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Post by ka9q on Jan 17, 2011 20:08:19 GMT -4
Hi Bob, on your page about the CM you say
Power: batteries; 20.0 kW, 1000 Ah
I don't understand that. The CM alone had five silver-zinc batteries. (Silver-zinc was the workhorse of the space program; every battery on the Saturn, LM and CSM was of that type.) Two were small 'pyro' batteries dedicated to firing the various explosive devices to isolate the big current spikes from the other CM systems. Three were 'entry' batteries that powered the CM after SM jettison shortly before entry, and also covered peaks when the fuel cells alone could not provide enough power, particularly the power-hungry gimbals in the Service Propulsion System engine. The batteries were recharged after each peak load period.
They had a nominal voltage of 28V and a nominal capacity of 40 AH each, though both numbers were a little conservative; 30V and 45 AH were closer to the actual values. The conservative figures would give the three batteries a total energy capacity of 3.36 kWh.
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Post by ka9q on Jan 17, 2011 20:57:27 GMT -4
On the next page on the Service Module you say
Power: fuel cells; 670 kW total, 6.3 kW average
The three fuel cells in the SM produced between 400 and 1420 watts each, or 1200-4260 watts total, at 27-31 VDC. Each cell had to operate at a minimum of 400 W to make enough waste heat to keep the electrolyte molten; if shut down in flight a fuel cell could not be restarted and would likely suffer damage.
Demand peaks above 4 kW were covered with the entry batteries in the CM, which had to be recharged later.
The total fuel cell energy depended on the mission. On Apollo 11, which lasted 195 hours, the three fuel cells supplied 393 kWh at an average of 68.7 amps (22.9 A/fuel cell) and an average bus voltage of 29.4V. That's an average load of about 2 kW, within the nominal load when the load is divided equally among the three fuel cells. Two fuel cells would have been enough to complete the mission and one is enough to conduct a safe abort.
Starting with Apollo 14, the SM carried a 400 A-h silver-zinc auxiliary battery of the same type used in the lunar module descent stage. This was to provide an additional safety margin in the event of a serious problem with the fuel cells as happened in Apollo 13 when the entry batteries were deeply discharged before the CM could be shut down.
Two more aux batteries were added to the Skylab SMs. Since Apollo's fuel cells could not be shut down to conserve reactants, and the SM could not carry enough to last for the entire stay, they shut down after several weeks. The aux batteries were used to power the return from the station.
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Post by ka9q on Jan 17, 2011 21:02:12 GMT -4
Another minor comment on the SM: I believe the RCS was fueled with MMH (monomethyl hydrazine), as opposed to the use of Aerozine-50 in the SPS. The LM, on the other hand, fueled its RCS with Aerozine-50 so it could share supplies with the main engines (APS and DPS).
I'm not sure why, but MMH seems to be the fuel of choice for small maneuvering engines while Aerozine-50 is the usual choice for larger engines.
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Post by ka9q on Jan 17, 2011 21:26:21 GMT -4
And on the LM descent stage, you give the electrical power as:
Power: batteries; 33.0 kW, 1600 Ah
1600 Ah (actually four 415 Ah silver-zinc batteries) is correct for the LMs through Apollo 14; the J missions (Apollos 15-17) carried a fifth 415 Ah battery for a total of 2000 Ah. At a nominal 28 V and 1600 Ah, that's 44.8 kWh for the pre-J LMs and 56 kWh for the J LMs. Because the buses usually operated at a higher voltage, the actual delivered energy was somewhat greater.
Because the LM depended entirely on batteries, power was far more critical than on the CSM with its fuel cells. When fully powered up during powered descent, Apollo 11's LM Eagle drew 2.3 kW but it was powered down to conserve the batteries at other times. On the way to the moon the LM drew power from the CSM via an umbilical to conserve the LM's batteries. During the Apollo 13 emergency, Aquarius drew only about 300 W to power life support and communications.
Each 415 Ah battery weighed 135 pounds (61.2 kg). That's about 200 watt-hours/kg, which is actually pretty good even by today's standards. Had the SM also used these batteries, it would have needed 2 tonnes of them to power the Apollo 11 CSM through its mission. The fuel cell really made a difference.
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Post by ka9q on Jan 17, 2011 22:13:42 GMT -4
...and on the ascent stage, you give:
Power: batteries; 17.0 kW, 800 Ah
The ascent stage carried two 296 Ah Ag-Zn batteries weighing 125 lb (56.7 kg) each, for a nominal capacity at 28V of 16.6 kWh. I believe this was constant through all the production models.
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Bob B.
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Post by Bob B. on Jan 17, 2011 23:49:27 GMT -4
Rather than me trying to digest everything you just wrote about the batteries and fuel cells, can you make it simple and just tell me what I should write in those four placeholders? It looks like the answer should be:
Command module: batteries; 28 V, 3.36 kWh Service module: fuel cells; ? ? LM descent stage: batteries; 28 V, 44.8 kWh, uprated to 56 kWh for J-series LM ascent stage: batteries; 28 V, 16.6 kWh
I'm not sure what to put in there about the fuel cells, can you give me the short version?
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Bob B.
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Post by Bob B. on Jan 18, 2011 0:36:30 GMT -4
Another minor comment on the SM: I believe the RCS was fueled with MMH (monomethyl hydrazine), as opposed to the use of Aerozine-50 in the SPS. I've seen conflicting information on that. The Apollo Spacecraft News Reference, which was prepared by North American Aviation, says the fuel was a "50-50 mixture of hydrazine and UDMH", which is Aerozine 50. However, I've seen other sources that say it was MMH. I'm not sure why, but MMH seems to be the fuel of choice for small maneuvering engines while Aerozine-50 is the usual choice for larger engines. MMH and Aerozine 50 both yield similar specific impulse, so there is no real advantage of one over the other in that regard. MMH's advantage is in its freezing and boiling points, which are -52.4 C and 87.5 C respectively. For comparison, Aerozine 50's freezing and boiling points are -7 C and 70 C. The wider liquid range of MMH (particularly its low freezing point) makes it better for use in space because it's an easier thermal control problem. On the other hand, MMH lacks the thermal stability to make it suitable for regenerative cooling. In large regenerative cooled hypergolic engines, such as those on the Titan II, UDMH or UDMH-hydrazine blends are used. You'll only see MMH used in engines that don't use regenerative cooling. Most spacecraft engines and RCS thrusters are cooled by radiation or ablation or some combination. If Apollo were done today, I'd guess the SPS, DPS and APS would all use MMH, as that seems to be more common these days. I'm speculating that Aerozine 50 was used because we had more experience with it back then, having been used in the Titan missile. It was Aerojet, the builder of the SPS engine, that developed Aerozine 50 and built the Titan engines.
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Bob B.
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Post by Bob B. on Jan 18, 2011 1:45:52 GMT -4
The total fuel cell energy depended on the mission. On Apollo 11, which lasted 195 hours, the three fuel cells supplied 393 kWh at an average of 68.7 amps (22.9 A/fuel cell) and an average bus voltage of 29.4V. That's an average load of about 2 kW, within the nominal load when the load is divided equally among the three fuel cells. Two fuel cells would have been enough to complete the mission and one is enough to conduct a safe abort. Here what I've come up with... Apollo 11 used 34.8 pounds of hydrogen, so based on your numbers, that's about 11.3 kWh per pound. The H-missions were loaded with an average of about 53 pounds of hydrogen, so if all the reactant was used, that's about 600 kWh. The J-missions were loaded with an average of 80 pounds of hydrogen, so that gives about 900 kWh if all were used. I've therefore edited the page to read: Power: fuel cells; total 600 kWh H-series, 900 kWh J-series; 2 kW average, 4 kW peak If that's not correct, how do you recommend I edit it?
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Post by ka9q on Jan 18, 2011 4:56:05 GMT -4
MMH's advantage is in its freezing and boiling points, which are -52.4 C and 87.5 C respectively. [...] On the other hand, MMH lacks the thermal stability to make it suitable for regenerative cooling. That explains it very nicely! That suggests the SPS, DPS and APS engines were regeneratively cooled, though I haven't checked. Otherwise MMH would have been the fuel of choice, especially in the LM given the severe thermal environment of the lunar surface, without the CSM's ability to reorient itself or conduct a barbecue roll. I think even some smaller pressure-fed engines are regeneratively cooled. AMSAT has flown three surplus MBB 400N engines of the type used on Galileo, and IIRC I saw a fuel line running to a cooling jacket around the throat. That would explain why they didn't use MMH.
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Post by ka9q on Jan 18, 2011 5:22:54 GMT -4
just tell me what I should write in those four placeholders? It looks like the answer should be: Command module: batteries; 28 V, 3.36 kWh Service module: fuel cells; ? ? LM descent stage: batteries; 28 V, 44.8 kWh, uprated to 56 kWh for J-series LM ascent stage: batteries; 28 V, 16.6 kWh I'd say something like: The CSM and LM had 28V DC electrical systems. Inverters produced 115V 3-phase 400 Hz AC for equipment needing it. Service module power supply: three H2/O2 alkaline fuel cells, 400-1420 W each; 415 Ah Ag-Zn aux battery added to Apollos 14-17. Command Module: three Ag-Zn batteries, 45 Ah each (about 3.9 kWh total). Used during demand peaks and recharged from fuel cells; provided all CM power after SM separation shortly before entry. Average CSM power consumption: 2 kW (Apollo 11) LM descent stage: four 415 Ah Ag-Zn batteries through Apollo 14 (48 kWh total); five 415 Ah Ag-Zn batteries Apollos 15-17 (60 kWh total) LM ascent stage: two 296 Ah Ag-Zn batteries (17 kWh total) Peak LM power consumption: 2.3 kW (Apollo 11 during powered descent) I figured the kilowatt-hour totals for 29 V, which seems like a good average.
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Post by ka9q on Jan 18, 2011 5:53:10 GMT -4
The J-missions were loaded with an average of 80 pounds of hydrogen, so that gives about 900 kWh if all were used. Sounds about right. All the missions had generous margins on fuel cell reactants. As I recall, they generally had about half left at SM jettison. It takes eight kg of oxygen to oxidize one kg of hydrogen and make nine kg of water. The enthalphy of formation for water is -241.82 kJ/mol. A mole of water is 18 grams, so that's 13.434 MJ or 3.73 kWh per kg of H2O produced or 120.91 MJ or 33.59 kWh per kg of H2 consumed. Your figure of 11.3 kWh/lb H2 corresponds to 24.9 kWh/kg, for a fuel cell efficiency of 74%. The remaining 26% kept the fuel cell at operating temperature and was dumped to space through the EPS radiators. The Ag-Zn batteries had an energy density of about 200 watt-hours/kg, so even at 74% efficiency the fuel cell reactants have about 18.7 times the energy density of the batteries. Of course that overstates things since I haven't included the weights of the H2 and O2 tanks, their associated piping, and the fuel cells themselves. But I'm sure the fuel cells would still come out well ahead of batteries. (I have the Apollo fuel cells as 100 kg each, but I have no figures for the tanks and piping. The H2 tanks can be charged to the electrical power system, but at least some of the O2 tank mass can be allocated to the life support system.) Given that the crew needed water to drink and the CSM needed some for backup cooling, water would have had to be carried even if electricity was generated in some other way. So you might as well launch hydrogen and oxygen and get some energy in the process. As temperamental as those fuel cells could be, they were a pretty clever win-win situation for Apollo.
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Bob B.
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Post by Bob B. on Jan 18, 2011 10:11:22 GMT -4
That suggests the SPS, DPS and APS engines were regeneratively cooled, though I haven't checked. Otherwise MMH would have been the fuel of choice, especially in the LM given the severe thermal environment of the lunar surface, without the CSM's ability to reorient itself or conduct a barbecue roll. Actually the engines were not regenerative cooled. That's why I suggested that perhaps the choice of Aerozine 50 was due to greater experience and comfort level. Particularly considering that Aerojet, who had developed the Titan engines, also built the SPS engine. Other than that, I don't know why Aerozine 50 was picked over MMH, as MMH seems to me to be the better choice. Also, Gemini used MMH, so it wasn't without precedence. All three engines - SPS, DPS and APS - had thrust chambers lined with an ablative material to dissipate heat. The SPS and DPS engines had nozzle extensions that were radiation cooled. The APS engine had an ablative nozzle extension. As a side note, the Space Shuttle uses MMH in both its OMS and RCS. Most other post-Apollo-era spacecraft that I've investigated also use MMH, with nitrogen tetroxide as the oxidizer. That seems to be the standard today. On the other hand, all the big launch vehicles with regenerative cooled engines use UDMH or blended fuels - Titan (Aerozine 50), Ariane (UH25), and Chang Zheng (UDMH). UDMH has the thermal stability for regenerative cooling but lacks the performance of MMH or hydrazine. Aerojet figured out that by mixing UDMH with hydrazine, they could get a performance comparable to MMH while still being stable enough for regenerative cooling. However, they sacrificed some freezing point in doing so. Straight hydrazine has a performance better than either MMH or Aerozine 50, but it has freezing point issues at 1.4 C. Hydrazine is rarely used outside of monopropellant thrusters, for which it is ideal. Hydrazine tanks often require heaters to keep it from freezing. That's fine for a small RCS system, but you don't want to have to heat your main propellant tanks. I recently purchased Tom Kelly's book about the building of the LM, which I haven't had a chance to read yet. Perhaps in it he discusses the reasons for choosing Aerozine 50.
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Bob B.
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Post by Bob B. on Jan 18, 2011 10:24:18 GMT -4
just tell me what I should write in those four placeholders? It looks like the answer should be: Command module: batteries; 28 V, 3.36 kWh Service module: fuel cells; ? ? LM descent stage: batteries; 28 V, 44.8 kWh, uprated to 56 kWh for J-series LM ascent stage: batteries; 28 V, 16.6 kWh I'd say something like: The CSM and LM had 28V DC electrical systems. Inverters produced 115V 3-phase 400 Hz AC for equipment needing it. Service module power supply: three H2/O2 alkaline fuel cells, 400-1420 W each; 415 Ah Ag-Zn aux battery added to Apollos 14-17. Command Module: three Ag-Zn batteries, 45 Ah each (about 3.9 kWh total). Used during demand peaks and recharged from fuel cells; provided all CM power after SM separation shortly before entry. Average CSM power consumption: 2 kW (Apollo 11) LM descent stage: four 415 Ah Ag-Zn batteries through Apollo 14 (48 kWh total); five 415 Ah Ag-Zn batteries Apollos 15-17 (60 kWh total) LM ascent stage: two 296 Ah Ag-Zn batteries (17 kWh total) Peak LM power consumption: 2.3 kW (Apollo 11 during powered descent) I figured the kilowatt-hour totals for 29 V, which seems like a good average. That's all great information, but it might be a little too much for the particular slides that you reference, which are supposed to included just a few quick bullet points. I discuss some of the other details, such as type of batteries and the use of inverters, on the "Electrical power" slide (#61). However, I do have the following web page: www.braeunig.us/space/specs.htmthat gives more detailed information and specs about the spacecraft. The new data you've given me is perfect for this; I'll revise the Apollo pages accordingly. I don't remember what the source of the original bogus data was.
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Bob B.
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Post by Bob B. on Jan 18, 2011 10:35:03 GMT -4
Sounds about right. All the missions had generous margins on fuel cell reactants. As I recall, they generally had about half left at SM jettison. I checked the hydrogen consumption for Apollos 11-14, 15-17 and on average they used about 74% of the initial supply. They might have had a larger margin on oxygen, but I didn't check that because I was only interested in the fuel cells. I don't have the information to know how much of the oxygen went to just the fuel cells versus the environmental control system.
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Bob B.
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Post by Bob B. on Jan 18, 2011 11:46:50 GMT -4
I kind of remember there being an incident on Apollo 11 where the LM fuel lines started to freeze while they were on the Moon. It caused some anxious moments before they eventually warmed up and melted prior to ascent. Does anybody recall anything about that and can confirm, deny, or elaborate on the incident?
(Had those lines frozen, I bet they wished they used MMH instead of Aerozine 50.)
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