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71 votes
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Could we wait for Mars to come to us?

Maybe some visual intuition for what actually happens in the Hohmann transfer helps? It's already very close to what you are describing. In the top arc, the spacecraft (yellow), is going a bit slower ...
SE - stop firing the good guys's user avatar
67 votes
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Why can't you travel to Mars in a straight line?

Only answering your first question here, and in qualitative terms: You can't travel to Mars in a straight line for the same reasons you can't throw a ball in a straight line: gravity. If you wanted to ...
Anton Hengst's user avatar
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38 votes

Could we wait for Mars to come to us?

It depends whether or not you want to orbit or land softly upon Mars, or just hit it. For the former, you have to match orbits with it, that probably means burning more fuel. For the latter, you can ...
Starfish Prime's user avatar
24 votes
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Mars versus the poles of Mercury WRT colonization

Your delta-v analysis doesn't account for the landing delta-v. On Mars, only a fraction of a km/s has to be done propulsively, on Mercury the entire landing will be propulsive. You also don't account ...
Christopher James Huff's user avatar
24 votes
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If one Starship can transport 100 people to Mars, how many could it safely land near Mercury's north pole after one Hohmann-like transfer?

I take "after one Hohmann-like transfer" to mean a direct Earth to Mercury interplanetary transfer (i.e., no Venus shenanigans), please let me know if that is not the case. Unless serious ...
BrendanLuke15's user avatar
22 votes

Why can't you travel to Mars in a straight line?

Disclaimer: this answer makes some coarse simplifications in order to keep it focussed on the crucial points. If you feel like pointing out something about orbit eccentricities or whatever in the ...
leftaroundabout's user avatar
19 votes
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What's the reasonable "window" in days for a Mars-Earth Hohmann transfer?

Techically a Hohmann transfer is a single instant, but as you mentioned, there is a window about the Hohmann that is nearly optimal. Just to give you an idea, here is the launch window sizes for some ...
PearsonArtPhoto's user avatar
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17 votes

Could we wait for Mars to come to us?

could we launch a vehicle in space for Earth, stop right on Mars trajectory... Yes, you could have a trajectory that came to a stop (briefly) in the path of Mars like how a ball thrown upwards stops (...
uhoh's user avatar
  • 149k
17 votes

If one Starship can transport 100 people to Mars, how many could it safely land near Mercury's north pole after one Hohmann-like transfer?

The delta-v to Mercury is 2.5x greater than to Mars; given the dry mass of Starship I doubt that it could land any payload at all without some heliocentric fuel depot, which would be quite tricky and ...
antlersoft's user avatar
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16 votes

Which of the 2 delta-v calculations for a Mercury orbit insertion from LEO is right and which is wrong?

Your $v_{inject}$ and $v_{insert}$ equations assume that both the departure and destination worlds are in coplanar, circular orbits. This is manifestly not the case with Mercury. Mercury is in an ...
notovny's user avatar
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15 votes

Orbital Rendezvous vs Hohmann Transfer

Circular orbits at different altitudes require different speeds, so if you start with a radial separation, the spacecraft and station will tend to drift further apart unless they accelerate themselves ...
Russell Borogove's user avatar
14 votes

~15.5817 -- Is there a name for this number?

For reference, that number is: $$5+4\sqrt 7 \cos\left({1\over 3}\tan^{-1}{\sqrt 3\over37}\right)$$ It is the positive root of: $$x^3−15x^2−9x−1=0$$ If we take the equation for the total $\Delta V$ ...
Mark Adler's user avatar
  • 58.2k
13 votes

If one Starship can transport 100 people to Mars, how many could it safely land near Mercury's north pole after one Hohmann-like transfer?

Landing on Mercury is supremely difficult due to how deep Mercury lies in the Suns gravitational well. In addition it has no significant atmosphere so retro propulsive landings would be required ...
Slarty's user avatar
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11 votes

Thrust and rotation strategy to circularize a standard GTO orbit using ion propulsion?

TL; DR: Trajectory optimization for continuous thrust is difficult and this field is very active in research. 2021 clarifications: Methodology For the least amount of fuel, the best is the thrust the ...
ChrisR's user avatar
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11 votes
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Which of the 2 delta-v calculations for a Mercury orbit insertion from LEO is right and which is wrong?

I think I have it sorted: this answer giving the lower values is "correct". The key part you have missed is emphasized (by me) here: That means a delta-v cost of 4.37km/s to get captured ...
BrendanLuke15's user avatar
10 votes
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Ratio of low-thrust slow spiral to Hohmann transfer $\Delta V$?

This answer has the two-impulse Hohmann transfer $\Delta V$. It is: $$\sqrt{2x\over x+1}+\sqrt{1\over x}-\sqrt{2\over x\left(x+1\right)}-1$$ where $x$ is the ratio of the higher orbit radius to the ...
Mark Adler's user avatar
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10 votes
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When is the supersynchronous orbit more efficient than a typical Hohmann transfer where inclination change and circularization are simultaneous?

(mostly recycled from What are the benefits of supersynchronous transfer orbits?) Excuse the wall of plots but I really do think they describe it better than my words ever could :) The total $\Delta ...
BrendanLuke15's user avatar
9 votes
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How to intuitively explain that reaching geostationary orbits consume more Delta V than escape orbits?

Intuitively: going from one circular orbit to another requires two burns: one to raise the apogee and another to raise the perigee. To achieve escape, you need to raise the apogee until your orbit '...
Hobbes's user avatar
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9 votes
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Is bypassing LEO more efficient when launching to Venus or Mars?

No. It is not more efficient to bypass LEO. You can't bypass getting out of earth's gravity well. 2.9 km/s is what it'd take to enter a Mars transfer orbit if the ship were outside of earth's gravity ...
HopDavid's user avatar
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9 votes
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How is 80 to 150 days to Mars possible at 6 km/s TMI Delta V?

My Hohmann spreadsheet shows TMI from LEO is around 3.6 km/s. Here is a Non Hohmann transfer sheet from earth to Mars. Into the pink cells I type .9 A.U. for the transfer orbit's perihelion and 1.6 ...
HopDavid's user avatar
  • 15.8k
9 votes
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How much delta-v could be saved with a Hohmann-like transfer orbit to the aphelion of Mercury 's orbit instead of its perihelion?

All the relevant velocities can be obtained from the vis-viva equation. The transfer/capture costs can then be derived from the $v^2 = v_e^2 + v_{\infty}^2$. I'm going to ignore Mercury's 7 degree ...
SE - stop firing the good guys's user avatar
8 votes
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How can Bielliptic transfers be more efficient than Hohmann transfers?

Citing Wikipedia, some bi-elliptic transfers require a lower amount of total delta-v than a Hohmann transfer when the ratio of final to initial semi-major axis is 11.94 or greater, depending on the ...
SF.'s user avatar
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8 votes
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Why is the Chandrayaan lunar insertion so long?

It didn't have enough thrust. Small rocket engines are easier to build than large ones, and weigh a lot less. That probe (I assume you mean Chandrayaan 1) had a 440 N main thruster that it used to ...
Michael Stachowsky's user avatar
7 votes

How to intuitively explain that reaching geostationary orbits consume more Delta V than escape orbits?

Let's start at the point which is common to the two maneuvers: we're at perigee of GTO - Hohmann transfer orbit from LEO (or even suborbital flight!) to GEO; an elongated orbit with perigee of ~200-...
SF.'s user avatar
  • 55k
7 votes
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Do "partial" Hohmann transfers exist?

Yes this is absolutely a thing. Delta-V can indeed be traded off in a continuous fashion for total flight time (i.e. uses more dV, but lowers transfer times, relative to a Hohmann transfer). ...
ANone's user avatar
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7 votes

Could we wait for Mars to come to us?

There is no "stopping" in space - no matter how far you are from other celestial bodies, the force of gravity will always be tugging on you, pulling you in some direction. Within the Solar System, ...
Nuclear Hoagie's user avatar
7 votes

Mars versus the poles of Mercury WRT colonization

You can't land on the day side of Mercury nor on a peak of perpetual light because it's far too hot (800 deg F / 430 deg C), even if not as hot as on Venus. A crewed mission must land either on the ...
LoveForChrist's user avatar
7 votes
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How to keep the phase angle with an Hohmann transfer?

To see why a Hohmann transfer is applicable for a symmetric jump preserving phase angle, let's consider what the semi-major axis of the transfer ellipse is: $$a_{H} = \frac{periapsis_H + apoapsis_H}{2}...
SE - stop firing the good guys's user avatar
6 votes
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Voyager Earth to Jupiter journey time

It wasn't a Hohmann transfer. The rocket simply boosted it to a higher velocity from Earth in order to enable a Jupiter flyby that would send the spacecraft to Saturn. You can get the approximate ...
Mark Adler's user avatar
  • 58.2k
6 votes

Is bypassing LEO more efficient when launching to Venus or Mars?

Yes. Consider a simplifying case of a direct escape vs. a parking orbit departure from an airless, non-rotating body using instantaneous maneuvers. The most efficient way to get to the parking orbit ...
Mark Adler's user avatar
  • 58.2k

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