115 votes
Accepted

Why is it easier to escape the solar system than get to Mercury or the Sun?

Because the earth goes very fast around the sun. If you want to get to the sun, you need to slow down almost completely so that your speed relative to the sun becomes almost zero. If you don't slow ...
Speedphoenix's user avatar
  • 5,324
47 votes
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Delta-v to hit the moon: is reaching Lunar L1 enough?

This is an excellent and fun question, bravo. First, the table is being read incorrectly, here's how to properly read it: For example, to go from LEO-Ken to EML1 is given as 3.77 km/s, not 0.77 km/s. ...
BrendanLuke15's user avatar
46 votes
Accepted

Do you need 0 km/s velocity to crash into the sun?

Wouldn't i inevitably spiral to sun surface even if i was faster than 0km/s ? No. On reasonable timescales, an orbit will have a fixed distance of closest approach, called "periapsis." (These ...
Erin Anne's user avatar
  • 11.3k
37 votes

Why is it easier to escape the solar system than get to Mercury or the Sun?

Changing orbits requires delta-v. To reach the Sun, you need to subtract delta-v such that your velocity relative to the Sun is near zero, which allows you to "fall straight down" into the ...
Nuclear Hoagie's user avatar
37 votes
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What does `ln` mean in the Delta V equation?

ln is a math function, the "natural log" Most scientific calculators have a key for it. See https://en.wikipedia.org/wiki/Natural_logarithm
Organic Marble's user avatar
37 votes

Going over the Apollo fuel numbers and I have many questions

Data isn't wrong. The LEM was a 2 stage vehicle, with separate engines and tanks for landing and ascent. The bottom part was the landing stage, the top part was the ascent stage. The fuel figures were ...
GdD's user avatar
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34 votes
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How could 99942 Apophis, in 2029, be captured and brought into a low Earth orbit?

The edge of Earth's Hill Sphere is about 929000 km, so in order to capture Apophis, it needs to decelerate from a hyperbolic orbit to an elliptical one with an apogee of at most 929000 km. In 2029, ...
WarpPrime's user avatar
  • 3,125
32 votes

Is it really ~648.69 km/s delta-v to "land" on the surface of the Sun?

Addressing is the sun's mass and other quantities known well enough for this to be absolutely accurate? Well, the key to this is the vis-viva equation in your question. It's not actually important ...
hobbs's user avatar
  • 947
30 votes
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How easy is it to take off from the Moon?

The amount of delta-V needed to get from the Moon to low Moon orbit is only 1.87 km/s or about 1/5 of that needed to get from Earth to LEO. That amount is easily attainable even if your ascent vehicle ...
Hobbes's user avatar
  • 128k
28 votes

Is it really ~648.69 km/s delta-v to "land" on the surface of the Sun?

The Vis-viva equation is $$ v = \sqrt{ GM \left(\frac{2}{r} - \frac{1}{a} \right) }, $$ The $GM$ product for the Sun is 1.327E+20 m^3/s^2. If 1 AU is 150E+09 meters, then when you are in a ...
uhoh's user avatar
  • 149k
25 votes

Why is it easier to escape the solar system than get to Mercury or the Sun?

Escaping the solar system requires adding orbital velocity to the spacecraft. Similarly, getting closer in the solar system requires removing orbital velocity. It turns out Earth is more out of the ...
Phil Frost's user avatar
  • 1,033
24 votes
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Does it take more energy to get to Venus or to Mars?

To flyby or impact Venus varies from 3.45 to 3.6 km/s from LEO for the optimal time every 19 months. Mars varies from 3.55 to 3.9 km/s for the optimal time every 26 months. So on average, getting to ...
Mark Adler's user avatar
  • 58.2k
24 votes

Going over the Apollo fuel numbers and I have many questions

According to the numbers just the fuel in the LM weighed 53,000lbs. I'm intimately familiar with delta-V, so big???Roughly 6380 gallons, which would require a tank 4ft high by 90ft long to store. I've ...
Russell Borogove's user avatar
21 votes
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What are the benefits of supersynchronous transfer orbits?

This is a partially copied answer from this closely-related question: The other answerer focuses on the straight-up dV savings which occur when you're launching from a very inclined launched site. I'm ...
Anton Hengst's user avatar
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20 votes
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Delta-v to move from GEO to GEO

Theoretically, you can go anywhere in GEO for an arbitrarily small ∆v - you raise your apogee a little bit, which slows you down, wait until you've phased to your destination latitude, then re-...
Russell Borogove's user avatar
19 votes
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What would it cost to bring a 1 gram payload to the CMBR rest frame (i.e. Δv 368 km/s ?)

With current technologies, this is unfortunately well outside our reach. However, there is promise on the horizon! Chemical Engines The Tsiolkovsky equation is always your friend when calculating Δv ...
Jack's user avatar
  • 9,976
19 votes

Do you need 0 km/s velocity to crash into the sun?

You need below 2866 m/s of orbital velocity at 1 AU to crash into the Sun. You technically don't need to slow down exactly to 0 m/s relative to the Sun in order to crash into it. Let's calculate the ...
Star Man's user avatar
  • 5,928
19 votes

Has there ever been a completely solid fuelled orbital rocket?

There have been a few purely solid-fuel orbital rockets over the years. The first was the Scout from 1961; the only ones in current use are the Long March 11 and the Minotaur/Minotaur-C family. ...
Mark's user avatar
  • 15.3k
19 votes
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What is the equation that relates delta v of a rocket to time taken to complete an orbit?

There is actually no relationship between the two. In a simple 2-body problem, each orbit starts and ends in the same place with the same velocity vector. The "delta" in "delta-v" ...
Woody's user avatar
  • 21.6k
19 votes

At which direction do you apply thrust to perform an inclination change maneuver?

To change an orbit's inclination, you need to apply thrust in the normal/anti-normal direction. The normal/anti-normal direction are some of the 6 main directions you can change your velocity. There ...
The Rocket fan's user avatar
18 votes
Accepted

Will crewed vehicles ever follow multi-flyby trajectories?

Given the mass costs in terms of consumables and the risk and support costs of keeping humans in space for longer, it seems unlikely that the multiple Earth-Venus flybys used by a lot of robot probes ...
Steve Linton's user avatar
  • 19.6k
18 votes

Going over the Apollo fuel numbers and I have many questions

The table on page 295 that you are looking at in Apollo by the Numbers is in fact just showing propellant consumed by the descent module. Since you are looking only at the Apollo 11 numbers then I ...
Steve Pemberton's user avatar
17 votes

Why do delta-v and delta-v both use the same term?

There's no conflict here. Because delta-v is a scalar figure, the result of applying velocity changes in different directions to a single object in 3 dimensional space over a period of time doesn't ...
Russell Borogove's user avatar
17 votes

Do you need 0 km/s velocity to crash into the sun?

And note that if you want to hit the sun the cheaper (but slow!) way to do it is to head out. 12.32km/sec will take you to infinity, at infinity a burn of 0m/sec will kill your orbital velocity and ...
Loren Pechtel's user avatar
16 votes
Accepted

Why is the Orion capsule using 2 burns to transfer from the moon back to earth instead of one?

The drawing in this "flight plan" indeed looks like it would be trivial to get into a direct Earth return trajectory without doing two burns and a low fly-by at the Moon. The actual ...
asdfex's user avatar
  • 15k
16 votes
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Delta V required to land and then ascend from the surface of every celestial body

Most search results I get for "solar system delta-v map" includes this information. I quite like this one: To read out the ascent cost from these maps, you take the single number between ...
SE - stop firing the good guys's user avatar
15 votes

What would it cost to bring a 1 gram payload to the CMBR rest frame (i.e. Δv 368 km/s ?)

Jack did a great job describing how to do it using propulsive engines. I have a different answer: We already (almost) planned to do it (inadvertently). The original plan for the recently launched ...
asdfex's user avatar
  • 15k
14 votes

Does it take more energy to get to Venus or to Mars?

The second table here essentially answers your question. Venus transfer from Low Earth Orbit is 3.5 km/s, Mars transfer is 3.6. This will allow you to impact either body (on Venus you will need to ...
Steve Linton's user avatar
  • 19.6k
14 votes

Why is it easier to escape the solar system than get to Mercury or the Sun?

Based on the calculations presented by @uhoh I generated a plot showing the necessary delta-V for a fly-by mission, i.e. entering into a Hohmann transfer with a far point intersecting the orbit of a ...
asdfex's user avatar
  • 15k
14 votes

What does `ln` mean in the Delta V equation?

It's the logarithmus naturalis. That wikipedia page was the first result when I googled "ln". If you encounter situations like this in the future and you're concerned that two letters aren't ...
Acccumulation's user avatar

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