges up to 304 s (depending on nozzle area ratio). You could improve the Isp by extending the nozzles. I believe these thrusters have radiation cooled nozzles, so this would involve no change to the rest of the engine. For control, you could use shuttle RCS vernier engines (111 N thrust). Paul ------------------------------ To: uunet!cs.rochester.edu!dietz@uunet.UU.NET Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM, gwh@soda.berkeley.edu, gwh@lurnix.COM Subject: Re: Request for Comments on this paper... (LONG) Date: Tue, 18 May 93 18:43:05 -0700 From: gwh@lurnix.COM >How about the shuttle RCS engines? The primary thrusters have a >thrust of 3870 N, and Sutton says the Isp ranges up to 304 s >(depending on nozzle area ratio). You could improve the Isp by >extending the nozzles. I believe these thrusters have radiation >cooled nozzles, so this would involve no change to the rest of the >engine. Can someone else confirm or deny that Isp rating? My information gave them as 274 for the vernier engines and 285 for the main RCS engines. If that's wrong, let me know... (and it sounds bad 8-) >For control, you could use shuttle RCS vernier engines (111 N thrust). ...or just pivot the main engine like I was doing with the 8kn motor. How much do the RCS mains mass, anyway? 12kg? [I don't want to try and compare the lower motor mass vs Isp loss tradeoff at this point, I'm fuzzy from work and other chaos. I'll do it when I get home...] -george ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 18 May 93 22:17:54 EDT To: George William Herbert Cc: space-tech@cs.cmu.edu Subject: Re: Request for Comments on this paper... (LONG) >...or just design the stage so that it can handle spinning up >a bit or down a bit. Shouldn't be too hard, the hard part will >be control laws to handle it. If the rate doesn't get *too* high, the guidance system should be able to just keep up with it. On the Mariner 2 launch, one of the Atlas's vernier engines malfunctioned in the worst possible way -- hard over -- and after booster-engine separation the roll rate built up to nearly 60RPM before the vernier engine unstuck and roll control was regained. The Atlas's control system coped successfully throughout, to everyone's amazement. The Sidewinder missile actually takes your approach: just cope with it as it happens. However, it also has rollerons to limit roll rate. You need to carefully assess things like the response rate of your engine actuators. If they can't keep up at maximum roll, you're in trouble. One serious advantage of having a bit of spin at the start is that it will keep you stabilized between Pegasus separation and your burn start, if there's a delay in between. (The Pegasus can supply the spin.) >>... how are you planning to separate propellants from pressurant >>for free-fall startup? > >Ahh, forgot that too. The simple answer to that is a pair of >C model rocket motors ;-) Just as well you're not flying on the shuttle, or you'd have to file six inches of paperwork and $10k of test results to prove they're safe! Again, a good reason for having a bit of spin at startup time -- you don't want a thrust imbalance between those Cs tumbling your stage, and you won't be able to put them close to the axis. The alternative, which was what was in my mind at the time of my earlier message, was a little cold-gas system to supply some ullage thrust for startup and then roll control during the burn. I haven't done the numbers on weight, though. Or maybe just a slightly bigger pressurization system, and tap some gas from that? Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 18 May 93 22:33:37 EDT Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM To: gwh@soda.berkeley.edu Subject: Re: Request for Comments on this paper... (LONG) >Can someone else confirm or deny that Isp rating? >My information gave them as 274 for the vernier engines >and 285 for the main RCS engines... The STS Reference (not the News Reference, but a much fatter NASA manual) says 280 for the mains and 265 for the verniers, in vacuum with the shortest nozzle lengths (the lengths vary with position on the orbiter). Note also that the RCS mains are only rated to fire continuously for a maximum of 150 seconds, although some of them are rated for a contingency continuous firing of 800s (for RCS retrofire, I would guess). The manual inconsiderately doesn't mention how much they weigh. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: George William Herbert cc: space-tech@cs.cmu.edu, Dominic.Herity@cs.tcd.ie Subject: Re: Request for Comments on this paper... (LONG) Date: Wed, 19 May 93 10:10:30 +0100 From: Dominic Herity X-Mts: smtp > 12 small spherical fuel tanks. Wouldn't two cylindical tanks be lighter and more compact ? > a thermal shield and insulation blanket seperates the engine from the fuel > tanks; this is massed at 3.0 kg. If the payload (capable of surviving aerocapture, so no extra shielding required) were between the engine and tanks, this would be unnecessary. Of course, seperation would be more complicated ... What do you think ? -----------------------------------------------------------|"Nothing travels | | Dominic Herity, dherity@cs.tcd.ie, |faster than light, | |Computer Science Dept, Trinity College, Dublin 2, Ireland.|except possibly bad| | Tel : +353-1-6772941 ext 1720 Fax : +353-1-6772204 |news"-Douglas Adams| ------------------------------ Date: Wed, 19 May 93 07:49:48 EDT From: Paul Carr X-To: AITGW::"space-tech@cs.cmu.edu" Subject: off-the-shelf samll engines To: space-tech@cs.cmu.edu > I also missed some note on the specific merits of this engine. Given its > bulk and the relatively short burn time, in an ideal world one would opt > for lower thrust to reduce engine mass and bulk. (I conjecture that the > answer is "there aren't many off-the-shelf small engines with good exhaust > velocities".) > > Henry Spencer at U of Toronto Zoology > henry@zoo.toronto.edu utzoo!henry > Royal ORdnance will sell you its 500N engine, which they can push to an Isp of about 325 for about $400K per. Very reliable, flight proven engine. You can get it at an Isp of 314 fro about $225 K. The mass is about 5 kg. IT burns NTO and N2H4, so you can use a single hydrazine tank to provide fuel for a roll control RCS. ------------------------------ Date: Wed, 19 May 1993 10:14:33 -0400 (EDT) From: Kevin William Ryan To: space-tech@cs.cmu.edu Subject: Re: Request for Comments on this paper... (LONG) Cc: What about mid-course corrections? I would think that an accurate aero-braking would require high accuracy atmospheric insertion. Secondly, data transfer back to earth: most of the micro-rover papers I have seen suggest a low power not very directional link between the micro-rover and an orbiter, with the orbiter having the antenna and power to connect back to the DSN or other terran transceivers. Will a 10K microrover be able to support the radio and antenna needed for two way communication? kwr Internet: kevin.ryan@cmu.edu ------------------------------ To: Kevin William Ryan Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM Subject: Re: Request for Comments on this paper... (LONG) Date: Wed, 19 May 93 16:43:07 -0700 From: gwh@lurnix.COM >What about mid-course corrections? I would think that an accurate >aero-braking would require high accuracy atmospheric insertion. Yes, but this is a payloads' problem, not the transfer stages'. All this vehicle does is set it up on the transfer; what it does on the way is its own problem 8-) > Secondly, data transfer back to earth: most of the micro-rover >papers I have seen suggest a low power not very directional link between >the micro-rover and an orbiter, with the orbiter having the antenna and >power to connect back to the DSN or other terran transceivers. Will a >10K microrover be able to support the radio and antenna needed for two >way communication? Probably not, but you can relay through the transponder on Mars Observer or send a dedicated comm relay sat if that ends up not working out. -george william herbert ------------------------------ To: Dominic Herity Cc: George William Herbert , space-tech@cs.cmu.edu Subject: Re: Request for Comments on this paper... (LONG) Date: Wed, 19 May 93 17:15:20 -0700 From: gwh@lurnix.COM >Wouldn't two cylindical tanks be lighter and more compact ? No. Two spherical tanks would be, if they'd fit, but the design is highly constrained by the shroud diameter on the Pegasus. The vehicle ends up being about 42 inches in diameter, in a 46 inch envelope. Both length and width are real problems. When I get the postscript graphics that go with it done and online, you'll see what I mean, but roughly... Payld. motor & nozzle vvv vvvvvvvvvvvvvvvvvv ________________ /-\/-\ ----------__ ----] \_/\_/ _____------| |<- payload envelope limit 3rd ] ##---/ | | st ] ##---\_____ | | ----] /-\/-\ ------| | \_/\_/ __________-- ---------------- ^^^^^ ^^^^^^ Peg. tanks The motor is 5 feet long total, from a 6 foot payload envelope length. The motor's nozzle diameter is significant, as is the space needed around the comustion chamber to allow the motor to swivel. There's no other way to fit fuel tanks in except the set of small tanks around the outside... >> a thermal shield and insulation blanket seperates the engine from the fuel >> tanks; this is massed at 3.0 kg. > >If the payload (capable of surviving aerocapture, so no extra shielding >required) were between the engine and tanks, this would be unnecessary. >Of course, seperation would be more complicated ... What do you think ? If the tanks didn't have to be around the motor, and if the payload was thermally resistant, then maybe that would work. I'm not willing to design a stage presuming that, though 8-) customers would be unhappy. -george ------------------------------ To: Paul Carr Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM Subject: Re: off-the-shelf samll engines Date: Wed, 19 May 93 16:57:55 -0700 From: gwh@lurnix.COM >Royal ORdnance will sell you its 500N engine, which they can push to >an Isp of about 325 for about $400K per. Very reliable, flight proven >engine. You can get it at an Isp of 314 fro about $225 K. The mass >is about 5 kg. IT burns NTO and N2H4, so you can use a single hydrazine >tank to provide fuel for a roll control RCS. My information stated it has lower Isp than that... I'll contact Royal Ordinance for more details after the conference. If that mass is right, even at the lower Isp range, it's within the range producing a higher payload than the 8kn SEP engine vehicle will, if the longer burn is within that motors endurance and gravity losses don't become significant due to the burn time. I ran a tradeoff last night and it indicated that the shuttle's RCS motors at 304 have a negative payload impact (relative to the SEP motor) if they mass more than 4 kg, which I'm pretty sure they do... One reason to stay with the SEP motor is that the stage can have more tankage added and be used for GTO transfers off a Taurus or something if the motor size stays high. I'll explain that in the final version. On related matters, the vehicle is now spin stabilized at 30+- a bit RPM; a first order stability calculation indicated that that was the best spin stability region, where i can leave off a roll system and accept changes in the roll rate during the burn. Using a tridyne roll control system was an extremely poor option. A bipropellant one is ok, but not light, and I'd prefer to leave it off entirely. The Pegasus third stage can set up that roll rate, according to its payload specs, so the orientation and roll spinnup are both accomplished before seperation. -george william herbert ------------------------------ Date: 20-MAY-1993 10:42:45.41 From: "Gordon D. Pusch (708)252-3843" Subject: RE: off-the-shelf small engines To: SPACE-TECH@BITNET.CC.CMU.EDU Content-transfer-encoding: 7BIT X-ANJE-To: SPACE-TECH X-ANJE-Cc: GATEWAY::"gwh@soda.berkeley.edu", GATEWAY::"gwh@retro.com", PUSCH > ...On related matters, the vehicle is now spin > stabilized at 30+- a bit RPM; ... > -george william herbert Given this spin-rate, you may have eliminated the need for an ullage system. The back of my envelope says that the tank centre will be experiencing about .34g of centripetal acceleration; I suspect that this will be sufficient to settle the propellant :-T. As long as your tanks are more than half full initially, I believe you'll be guaranteed a positive feed... Gordon D. Pusch ------------------------------ To: space-tech@cs.cmu.edu Cc: gwh@soda.berkeley.edu Subject: Paper revised, will be FTPable with diagrams Date: Fri, 21 May 1993 19:55:14 -0700 From: George William Herbert The Zephyr paper has been revised based on input from several persons on this list (thanks all), and the final version which is going to be presented at Case for Mars V on Friday, May 28 at 10:40am (barring schedule shifts) and the diagrams will be in the directory pub/Space/Zephyr at the ocf.berkeley.edu ftp site shortly thereafter. -george william herbert Retro Aerospace ------------------------------ End of Space-tech Digest #149 *******************