Sunday, August 01, 2004
While Discovery has been my main project I haven't forgotten the other fictional option - taking a Saturn Orbital Workshop - Skylab - to Mars and back. To boost inwards to Venus from LEO takes ~ 3.5 km/s, takes 0.4 years then 0.55 years back out to Mars. To save fuel a highly elliptical orbit around Mars is needed - Mars arrival takes ~ 1.5 km/s and departure [sans 25 tons of MEM] takes 0.9 km/s for an Earth return.
The major problem in that case is choice of fuel - a Saturn IV-B uses LH2/LOX, but LH2 boils away in its tanks and needs to be vented or else the tank will rupture. Over a year plus much of the initial fuel will be gone. Passive cooling in insulated tanks needs the slow vent of cryogens to work, while active cooling takes energy and machinery, but avoids venting.
Perhaps trusty old UDMH/N2O4 is the way to go. Certainly storable indefinitely but the ~2.5 km/s dv at Mars will need ~ 130 tons fuel. Which means more propellant to boost out of LEO.
Yesterday I discovered happily that the Apollo CSM can easily brake the stack into a Highly Elliptical Earth Orbit (300 km x 200,000 km say) - a 100 ton cluster gets a dv ~ 650 m/s out of the 18,413 kg fuel in the CSM's tanks. Only about ~ 570 m/s is needed, the rest is to put the CSM into a re-entry orbit.
Friday I figured out how much propellant a Saturn V could boost to LEO in the tanks of the IV-B stage, with just an aerodynamic shroud. Approximately 96 tons useable propellant will remain in the tanks after Orbital Insertion. So not much modification would be needed to launch suitable boosters for the Mars or Titan missions.
Posted at 10:21 am by Adam
Thursday, July 29, 2004
Last entry I linked to the idea that the Shuttle Orbiter should carry a Soyuz in its cargo bay as an emergency re-entry vehicle [ERV] - in reality all that is needed is the re-entry capsule and the engine unit to give the de-orbit burn and escape maneuvering. Such a configuration is not too far off the Zond configuration for circumlunar flights...
So I wonder what does a Soyuz need to get to the Moon? Or at least a looping orbit around the Moon - a free return trajectory - which has recently been advocated for a Space Tourism option, here...
July 27 Astronotes
A Moon-bound Soyuz needs a fairly large boost to get there - it's about a ~ 3 km/s boost from LEO and a Soyuz/Zond masses ~ 5.4 tons. Here's the Soviet-era circum-lunar orbital system that successfully flew a few flights around the Moon, but only managed one successful re-entry that wouldn't have killed/injured its crew...
Lunar 1 configuration
...obviously needs some work before it becomes a tourist option. Twenty-gee ballistic burns through the atmosphere would be a battering for anyone - much better to have a controlled "double-dip" re-entry just like Apollo. However the Block D stage that is the main-engine has been launched successfully for years - unlike the explosive hiccups in its early days - and has proven reliabilty...
...all in all the system masses the 13,360 kg of the Block D, plus the 5,400 kg for the Soyuz/Zond. About ~ 18,800 kg all connected up. This also fits neatly into the Shuttle's launch bay and is well under the maximum payload mass of ~ 24,400 kg. The actual Zond circum-lunar stack massed ~ 18.2 tons at the start of its translunar burn, so a tourist version might mass about the same. The system was ~ 11 metres long so there would be plenty of room in the Shuttle Orbiter's 18.3 metre long payload bay.
Posted at 1:06 pm by Adam
Wednesday, July 28, 2004
Here's a good concept - if NASA's so afraid of going to service the Hubble Space Telescope because the astronauts have no bail-out option in case of Orbiter damage, then why not take an emergency ride home? Here's a post at Save Hubble...
Save Hubble Posts
...simply: a Soyuz fits neatly in the Shuttle's payload bay, so why not take your spare spacecraft with you? Not an insane idea at all. Two Soyuz could easily fit in the payload bay and only mass 7 tons each, which is half the Shuttle's maximum payload. They're only 7 metres long so a docking node or Hubble repair gear could fit in there with them too.
Really radical idea, since the Russians sent stripped back Soyuz around the Moon as Zonds, is to use Soyuz/Zond vehicles for crew transfer to a Moon Station... hmmm...
Posted at 2:06 pm by Adam
Stephen Baxter's Discovery recapitulates Cassini's original flight-plan to the day, but in reality this is highly unlikely. Cassini launched in 1997 while Discovery launches in 2008 - and all the planets involved would have moved into new relative positions between those two dates. Multi-planet gravitational assists that can reach Saturn do happen frequently - several flight-plans were available for Cassini if the early launch window was missed. Fortunately Cassini was launched close to on time as the later flight-plans all took substantially longer, but eventually a shorter flight configuration would reappear.
If Discovery were in Cassini's current orbit (periapsis 1.33 Saturn radii, apoapsis 150.5 radii) and then able to change its periapsis to meet Titan directly the total time from SOI would be about 18 weeks and the aero-braking speed a mere 3.18 km/s - to enter a 4 hour orbit Discovery would then need to aerobrake by a mere 1,295 m/s. The periapsis raise would cost ~ 728 m/s of dv and rocket-braking into that 4 hour orbit would take 1,326 m/s, so aerobraking is definitely better. To do so I am not so sure how the Discovery could be configured to aerobrake - it has two Apollo Command Modules hanging off a docking node in the payload bay.
An Apollo CM is 12'10" (3.91 m) wide, and certainly short enough to fit the 4.6m x 18.3 m bay. But there are Topaz reactors, Spacelab and ISS modules squeezed in there too. However the aerobraking at 3.18 km/s need not be as fiery as a full re-entry and a few dips through the atmosphere might do the job.
Posted at 1:32 pm by Adam
Tuesday, July 27, 2004
Discovery arrives at Titan...
Back to our major theme - the fictional Discovery mission to Saturn and Titan. Once Discovery crosses into Saturn's Hill Sphere it has some important tasks:
deccelerate from hyperbolic velocity
change into Titan's orbital plane
line up an intercept orbit for Titan
deccelerate into orbit around Titan
land on Titan
Cassini - and Baxter's Discovery presumably - deccelerated into a highly elliptical orbit, then maneuvered into a Titan approach orbit. Cassini's current orbital path - modified from the originally planned Titan approach to improve communication with Huygens - needs two major maneuvers:
Saturn Orbital Insertion (SOI) - about 626 m/s
Periapsis raise - about 391 m/s
The SOI was completed successfully, though Cassini plowed through substantial amounts of dust nothing fatally injured the vehicle. The periapsis raising maneuver should go smoothly as well being a shorter main engine burn than the SOI.
The fictional Discovery suffered a decompression from a ring-plane impact, but otherwise successfully arrived. Baxter's described decceleration level was actually about 10 times too high - 1/10th of a gee instead of the actual 1/100th. The Shuttle's OMS is actually only capable of about ~ 2/100th gee. Poetic license I guess.
A direct braking into a Titan approach orbit would take a dv of ~ 1,400 m/s, but Titan is unlikely to be so obligingly in the right part of its orbit to be approached so directly. And I can't currently work out where Titan really would be with respect to a real Discovery in circa 2014 AD, but I'm working on it ;-)
However a direct approach is not the best. To minimise propellant load Discovery needs to aerobrake in Titan's upper atmosphere to shed its relative velocity and a direct orbit doesn't minimise that relative velocity. An indirect approach from a higher orbit with Titan's radius as the periapsis has a better chance of minimising the relative velocity.
Cassini's current orbit would have required aerobraking at 8.32 km/s (29,950 km/h) which is higher than the Shuttle's usual aerobraking range.
A direct approach would require aerobraking at 5.38 km/s (19,400 km/h.)
An apoapsis at 210 Saturn radii with Titan at the periapsis (20.273 radii) means aerobraking at a mere 3.26 km/s (11,740 km/h.)
Shedding a mere ~ 1,375 m/s for a 4 hour parking orbit means a lower heating load. Propellant needed for the two maneuvers is also lower than the direct orbit, saving about 320 m/s of delta-v. The penalty is the +30 weeks spent getting into the right orbital positions.
Posted at 3:15 pm by Adam
Monday, July 26, 2004
In the late 1960s physicist Peter Glaser proposed to build huge solar energy collecting satellites, but no one took him seriously in the age of cheap oil and "safe" nuclear power. Then the Oil Crunch of the mid-70s hit and the US Department of Energy (DoE) and NASA took him seriously. The Solar Power Satellites (SPS) they designed were typically capable of delivering 10 gigawatts power to the ground, with about 62.9% efficiency. In space that meant 129 km^2 arrays of solar cells massing perhaps ~ 80 - 100 thousand tons, beaming power to the ground as microwaves.
Their designs made a lot of assumptions that made them difficult -
on-orbit assembly was assumed with a huge work-force and inter-orbital transportation system requiring everything haulled from the ground, cargo haulled to GEO via giant ion-drive space-tugs and faster chemical fuel tugs delivering workers
arrays were supported by rigid frameworks that had to be assembled on-orbit
power transmission was at lower microwave frequencies that minimised atmospheric absorption, but need huge transmitting antennae
the solar array had to track the Sun continuously and the antenna had to point to the power receiver continuously
huge dedicated cargo delivery aerospace vehicles had to be developed for the program to deliver equipment to LEO
Each assumption complicates design and increases cost and mass delivered to the target orbit. Geoffrey Landis, a physicist who works for NASA and writes SF, has discussed SPS alternatives extensively. He has several interesting papers at his web-page...
Landis technical papers online
...scroll down and you will find the SPS section. The first article in the list is the most recent and well worth a read. He proposes a serious rethink of all assumptions about SPS and then describes a system that is nearly viable for power supply today.
His next article on Super-Synchronous SPS provides a good description of an ultra-light weight system. Compare it to the old DoE/NASA SPS satellites - its inflatable, uses advanced concentrators, doesn't need a separate, rotating transmitter, can supply power from the beginning and only masses 1,300 tons for 1 to 2 gigawatts power supplied to the ground.
An alternative for transporting SPS is to deploy an array sub-unit at LEO and use its power for an ion/plasma drive to move it to GEO and/or L1/2. Potentially this approach will cut mass delivered to LEO to a mere 30% of trying to do it all via chemical rockets. The apparently defunct PowerSat corporation tried to patent this concept, but I believe that this article...
Electric Propulsion for SSPS
...indicates a prior useage.
Posted at 12:58 pm by Adam
In some ways the Russian Space Shuttle Buran is a superior vehicle to the USA's Shuttle. More payload, more fuel, better Isp, less dead weight. And no clunky Main Engines on the Orbiter - which doesn't use them in space. However that's the liability as well - can't launch it except via an Energia, and there aren't too many of them around. Fewer than the Saturn Vs.
But despite the sole Energia still sitting in its hangar while its boosters are being used by Sealaunch as Zenit launchers, Buran is a nice machine. Its OMS equivalent uses LOX/Kerosine with an Isp ~ 362 seconds. Now I'm not sure what the long term storage capability of LOX is - it is cryogenic, but it's not a "hard" cryogen like hydrogen. I don't think it has the boil-off issues that hydrogen does and certainly actively cooling it won't take as much power or equipment as LH2. For the deep space maneuvers needed to cruise to Saturn Cassini-style and to reach Titan safely, about 1,650 m/s dv is needed. An exhaust velocity of 3,550 m/s is better than the Shuttle's 3,072 m/s, needing less fuel and tankage.
Buran's payload bay is also a bit bigger, but I think its power system is a bit behind the Shuttle's. All in all it would be potentially a better vehicle to send to Saturn than the NASA Shuttle Orbiter.
Encyclopedia Astronautica on Buran
Posted at 12:37 pm by Adam
Sunday, July 25, 2004
New Scientist has just published a possible experimental falsification of several interpretations of Quantum Mechanics, notably the Copenhagen and Many Worlds Interpretations. What remains viable is John Cramer's Transactional Interpretation - that the quantum wave-function travels forward and backward in time in any interaction, creating a "handshake" to exchange energy/momentum/spin etc. Cramer's daughter, Kathryn, has placed a number of links and discussions of the new claim online here...
Kathryn on Quantum
...the Physics community has been abuzz about this one since at least April. Would be nice to see more replications of Shahriar's experiment.
John's site is here...
...he discusses his Interpretation here...
Transactional Interpretation Explained
Explained Clearly [this is the more accessible explanation]
...if this pans out then it will have interesting consequences. Many Worlds has a lot of advocates and won't die easily. But for my money it's a philosophical mess. Creates a new World for every quantum choice which seems rather excessive. But have a read of this one...
Many Worlds in one
...in an infinite Universe all physical histories are physically real - somewhere Out There.
John Cramer has written extensively on wormholes at a popular level. Here's an interesting discussion from a few years ago...
Posted at 12:32 am by Adam
Spin Gravity Adaptable...
A common criticism of 2001: A Space Odyssey is the size of the 1/6th gee centrifuge - "it's too small". At 35' across (radius = 5.334 m, g = 1.622 m/s^2) the centrifuge is spinning at ~ 5 rpm. Common objection is that Dave and Frank will be unable to adjust to the Coriolis forces, chucking up their SpaceFood packs for the whole trip. However Greg Benford and George Zebrowski in their anthology Sky Life note that 6 rpm is fine and up to 10 rpm is adaptable, based on the only real biomedical work done in the '60s. Well now it seems that the old data was right - NASA's latest news update...
Spin Gravity News
... test subjects could adjust to spin rates as high as 25 rpm. For seated tasks, mind you, but there is no in principle objection to walking around at lower spin-rates. At 6 rpm a one-gee centrifuge has a 25 metre radius, and a Mars-gee (~ 0.378 gee) just 10 metres. At 10 rpm the respective radii are ~ 9 metres and 3.4 metres. Providing substantial spin gravity should be feasible for relatively small vehicles and habitats - thus the "zero-gee is bad for us" objection to in-space living is missing the point: no one needs to live in zero-gee all the time.
Another point is that it makes inflatable habitats more attractive - toroids ("inner tubes") were extensively studied in the early 1960s. Easy to inflate and rotate. Shield it within a plasma sheath and the habitat will be safe from solar storms and the Jovian magnetosphere. And you can use the plasma shield as a plasma sail to gradually spiral out to any of the outer planets and their extensive moon systems.
Posted at 12:26 am by Adam
Friday, July 23, 2004
Discovery - a first estimate
More on flying to Titan in a Shuttle.
Firstly, how much does Discovery weigh?
77,564 kg empty, with main engines (at launch.)
68,682 kg empty, without (after launch.)
A Shuttle can carry 10,830 kg of OMS propellant, plus 3,273 kg RCS propellant - 14,103 kg total. Both systems use N2O4/MMH which is indefinitely storable in space, just like Cassini's rocket system.
The fictional Discovery's payload consists of :
ISS Habitation module (~ 10,000 kg)
Spacelab pressurised module as CELSS farm (~ 7,000 kg)
docking node (~1,800 kg)
2 slightly modified Apollo Command Modules (5,900 kg each)
several Russian Topaz nuclear reactors (~ 1,000 kg each)
All up, considering how vague Baxter was, that's about ~ 33,400 kg extra. The CELSS farm is supposed to recycle the crew's supplies, their waste dissociated by a Super Critical Water Reactor - extremely hot steam at high pressure which breaks up virtually all organics into basics. Oxygen can be cracked from carbon dioxide via electricity, the CO2 itself adsorbed by zeolite beds in the air system.
If we throw in supplies for five for ~ 600 days @ 1.27 kg/day.pp that's another 3,810 kg. Plus other odds and ends make it ~ 50,000 kg for life-support/Titan Base. Hence Discovery is about 133,000 kg, and about ~ 130,000 kg less the tailplane.
Fuel wise Discovery needs about 1,600 m/s of maneuvering to emulate Cassini's gravitational ballet to get to Saturn and deccelerate into a Titan approach. Using the OMS for maneuvers gives an Isp ~ 313.2 seconds (a kilo of propellant creates a kilo of thrust - 9.8 newtons - for 313.2 seconds) which means a 130,000 kg vehicle needs ~ 88,700 kg of propellant plus the propellant needed to shift the fuel tanks as well. Including the tankage (~0.11 fuel mass) it's about 106,700 kg total - less tankage saved by fuel kept in the OMS pods, means ~ 105,000 kg total. Baxter was about right with his vague two hundred thousand pounds which means we're on the right track.
Posted at 12:58 pm by Adam