Why does it take 3 days to get to the moon?

[askold]

Earth escape velocity is about 25,000 mph, the moon is about 250,000 miles away - sounds like a 10 hr trip. Less time than an LA to Tokyo flight.<br /><br />

 

[harmonicaman]

Well; when you achieve escape velocity that just puts you into orbit around the Earth; from orbit, you then need to add more thrust to actually move away from the Earth.

 

[CalliArcale]

You mean orbit alongside the Earth, right? If you've escaped Earth, you're orbiting the Sun. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[askold]

I thought that orbital velocity was 17,500mph. And "escape velocity" was what took you out of orbit and flying free in space.

 

[askold]

OK - so why the 3 days? Did Apollo take the scenic route??

 

[vogon13]

Apollo was constrained by using the moon in a figure 8 orbit for a possible return to earth with out lunar orbit (Apollo 13 ring a bell?)<br /><br />Also, faster you are going as you pass the moon , the more fuel it takes to go into orbit. Apollo was rather constrained, delta v wise, hence the pokey trip.<br /><br />Mariner 10 crossed the moon's orbit (and fairly close to the moon, too, but it didn't stop) in ~10 hours while on its' way to Mercury, so such flight plans do exist and have been flown.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[askold]

Ok - that makes sense. Thanks.<br /><br />Too bad the short route is undesirable due to the constraints. Otherwise you'd barely be able to get in two meal services on the way to the moon ...

 

[Leovinus]

Also, don't forget that the faster you want to go, the more fuel you need. We built a hell of a launch vehicle and we needed every drop of fuel to send the payload at 25K mph to the moon. As the payload climbed out of the gravity well, it slowed down all the way. I think I read somewhere that by the time it entered the Moon's sphere of influence, the CSM/LEM was down to 1000mph. Of course, then it picks up speed again as it falls to the Moon.<br /><br />Another factor is that the more speed you have going to the moon, the more speed you'll have when you get there. So that takes even more fuel for the slow-down burn and more fuel on launch to carry that extra slow-down fuel etc. A viscous cycle. It's a wonder that they even made it there at all. The math makes my head hurt. <div class="Discussion_UserSignature"> </div>

 

[CalliArcale]

Actually, it's worth pointing out that it wasn't quite that easy with Apollo 13. They were no longer on the free return trajectory when the accident happened! They had completed the first burn already, which set them up for lunar orbit insertion but made the free return impossible. That's why they had to burn the LEM's engine. Engineers on the ground made careful calculations to use the moon's gravity to get them back onto a free return trajectory. It was pretty dicey. There was a very real possibility of sending them hurtling out into heliocentric orbit, where they would die as soon as their life support ran out.<br /><br />It did set the Apollo 13 crew up for a rather interesting record: the three of them share the human altitude record, at 401,056 km. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[Leovinus]

Which crew has the speed record? Was it 13 as well? <br /><br />Edit: Answer follows<br /><br /> Class Record Setter Speed Date Set<br />Insect Australian Dragonfly 36 mph<br />(58 km/h) n/a<br />Bird (level flight) Red-Breasted Merganser 80 mph<br />(129 km/h) n/a<br />Bird (dive) Peregrine Falcon 217 mph<br />(349 km/h)<br />in a 45° dive n/a<br />Autogyro WA-116F 120.5 mph<br />(193.76 km/h) 18 September 1986<br />Rotorcraft Westland Lynx 249.10 mph<br />(400.55 km/h) 11 August 1986<br />Biplane Fiat CR42B 323 mph<br />(520 km/h) 1941<br />Piston-Powered Seaplane Macchi MC72 440.68 mph<br />(709.21 km/h) 23 October 1934<br />Piston-Powered Aircraft Grumman F8F Bearcat 528.33 mph<br />(849.55 km/h) 21 August 1989<br />Turboprop-Powered Aircraft Tupolev Tu-114 545.07 mph<br />(876.47 km/h) 9 April 1960<br />Jet-Powered Flying Boat Beriev M-10 566.69 mph<br />(911.24 km/h) 7 August 1961<br />Jet-Powered Aircraft Lockheed SR-71A 2,193.16 mph<br />(3,326.60 km/h)<br />Mach 3.3 28 July 1976<br />Rocket-Powered Aircraft North American X-15A-2 4,520 mph<br />(7,274 km/h)<br />Mach 6.72 3 October 1967<br />Winged Vehicle Space Shuttle Columbia<br />on re-entry ~ 17,000 mph<br />(27,340 km/h)<br />Mach 25 14 April 1981<br />Manned Vehicle Apollo 10 capsule<br />on re-entry ~ 24,790 mph<br />(39,885 km/h)<br />Mach 36 26 May 1969<br />Interplanetary Vehicle Voyager I ~ 38,600 mph<br />(62,070 km/h) launched<br />5 September 1977<br />Manmade Object Helios 2 ~ 150,000 mph<br />(241,350 km/h) 17 April 1976 <div class="Discussion_UserSignature"> </div>

 

[askold]

So, is it likely that NASA's new moon initiative is going to follow the same route - a "safe" trajectory that allows an orbit insertion abort and a return to Earth? Or have some parameters changed over the last 30 years?<br /><br />Time in flight takes away from time on the moon.

 

[nexium]

Escape velosity depends on the details of the definition and typically is not a very helpful number. The bottom line is we are climbing out of Earth's gravity well, until we get close enough to the Moon to be in the Moon's gravity well. To take 3 days, most of the trip (perhaps all) is at less than 25,000 miles per hour. The craft may be going slower than 1000 miles per hour when it makes the transition from Earth gravity well to Moon gravity well. Also the total distance traveled may be a million miles as this takes less energy than shorter routes. At an average speed of 14,000 miles per hour, it takes 72 hours = 3 days to reach the Moon over a path one million miles long. A trip to the moon can be done without orbiting Earth until you land on the Moon's surface. Neil

 

[nexium]

72 hours to the moon is an average speed of 4000 miles per hour, if the length of the path traveled is 288,000 miles. A higher average speed would take more fuel and would arrive at the moon going too fast, so lots more fuel would be needed to get in orbit around the moon or make a soft landing on the moon. Neil