# Tag Info

62

I've done a lot of work on this subject with researchers and engineers at JPL, NASA Langley, and NASA Ames. There are some interesting things that come out of high-fidelity CFM (Computational Fluid Mechanics) modeling of entries or re-entries, and also from flight experience. This FAA tutorial segment is a good general reference for the principles involved. ...

33

Spiraling down in the sense you mean is not possible, the reason is that when a spaceship is orbiting Earth, it is travelling extremely fast relative to the surface, it is not that space is so high up, but that a spaceship needs to travel very fast in order to orbit. So in order to reenter, it is not the velocity of falling that needs to be shed, but the ...

31

Rockets are cylindrical for the same reason maize silos are cylindrical: A circle has the largest area vs perimeter of any shape and also provides maximum strength from internal pressure. This means you can save on weight for the walls of a rocket when it is cylindrical. A cylinder is not the absolute best aerodynamic shape as the Drag Coefficient section ...

31

Rockets are much faster than airplanes for most of their flight. Here's a graph of a Space Shuttle launch: The red line is speed. It's in ft/s, 1000 ft/s is 1097 km/h. So At about 45 seconds, the Shuttle flies 1000 km/h which is faster than an airliner. At about 1:40 it crosses 3000 ft/s which is about Mach 3 (the speed of the fastest aircraft). ...

28

I'll give you the numbers. I'm breaking this up into 3 different terms. There's atmospheric drag, what I'll call the "hover" term, and the gravitational potential climb. I will more or less assume a flight directly up. You're welcome to use whatever term for velocity you want, as none of them will be representative. I'll take the Shuttle's speed at ...

26

For supersonic flow, the Sears-Haack body offers less drag than the shorter teardrop that's optimal in the subsonic regime. Sears-Haack is pretty similar to the German V-2 rocket body. (Note that the proportions of this particular example aren't part the definition of the Sears-Haack shape; for minimal drag you'd have to have an impractical body of infinite ...

26

In order for a combustion process to happen, you do not only need fuel, you also need an oxidizer. On Earth, that is usually the oxygen in the air. In Titan's atmosphere, there is no oxygen. This applies to other atmospheres too, like the hydrogen dominated atmospheres of Jupiter and Saturn. Hydrogen, just like the methane in Titan's atmosphere, is flammable ...

25

I think you answered your question in the first sentence. I am reminded of this image: Aerodynamics is but one (albeit a large one) of many concerns in the systems engineering of a rocket. Others include manufacturability, propulsion, changing flight regimes, safety, structures, economics, etc... All of these seem to have converged on a simple, more-or-...

25

First off, a stone does not skip off the surface of the water. It skips off the water. The stone has to bite into the water to use its lift to come back out. A skipping stone can, and sometimes does, go completely under the surface in the process of skipping. The air is a fluid as well, and a lifting body can in fact skip in the same way as a stone does ...

21

A rocket isn't automatically fast - a small firework rocket may be no faster than a car. The important point is that rockets carry their own oxidiser and aren't limited by the need to interact with the air. Most aircraft engines need to develop lots of thrust at low speed for take off, and they have propellers or large fans that cause drag at high speeds, ...

19

First of all, let's figure out what the drag actually is. For that, Heavens-above has a nice chart. Of some note is the fact that the atmospheric drag rate changes over time, most notably with the solar cycle, but it can change for a variety of reasons, especially for a body as dynamic as the ISS. With the current altitude ranging around 400. The time that ...

19

The key concept is that for a satellite at a fixed altitude, when the atmospheric temperatures below its altitude increase, atmospheric expansion pushes more atmosphere up above the satellite! At the satellite's altitude the pressure must increase to support the weight of that additional atmospheric mass above, and the increase in pressure outweighs the ...

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 "...

18

The Soyuz uses conical boosters because there's an aerodynamic advantage. According to The Red Rockets' Glare: Engineers gravitated to a conical shape primarily because of the aerodynamic advantages ...but also for 3 other reasons: the large size of the engines at the tail end, the possibility of imparting additional thrust to the central sustainer ...

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 ...

16

There is air inside the space station so, yes, anything moving within that air will experience air resistance. Whether that air resistance is enough to stop a thrown object before it hits the wall depends entirely on the object and how hard it is thrown. If the object is very aerodynamic (i.e., experiences little air resistance) and/or is thrown hard, it ...

15

There are three major constraints that have to be taken into account: maximum deceleration (equipment and structural elements can withstand much higher g's than the crew, so it's about medical limits); peak heat flux which allows one to determine the worst case temperatures that the spacecraft structure is heated to during re-entry (heat transfer and ...

15

I am Patrick Shober (the lead author of the study). Thanks so much for checking it out! If you check out Figure 10 in the paper, I have plotted the specific angular momentum in the Sun-centered frame. So this shows how the meteoroid (the rock) gained energy during the close encounter but then lost a fraction of it due to the atmospheric passage. This can ...

14

The term "friction" is a misnomer. The source of heat is adiabatic compression - gas on trajectory of the reentering object is compressed against its leading surface, and as result heats up. On molecular level you can think of it as number of molecules rising in given volume (compressed) and additionally speeding up (by elastic collisions against the fast-...

12

No: The heat produced by atmospheric reentry isn't a happy side effect of returning to the earth, it's a byproduct of the fact that your satellite/orbiter has enough kinetic energy to be circling the earth every 90 minutes and you want it to stop doing that and come down. To have something you've made for the purpose of harvesting energy reenter the ...

12

In the Shuttle Mission Simulator we used the Jacchia Reference Atmosphere, it's good to 2500 km. IIRC it's not good low down so we used a standard atmosphere model for atmospheric flight regimes and Jacchia above 182 km (600K feet).

12

I can see your reasoning, however it's not going to work. Aerodynamically the byproduct of lift is drag, meaning when you create lift you also create drag as well. An efficient wing creates less drag per unit of lift, but the drag is still there. A wing works because the pressure of the air above it lower than the pressure below, pushing the wing in the ...

11

Let's compare two rockets with somewhat similar specifications, but one very large difference. Falcon 1- Carries about 670 KG to LEO (See the User's Guide) Mass 38555 KG (Wikipedia). Launched from sea level. Pegasus- Carries about 450 KG to LEO (See Wikipedia). Mass 18,500 KG. Launched from 40,000 feet. This makes a lot of assumptions, but let's just ...

11

When you're in orbit you have velocity roughly parallel to the atmosphere, a flat-ish shape angled properly can "fly" across it in a lifting entry where you can dissipate energy in the thinner upper atmosphere before you hit thicker atmosphere. The first stage of a rocket uses most of its energy to gain altitude, getting above the thickest part of the ...

10

Edgar Andreas made a nice chart for water sublimation in a vacuum at various temperatures: At 270 K it looks like a square centimeter of water ice surface sublimates 100 grams per hour. A typical snow flake masses 3 milligrams. Unless the snowflakes were cryogenic, they'd quickly sublimate Root square mean speed of water molecules at 270 K would be ~.618 ...

10

The scale height is proportional to temperature. As the scale height increases, the density above about one scale height increases. (The density below that decreases.) This is what you'd expect if the whole atmosphere gets heated. In short, the atmosphere blooms, so you are simply getting more particles higher up. The reality is way more complicated than ...

10

There is a fixed amount of energy which has to be dissipated. You can, to some extent, choose how fast this is done -- more air resistance (either by getting into thicker air or having a bigger surface) dissipates it faster, with higher g forces. Less air resistance dissipates it slower, but you do have to make sure to get rid of it all before you hit the ...

10

Rockets don't actually mostly go up, they try quite hard to go up as little as possible. While flying, gravity is always accelerating you downwards at 9.8 m/s^2. This means that any fuel spent accelerating upwards is wasted, as gravity will pull you back to earth eternally, no matter how much fuel you burn (unless you reach escape velocity, but it will ...

10

A starting point for checking orbital stability is the Sphere of Influence for short term stability (or rather, to select a suitable frame a reference in the patched conic approximation), and the Hill sphere for more long term stability (satellites). $$r_{SOI} \approx a\left(\frac{m_{satellite}}{m_{parent}}\right)^{2/5}$$ For a reference spacecraft, I'm ...

10

That's a great software-based experiment! What is this about? It's about drag and Newton's 2nd law of motion! $$F = \frac{dp}{dt} = ma$$ but in the context of orbital mechanics. We can re-arrange Newton's law as $a = F_D/m$ where $F_D$ is the drag force, and the drag equation is $$F_D = \frac{1}{2} \rho v^2 C_D A$$ where $\rho$ is the density at that ...

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