HopDavid
  • Member for 7 years, 10 months
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How fast will 1g get you there?
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49 votes

Assuming acceleration is constant, $d=(1/2) a t^2$. So plotted over time, distance traveled is a nice parabola. If you want the time it'd take for a specific distance, it's easy to manipulate $d=(1/...

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Why don't we launch spacecraft from the Moon?
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41 votes

First a few terms: Low Earth Orbit (LEO) All spacecraft must first achieve low Earth orbit. This is true whether you're sending stuff to the Moon or Mars. Trans Mars Insertion (TMI) The burn needed ...

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What's the path of something dropped from a space elevator
24 votes

After being released from an elevator, the payload and earth are a 2-body system. The paths in 2 body systems are conic sections: ellipses, parabola, and hyperbolas. What sort of conic section depends ...

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Why did SpaceX attempt to land the booster on a barge instead of somewhere on land?
23 votes

Edit Jan. 25, 2015: Geoffcc recently provided a link to words straight from the Horse's mouth, Elon Musk. Musk says the payload hit for RTLS (Return To Launch Site) is 30% vs a 15% payload hit for ...

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Gravity Assist braking
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23 votes

The speed of the probe doesn't change with regard to the assisting body. It is direction that is changed. If a hyperbolic orbit about the sun comes in with a Vinfinity of 5 km/s, it will exit with a ...

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What's after Pluto for New Horizons?
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22 votes

Edit as of August 31, 2015: Looks like 2014 MU69 will be the post Pluto target. Paul Gilster has a nice article on Centauri Dreams. Edit as of Jan. 3, 2015: This article says 3 potential candidates ...

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What is the rarest launch window?
21 votes

Hohmann launch windows occur each synodic period. Or a more general version of a Hohmann transfer would be a transfer orbit tangent to both departure and destination orbits. This also occurs each ...

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How long will a 500 km altitude satellite spend in Earth - shade per orbit
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21 votes

In this pic the angle $\phi$ is asin(6378/6878) or about 68º. It stays in the shade over 2 * 68º or about 136º. That's about 38% of the period. That's 2145 seconds or about 36 minutes. This is what ...

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Reverse Lunar Space Elevator
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18 votes

Here's a pic of the tether proposed by Liftport. Go 160,000 km beyond EML1 and drop a payload from that point. It will follow an elliptical path whose perigee grazes low earth orbit. A 3 km/s burn at ...

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Could the sun be used as a gravity assist outside the solar system (with current tech)?
16 votes

Hefty gravity wells can give a healthy Oberth benefit. Doing a burn deep in Neptune's well makes sense. Suggesting an Oberth maneuver near a Pluto sized object is pretty silly. Don't know if Randall ...

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Is a Moon-base inherently more dangerous than a Space Station?
15 votes

If it is more dangerous, how much more and in what ways? Proffesorfish and Eli Skolas have both given thoughtful answers comparing the hazards of I.S.S. vs Moonbase. If humans were preceded by robots ...

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Oberth effect for Earth vehicles
15 votes

Kinetic energy is 1/2 mv2. Here's my visualization: When you add to that velocity v, you get a bigger square: Here Vb is velocity imparted by your burn. So for example, if you accelerate a kilogram ...

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Is a railgun up Olympus Mons possible?
14 votes

I've been wondering this for years, not knowing how to approach the problem. Then at a NasaSpaceFlight thread a poster who calls him(her?)self R7 told me about dynamic pressure, also known as q. $q=....

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Where could we build a space elevator today (2014)?
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14 votes

Earth Carbon nanotubes might endure the enormous stress of an earth elevator but only short lengths have been manufactured so far. It would be a mega engineering project that would dwarf earlier ...

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Did the space shuttle fly straight "up" when leaving Earth?
13 votes

The major burn is a 7.7 km/s acceleration in the horizontal direction. But the first part of the trip is usually a vertical climb: Apollo Ascent profile from a NASA history page. Rescaled to show how ...

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How feasible would it be to move a Kuiper Belt object (a good sized one) into an orbit around Mars?
13 votes

Here Robert Zubrin and Christopher McKay talk about terraforming Mars. Have your browser find "Moving Ammonia Asteroids" and it will take you to the relevant section about 3/4 down the page. Zubrin ...

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What are speed limits for free fall, orbital trajectory, and unbound trajectory?
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13 votes

What they teach in high school physics is wrong. When you toss a penny, it does not follow a parabolic path. Here is a picture of the penny's actual path (neglecting friction from air and rock and ...

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Space Elevator Between Doubly-Tidally Locked Bodies
13 votes

One scheme for building elevators is to start from an anchor mass in synchronous orbit and to extend tethers down and up. The tether above will pull the anchor mass up and the tether below would pull ...

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Why is L2 Langrangian point stable?
12 votes

Gravity does not cancel out at any of the Lagrange points, not even L1. As you point out, at L2 both the central body and orbiting body are pulling the same direction. But there is a third character ...

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Are there any active or recent projects investigating skyhooks for orbital lift?
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12 votes

To my knowledge there has been very little done in the 21st century. Here is a subsection of a page on space elevators and orbital tethers that lists tethers missions. The last two paragraphs are ...

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Why have no reusable launch technologies been developed?
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12 votes

Mass fraction and Tsiolkovsky's rocket equation make for major hurdles. Mf, or the ratio of propellent to dry mass is given by: $Mf=1-e^{-delta V/V_{exhaust}}$ To climb out of a steep gravity well ...

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If we build a space elevator from Earth surface to GEO, could I step off it at GEO and remain in GEO?
12 votes

A Clarke style space elevator is a (very large) gravity gradient stabilized vertical tether. When in a rotating frame (as on a merry-go-round) you feel a tug. It's just inertia but feels like an ...

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Would it be more viable to process moon rocks into Helium-3 on the Moon than doing so on Earth?
11 votes

Two big ifs here. IF we achieved viable commercial fusion power (other than the sun) and IF Helium-3 was an indispensable part of this process. But for the sake of argument, let's say Helium-3 is the ...

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How unreachable are Jupiter's moons from Mars with the technology developed for going to Mars?
11 votes

I whomped up a spreadsheet to compare scenarios like this: Hohmann.xls. Typing Earth into departure planet cell and Mars into destination planet I get Launch windows open each 2.14 years (synodic ...

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Why there is no orbiter for Uranus or Neptune?
11 votes

Using my Hohmann spreadsheet you can get an idea of approximate delta V and trip times. The spreadsheet assumes circular, coplanar orbits. So it's ballpark, not exact estimates. LEO to Mars capture: ...

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Is the ideal transfer between two arbitrary planar orbits always a bi-tangential ellipse?
11 votes

Thanks for linking to my pdf! I have always assumed bi-tangential transfers took the least delta V. But your question has made me realize my assumption is conjecture. My goal is to find a general ...

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How far away will I get if I'm --just under-- escape velocity?
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11 votes

You are talking about an elliptical orbit about the earth. Two points on this ellipse are of interest. Perigee - point on the orbit closest to earth. Apogee - point on the orbit farthest from earth ...

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Are there any major flaws with the Mars Architecture used in the Martian?
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10 votes

There were some major flaws in the backstory. I believe these were the movie studio's fault and not Andy Weir. In Tyson's trailer for the Martian The Hermes is harbored in low earth orbit between ...

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How can delta-v and aerobraking as shown in this chart be explained?
10 votes

The calculations for Ulysse Carion's chart were done by Curious Metaphor. If you go the this Reddit thread, you'll see a conversation between Curious Metaphor and I. For years I made delta V charts ...

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Where will objects end up, after losing stability at Lagrangian points?
Accepted answer
10 votes

EML1 and EML2 From EML2 and EML1 it is possible to collide with earth or the moon. It is also possible to sail out of earth's Hill Sphere. Here are a range of pellets from EML2 nudged away from the ...

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