50 votes

Do you need to burn fuel between gravity assists?

Yes, Trajectory Correction Maneuvers (TCMs) are always performed during cruise phases, whether before or after gravity assist flybys. This NASA tutorial serves as a good general reference. One source ...
48 votes

What are "Ohms burns" in the context of Scott Kelly, KSP, and the Space Shuttle?

That's a mistranscription of OMS Burn, or Orbital Maneuvering System burn. The OMS system is how the shuttle changed its orbital characteristics. You can read about it here. One, two or more might ...
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48 votes
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Why is it so hard to build crewed rockets/spacecraft able to reach escape velocity?

Delta-V to LEO is about 10 km/s. From there to C3 (Earth escape) is another 3.2 km/s. It's just another 30% delta-V. The problem is the Tyranny of the Rocket Equation. More delta-V means more fuel. ...
  • 7,906
43 votes
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How often does ISS require re-boosting to higher orbit?

The easiest to see ISS orbital reboosts is by checking Height of the ISS (where with height they mean orbital altitude above mean sea-level) over at Heavens Above. For example, for the last year, this ...
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43 votes
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How does a spacecraft know that it is in orbit?

The Juno spacecraft has no means to directly measure and compute that it is in orbit. It did not send any such confirmation message. All it sent was an FSK tone indicating that it had completed the ...
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42 votes
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When the ISS accelerates, do the astronauts feel it?

If you watch these videos: ATV boost Zvezda boost ...you can see that the acceleration is quite gentle, but definite. The astronauts do need to hang on to something if they don't want to drift to ...
42 votes

When the ISS accelerates, do the astronauts feel it?

The answer to the question, "Do the astronauts feel the station moving?" is yes, definitely, but sometimes in an "indirect" fashion. During Space Shuttle mission STS-109, when floating in my sleeping ...
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39 votes
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How can I intuitively understand gravity assists?

Understanding the Principle Let's start by understanding the mechanism of a gravity assist. As a spacecraft approaches a planetary body, it gets affected by the planets gravitational pull. Getting ...
39 votes
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Why is it most efficient to change orbit inclination while crossing the equator?

Why is it the most energy efficient to change orbit inclination while crossing the equator? Specifically, it's most efficient to do a plane change at one of the two "nodes" where the origin ...
35 votes

How could a 90 m/s delta-v be enough to commit the space shuttle to landing?

Page 331 in the Shuttle Crew Operations Manual, an official NASA astronaut training document, confirms that The deorbit burn usually decreases the vehicle's orbital velocity anywhere from 200 to ...
35 votes

James Webb orbit insertion

How can JWST brake without turning around? Answer: JWST does not. From More Than You Wanted to Know About Webb’s Mid-Course Corrections!: One interesting aspect of the Webb launch and the Mid-Course ...
34 votes

Why is it so hard to build crewed rockets/spacecraft able to reach escape velocity?

It's not hard, it's just expensive. We know exactly how to do it. Compare this to building computer processors with 1nm transistors, or making reliable self-driving cars. Those are both things that we ...
34 votes

When burns are made during inefficient parts of the orbit, where does the lost energy go?

After writing my comments, I started writing a new answer. That got long, so here's a shorter one. The "energy of an orbit" may be poorly defined and depending on the definition, is not ...
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31 votes
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Is this really Rosetta's orbit around 67P?

This was one of the questions just now during the Rosetta press briefing. This video was shown during the presentation: The triangular trajectory are hyperbolic ...
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31 votes
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How could a 90 m/s delta-v be enough to commit the space shuttle to landing?

If you're just looking for an intuitive handle on it, try this: In circular LEO, your orbital period is about 90 minutes. If you apply a velocity change of 90 m/s, then wait half an orbit -- 45 ...
29 votes

Why is it most efficient to change orbit inclination while crossing the equator?

A great aid to intuition is to remember one principle about orbit changes: if the engine is off, the orbiter always returns to same point one orbit later. So for any orbit change, if you want to do ...
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25 votes
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How did NASA Conclude that the General Theory of Relativity was not Needed for Earth-Moon Flight Path Computation?

This conclusion MUST have been reached by NASA (or even NACA) in the leadup to the Apollo landings. Despite the name, getting people to the Moon is not rocket science. It's rocket engineering. ...
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23 votes
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Lowest delta-v maneuver to reverse the direction of a satellite in LEO?

Reversing direction is a special case of inclination- / plane-change. Here's one way to find a better upper bound on the necessary delta-V for a 180 degree plane change: Assume a 2-body system (i.e. ...
  • 9,959
23 votes
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Was SpaceX's launch of Formosat-5 more vertical than normal for any particular reason?

FORMOSAT-5 was deployed directly to a 720 km circular orbit, with only a single burn. In order to do a circular orbit so high, one has to have a more vertical ascent then would be typical. Basically, ...
  • 119k
23 votes
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This ISS trash deployment looks more like 2 feet than 2 inches per second, was it too fast or are these articles incorrect?

What's the root cause of the disparity between what the article says and what's shown in the video? There is no disparity. The article says you need a minimum velocity of two inches per second [bold ...
22 votes
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How will the ion thruster powered Dawn spacecraft enter orbit around Ceres?

Does it have any additional thrusters? Not to thrust towards its targets. For that, it's 100% ion thruster propelled. It does also have a set of 12 MR-103G variable thrust (0.9 N maximum) RCS (...
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21 votes
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What prompted Lovell on Apollo 8 to announce "there is a Santa Claus"?

The first trans-Earth injection from the moon was a high-stakes maneuver; if the Apollo's SPS didn't fire, the crew would have been stuck very far from home. I always took "please be informed there is ...
21 votes

How does a spacecraft know that it is in orbit?

Using attitude determination devices, (including doppler shift of radio signal from Earth), it can determine* its location and velocity relative to Jupiter, and from that data, and knowing Jupiter ...
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21 votes
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What is the smallest body in which a sling shot maneuver can be performed?

How small do you want to get? $F=G{Mm \over r^2}$ applies regardless of size. If you remove enough disturbances from other bodies you can get two neutrons to orbit a common barycenter on gravity alone ...
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21 votes
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What is the delta-v required to get a mass in Earth orbit into the sun using a SINGLE transfer?

Actually the answer is a bit more complex than "Earth orbits at 30 m/s, so you have to stop that velocity and drop in. Thus the delta-V is 30 km/s." The question states that you start from Earth orbit,...
20 votes
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What happens to JWST after it runs out of propellant?

This Northrop Grumman video (starting at 09:31) illustrates JWST's orbit in a non-rotating (normal) frame. It's really in an orbit around the Sun about 1% farther ...
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20 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 ...
  • 10.1k
19 votes
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What are "Ohms burns" in the context of Scott Kelly, KSP, and the Space Shuttle?

Since the questioner also asks "why are two needed" and the other answer didn't address that: Early shuttle missions flew a "standard insertion" ascent. This required two burns of ...
19 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-...
19 votes

What kind of mission objective would make a parabolic escape trajectory desirable?

Why zero excess velocity? Well, with almost zero excess velocity you can stay near Earth, but not too near. For example, the Spitzer space telescope did this to communicate with Earth while avoiding ...

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