In what little knowledge of space and rockets I have, it seems to me that the further the object your aiming for, the more fuel you’ll need, and the bigger engines you’ll need. Having compared the height of the Atlas and the Saturn rockets, the Saturn rocket appears to be much taller (~48.8m and ~110.6m respectively), it doesn’t really make sense to me as to why the smaller rocket went further. Could someone explain how/why it works?
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asked Sep 12, 2022 at 23:13
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24$\begingroup$ Hint: Compare the masses of the payloads each rocket had to deliver to their intended destinations. $\endgroup$– notovnySep 12, 2022 at 23:27
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$\begingroup$ Because they needed to deliver more on a mission that took more delta v @Blue Skin and Glowing Red Eyes $\endgroup$– Starship is go for launchMar 2 at 11:01
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1 Answer
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The first thing to keep in mind is that once you're moving in space, you keep moving in the same direction unless a force acts to change your trajectory. So distance, in itself, is not a factor.
What is a major factor is gravity. Both the Apollo lunar missions and the outer planet/extrasolar missions like Voyager (Titan IIIE) or New Horizons (Atlas) had to escape Earth's gravity. The outer planet missions had to build up enough speed to escape the sun as well, but the Apollo missions had to:
- brake into lunar orbit instead of flying right past;
- slow the orbiting lunar module further, to drop it to the moon's surface;
- brake while descending to the moon to land the LM softly;
- lift the LM's ascent stage against the moon's gravity, back to lunar orbit to rejoin the command/service module;
- boost out of the moon's orbit to head back to the Earth.
All of that takes a great deal of fuel, and instead of doing it with spacecraft massing less than one ton (478 kg New Horizons, 825 kg Voyager), it had to be done with spacecraft large enough to keep a crew alive, carry their experiments and moon rocks back and forth, and so on. The LM ascent stage alone was over two tons dry; the total mass of the command/service module and LM sent to the moon was over 40 tons.
answered Sep 13, 2022 at 0:41
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21$\begingroup$ It's also not just how much stuff you want to throw there, but how fast you want to throw it there. Capstone takes 5 months to get to the Moon, Apollo took less than 5 days. $\endgroup$ Sep 13, 2022 at 13:38
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6$\begingroup$ Just in case anyone is confused, once you're moving, you keep moving in the same direction unless a force acts to change your trajectory. This is true whether or not you're in space, it's just that space usually has less friction. $\endgroup$ Sep 14, 2022 at 17:34
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3$\begingroup$ All very true. The propellant mass needed to lift off from the Moon also needs to be landed on the Moon using propellant. Both need further propellant to get to the Moon and all three require more propellant still to get to Earth orbit in the first place. $\endgroup$– SlartySep 14, 2022 at 17:57
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$\begingroup$ @AdamChalcraft it should be noted that gravity is a force acting on all bodies in the solar system which is why you need fuel to get to places in space. $\endgroup$ Sep 14, 2022 at 20:58
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$\begingroup$ @AdamChalcraft Worth noting that this means nothing keeps moving in the same direction, ever. $\endgroup$ Sep 14, 2022 at 22:12