# Tag Info

46

Nearly all the velocity is cancelled by atmospheric deceleration of the descent module, before its parachutes are deployed. ISS orbital velocity is around 7700 m/s. An initial retro-burn of the Soyuz engines, of something like 115 m/s magnitude, is sufficient to lower the perigee of orbit into the uppermost part of the atmosphere. The orbital module and ...

43

The capsules designed to reenter the atmosphere have to slow down from about 8 km/s to zero by the time they get to the ground. They actually don't use the part that looks like a cone to do that. They all have flat bottoms that they face into the wind to do that. If you compare the Dragon capsule from your link to a Soyuz capsule, the Orion capsule, or the ...

25

The process is described here, which answers nearly all of your question. The reentry burn removes about 120 m/s of velocity from the capsule (that's your 1) and the final impact is 15 miles per hour (about 6 m/s). That's your 3. That leaves about 7.5 km/s for part 2. The only remaining question is the split between 2a and 2b, ie the velocity when the ...

23

Yes, a capsule cannot literally bounce off the atmosphere and its kinetic energy must be reduced by an encounter with the atmosphere, rather it would just pass through the atmosphere and back into space, having failed to lose enough velocity to stay in the atmosphere. After going partially around the planet it will reenter the atmosphere, that is actually ...

18

Why are LEO satellites not aerodynamically shaped? The need for electrical power overwhelms the need to reduce drag. That means a sizable cross sectional area to incoming solar radiation. Sometimes that cross section to solar radiation corresponds nicely (or not so nicely) with cross section to drag. What's worse, it's hard to claim that any shape is "...

17

I can't say that the terminology is consistent across all users, but where I work (at JPL) we use aerobraking to refer to many light dips to lower an orbit, aerocapture to refer to a single deep dip to bring a hyperbolic approach to an elliptical orbit, aeroentry or simply entry to refer to an entry into an atmosphere with no exit, and aeroassist as a ...

15

This works well in reality too, and helps to reduce heat loads. The reason this is not used is the fact that you must then pass through the Van Allen radiation belts multiple times, where you ideally want the crew to spend as little time as possible due to the high levels of radiation. Another reason is redundancy. If the heat shield is not able to the ...

15

The entry vehicle for the Apollo missions is the command module (CM), which has a symmetric body with an offset center of gravity (c.g.). This offset c.g. causes the CM to trim aerodynamically at an angle of attack with a resulting lift force as illustrated in figure 1. The magnitude of the lift force is not controllable; ...

14

For Gemini, Apollo, and Soyuz capsules, lift is achieved by offsetting the center of gravity of the reentry module from the center line of the craft. This is represented in your diagram by the "location of heavy equipment" callout, and results in the tilt of the capsule relative to the flight trajectory shown. The tilt causes the body of the spacecraft ...

13

Here's the original source of the diagram: https://www.reddit.com/r/space/comments/29cxi6/i_made_a_deltav_subway_map_of_the_solar_system/ Your concerns are also discussed there. 27000m/s is a very theoretical value, that takes into account the losses of a chemical rocket escaping the thick atmosphere of Venus. This makes it clear that launching a rocket ...

13

This is used in reality, although it is not as easy to do in real life as it is in Kerbal Space Program. Such passes dramatically increase the heat that the capsule is forced to deal with, and as a result, it might simply fail. The only time it is done is when orbiting bodies with an atmosphere, specifically Mars and Venus. What is done is that the ...

12

It most certainly won't hurt anything. From FAA's document on returning from space, there is a very interesting chart, which I've included below. So the maximum g load is almost always at around 4500 m, for this particular flight trajectory. From other charts, we find that a lower angle will spread out the re-entry forces, and specifically make the max ...

12

Not without propellant. Or at least not easily. However the "why" is a bit tricky. This is far from the whole story but one of the problems is that generating enough lift to keep you from diving too fast into the atmosphere is tricky. If you're going near orbital velocity you can scrub speed quite well as you don't need to generate much lift. At low speed, ...

10

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 where it took about the same to escape earth and Venus. But Metaphor pointed out that Venus has a much thicker atmosphere. But who would go to the surface of ...

10

Aerobraking is most definitely used at Mars. Going from about 6000 m/s down to 100 m/s is done entirely with aerobraking, first with a heat shield, and then with a parachute. The last bit from 100 m/s to about 1 m/s is done with rockets. (There seems to be a lot of excitement about the bit at the end for some reason, but by then nearly all the work has ...

9

The idea with aerobraking is usually to do something like the following: Make your orbital insertion burn as usual, but instead of slowing all the way down into your final orbit, slow down just enough to get captured by the planet. (This burn is executed close to the planet, using the Oberth effect to reduce the amount of fuel required.) You'll end up in ...

8

What you are referring to is a Free Return Trajectory. It references a paper called Trajectories in the Earth-Moon Space with Symmetrical Free Return Properties, which states that the right path can lead to flying between the Earth and Moon with no further fuel required. But you won't gain any momentum from that.

8

The two systems that I know of that were qualified for skip entry, Apollo and Shuttle, did so purely for cross-range capability. The skip entry would have been used for an emergency return that could not wait for a more favorable alignment with respect to the landing/splashing site. The purpose of qualifying those vehicles for skip entry was not related to ...

8

It is possible to do a slow and cool reentry. But it would be extreamly expensive. Just slow down from orbit speed to subsonic speed above the atmosphere and then fight gravity until it is possible to deploy parachutes. All that using rockets thrust to decelerate. No heat shield necessary. But you need a very large rocket with at least two stages in orbit ...

8

Take the super-optimistic 500 kJ/kg energy density of flywheel energy storage. In reality 10% of that would be a great result. $E_k = {1 \over 2} mv^2$ so 0.5*1kg*(8km/s)^2 = 32MJ per kilogram of orbital mass. If the craft was nothing but the flywheel, you'd still receive 64 times more energy than you can most optimistically contain. Your flywheel would ...

7

How many grains of sand does it take to form a heap? An orbiting spacecraft is flying many times faster than the speed of sound. It's starting in atmosphere too thin to sustain an audible shock wave. As it descends, it's going to be producing a shock cone continuously, but in the very thin atmosphere high up, the amplitude of the shock wave is too faint to ...

7

I don't know if there was propellant used in between, but I know of at least one mission that had a spacecraft use two lunar assists: STEREO. If you look at the first video on their web page explaining the orbits, you'll see that B (STEREO-Behind) gets a second gravity assist from the moon. (Disclaimer: I work for the STEREO Science Center)

7

A 250km periapsis Earth orbit won't produce significant atmospheric drag. The inbound-aerobraking-possible notations for orbits are assuming that you dip lower into the atmosphere in order to slow down into an eccentric orbit, which you then circularize once you've come back up out of the atmosphere. This is tricky; if you hit the atmosphere too high, you ...

7

Adding to @Rikki-Tikki-Tavi answer. For destinations where aerobraking is possible, and you have a sufficiently heat and acceleration hardened vehicle, it's possible to cheat. The map shows how much delta-v must be applied, not where it has to come from. The single most fuel efficient way of shedding speed is to smack into an atmosphere blunt end first. To ...

7

You can't just slow it down over many orbits I think the question is suggesting letting a little bit of drag slow the Cessna down until it's at a normal speed before gliding through the atmosphere. That's nice, but it won't work. Orbiting is ballistic flight. Ballistic flight without lateral speed (i.e. once you've started to really slow the Cessna down) is ...

7

According to https://www.flightclub.io/results/?code=SS10 , reentry Q for flights like SES-10 peak at almost 92kPa at T+434 sec, about 3x the ascent max Q. The reentry prep burn brings velocity down to about 1500m/s, about 1/5 of orbital speed, so it doesn't need the kind of thermal protection an orbital capsule would. An orbital capsule also comes in much ...

6

First of all, the heat shields aren't there just for the dense atmosphere with high deceleration, the upper atmosphere part of reentry also generates heat. If you had a craft that could fly in the upper atmosphere it would still heat up just from its forward speed. Hypersonic vehicles also heat up considerably even though they are just normal aircraft. The ...

6

A typical metric is that you want to exit with no more than 100 m/s of correction $\Delta V$ to get to your targeted conditions. (That does not include the deterministic $\Delta V$ required to raise periapsis assuming a perfect exit state.) That generally requires that the flight through the atmosphere be guided or drag-modulated to account for both ...

6

As I understand it, wrongly or not, temperatures in the sun's outer layers are about 6000K as opposed to the upwards of 100,000K ablative temperature of a heat shield during aerobraking. The relevant temperature is not the temperature of the star's atmosphere, it's the temperature to which everything is heated by friction. If you compare 0.1c with the ...

6

ESA has done aerobraking with Venus Express. NASA has done aerobraking at Mars several times, as well as at Venus. This will be the first time that ESA has done aerobraking at Mars.

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