71
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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 ...
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 ...
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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 ...
- 8,001
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 ...
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24
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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 ...
- 9,685
22
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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 ...
- 3,585
19
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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 ...
- 121k
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 (...
- 148k
17
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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?
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 ...
- 1,073
16
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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 ...
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15
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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 ...
- 167k
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 $\Delta V$ ...
- 58.5k
13
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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?
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 ...
- 8,974
11
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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 ...
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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 ...
- 9,685
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 ...
- 58.5k
10
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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 ...
- 9,685
9
votes
Accepted
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 ...
- 15.7k
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 ...
- 15.7k
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 '...
- 126k
9
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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 ...
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 ...
- 54.9k
8
votes
Delta-V to Low Mars Orbit
Let's break down your problem into a very simplified launch phase from the ground to low Earth orbit at 250 km altitude. Then we'll use patch conics to get an estimate of the $\Delta v$ necessary to ...
- 4,350
8
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Do transfer orbits toward the central star necessarily result in a higher velocity on arrival due to the star's gravity?
When you've reached escape velocity relative to the planet, the star becomes the dominant gravitational influence: you're in orbit around the star.
And as always when you're in orbit, your orbit's ...
- 126k
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 ...
- 6,250
7
votes
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Do transfer orbits toward the central star necessarily result in a higher velocity on arrival due to the star's gravity?
You didn't say anything about how you chose the transfer orbit, nor did you specify velocity relative to what. And of course, "appreciably" is not defined.
So we will have to make some assumptions.
...
- 58.5k
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-...
- 54.9k
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). ...
- 3,412
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, ...
- 3,687
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 ...
- 2,233
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