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Jeff Bezos' secretive rocket company just revealed its plans totower over SpaceX
JF Mezei wrote:
On 2016-09-21 14:42, Rob wrote: I understood that it is weight of fuel that matters because the weight determines the amount of energy you get from it, and the volume varies by temperature. Airplanes have limits on how heavy they can be at takeoff and landing. Fuel represent a huge percentage of total weight. Every kilogram of fuel you load means one less kilogram of cargo you can load. I understand that the total weight of the aircraft is important, but my understanding is that the weight of fuel is calculated for the trip because the aircraft requires a specified weight of fuel to cover some distance. (this cannot be a linear function, though, because taking on more fuel means the plane will become heavier and consume more fuel, requiring even more fuel to make up for that) So they know they need "10000 kg of fuel" (say 21000 pounds for americans) for the trip, then they order that from the supplier and somewhere along the line it has to be converted to liters or gallons. Error-prone. I would say, make the fuel trucks deliver in kilograms, and all trouble is over. Maybe wasn's to easy with an old mechanical counter, but should be peanuts on a modern microcontroller based counter. The density of fuel is needed because fuel pumps work with volume, and the tanks on a plane measure volume. So they need the density to get more accurate evaluation of it weight. What I wonder about is how significant the varying density of fuel will be relative to the total volume. When the density of the fuel varies by 3% and then you add 10% at the end for safety margin, most or all of the trouble to include density is wasted and you introduce a new risk of introducing an error. To get somewhat back on topic, SpaceX has made strides in getting "supercooled" kerosene and I assume LOX in order to get more fuel loaded in the same space. Anyone know the rought weight difference between a normal launch and one with super cooled fuel ? Yes that is interesting, and in a space launch it is more interesting because much more prediction, calculation and checking can be done on a space launce than on an airliner trip. |
#52
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Jeff Bezos' secretive rocket company just revealed its plans to tower over SpaceX
JF Mezei wrote:
On 2016-09-21 04:11, Rob wrote: While I understand that weight of fuel is more significant to performance of the aircraft than volume, Needed to determine how much luggage/cargo can be loaded on aircraft, calculate centre of gravity and the V1/V2 speeds during takeoff. Not just that. Fuel consumption is usually given in pounds of fuel in the performance manuals. For flight length, either will work because computer can display "pounds per hour" or "litres per hour" for fuel consumption. But for takeoff, the plane's specs are in weight. You don't just say 'fill er up' and go fly. The quantity of fuel required for the flight (and diverts) is calculated before fueling. Performance figures are in MASS of fuel required. It doesn't matter what the airplane can display in. I find it confusing that the two are being used in parallel, and manual conversions are being made by pilots and ground personnel. In the case of the Gimli glider, pilots *should* have known that if they want 22 pounds of fuel, the rough equivalent would have been 10 litres. It's 12.5 litres, not 10 litres. And since fuel is lighter than water, the number of litres should have been north of 10. So if their math yielded 5 litres they *should* have seen a problem. Except we're not talking about little numbers like 5-10 litres here... -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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Jeff Bezos' secretive rocket company just revealed its plans totower over SpaceX
On Sep/21/2016 at 2:08 PM, Fred J. McCall wrote :
Alain Fournier wrote: On Sep/20/2016 at 5:37 PM, Fred J. McCall wrote : JF Mezei wrote: On 2016-09-20 13:30, Fred J. McCall wrote: Still a conversion factor. And still not an SI unit. farenheight is defined by freezing and boiling points of water. You can get a farenheit temperature wiothout needing to refer to another scale. Nautical Mile is defined in terms of metres, a SI meaure. You cannot get a nm without refering to the metre because it is defined as 1852 metres. Still a conversion factor. And still not an SI unit. No it isn't a conversion factor. It is the definition. Nautical miles, inches, pounds etc. are defined based on SI units. Coming to an agreement to REDEFINE something in terms of something else based on what the required conversion factor was does not make the new ratio magically into something new. Still a conversion factor. No it is not just a conversion factor. There is a conversion rule to go from Fahrenheit to Celsius degrees. If the definition of a Celsius degree was to be changed, you would have to change the conversion rule. Because it is a conversion rule not a definition. Not so for the pound-mass versus the kg. If the definition of a kg was to change, which will likely happen in the not too distant future, you don't have to change to conversion rule to go from kg to pounds. It is the value of a pound that would change, because a pound is defined to be 0.45359237 kg, whatever a kg is. Same for inches vs meters. The inch is defined as 2.54 cm, if the cm changes, not likely to ever happen again, then the inch changes, not the conversion rule. Alain Fournier |
#54
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Jeff Bezos' secretive rocket company just revealed its plans to tower over SpaceX
JF Mezei wrote:
In the case of Gimli, their ordered in litres. Pilots made error converting from whatever they had calculated they needed into litres. BTWm the Flight did Montreal-Ottawa-Edmonton, and at the stop in Ottawa, they made the exact same mistake, ran out of fiew before reaching winnipeg. No, they didn't. They ordered in KILOGRAMS. The GROUND CREW used the wrong conversion factor to calculate the number of litres of fuel required. So fuel variations of 3% when you are managing your required 10% to bare minumum is a huge "error" and I suspect they are very careful about that. To get a 3% swing in fuel volume you would need a preposterous temperature swing. Coefficient of expansion of kerosene is something like 0.00099 per degree Celsius, so a 3% change in volume would require a temperature swing of some 30+ degrees Celsius from 'standard'. -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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Jeff Bezos' secretive rocket company just revealed its plans to tower over SpaceX
JF Mezei wrote:
On 2016-09-21 15:20, Fred J. McCall wrote: You don't just say 'fill er up' and go fly. Suggest you want a documentary called "Octopussy" where a british secret agent does just that after running out of fuel and landing near a gas station :-) Suggest you're enough of an idiot without trying to make jokes. want 22 pounds of fuel, the rough equivalent would have been 10 litres. It's 12.5 litres, not 10 litres. 1 litre of H2O = 1kg = 2.2 lbs. so 22 pounds is 10 litres. We don't fuel jets with H2O. We fuel them with Jet-A or similar, which has a density of around 0.8 kg per litre. Do the math. You'll get 12.5 litres for 22 pounds. Except we're not talking about little numbers like 5-10 litres here... They loaded about half the needed fuel on the plane. The the order of maginture of their error is far greater than the density difference between water and fuel. And less dense fuel would have taken more litres than if they had ordered the same weight of water. If you take pounds of fuel needed and divide by 2.2, you get litres of water to match those pounds. In fuel, the numbe rof litres would be somewhat higher. If your calculated number is half of what you need for water instead of being slightly higher, then you shoudl spot a mistake has been made. Except nobody checked after the fact to see what they had. This is normally the duty of the flight engineer. B767 has a 2-person cockpit crew (no flight engineer) and the duty of checking the fuel level was never reallocated by Air Canada for this type of aircraft. So it didn't matter what they used, since they didn't look. -- "Ignorance is preferable to error, and he is less remote from the truth who believes nothing than he who believes what is wrong." -- Thomas Jefferson |
#56
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Jeff Bezos' secretive rocket company just revealed its plans totower over SpaceX
Fred J McCall wrote:
So fuel variations of 3% when you are managing your required 10% to bare minumum is a huge "error" and I suspect they are very careful about that. To get a 3% swing in fuel volume you would need a preposterous temperature swing. Coefficient of expansion of kerosene is something like 0.00099 per degree Celsius, so a 3% change in volume would require a temperature swing of some 30+ degrees Celsius from 'standard'. That is why I question the reasoning that fuel has to be calculated in weight and then converted to volume "because of variations in the fuel density". There will be some difference but it will be very small. Important for space flight, but probably not for aviation. |
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