Hot answers tagged

71

The ascent engine was normally fired when the LM was sitting on the moon's surface, so the tankage was subjected to about 1/6 $g$, more than sufficient to separate the dense liquid fuel from the helium pressurant. Once the engine was ignited, its own thrust accelerating the spacecraft would maintain the ullage. The same question could well be asked of the LM'...


19

As suggested by OrganicMarble in a comment, nitrogen is miscible with oxygen (you can thus make liquid air). According to NASA Technical Paper 2464, this is a major concern because using "enriched air" instead of pure oxygen as the oxidizer degrades the performance of the engine: The transfer of liquid oxygen (LOX) from a storage vessel to a ...


19

Imagine your telescope optics looked like this red-hot glass! Herschel's instruments look at the world in the wavelength range of 55–672 µm. When plotted as a function of wavelength, the thermal spectrum of a black body peaks $ \approx 5 k_\mathrm B T$. The boiling point of liquid helium is 4.2 K. The peak wavelength for something at that temperature would ...


18

(Top edit: The Question asserts "Xenon and krypton are popular despite their heavy mass" and asks about exploring H or He ion propellants for improved Isp. This answer shows that lighter is not better for ion thrusters, because Isp is not the proper measure of a power-limited situation. Hence, although lighter atoms have been explored for other reasons, ...


18

Pumps are great at creating positive pressure, hundreds of bars. But with tank at ~1 bar, atmospheric pressure, they can only create a grand total of 1 bar of suction - can't go more vacuum than vacuum, can't create a negative density, can only go from original to zero - and 1 bar of pressure is a pretty meager amount, when this kind of flows is involved. ...


16

A good pressurizing gas needs to satisfy a few basic properties: it needs to stay gaseous at the temperatures and pressures your fuel and oxidizer are stored at, so that it won't just condense out when injected into the tanks; it needs to be inert enough to be safe to mix with both the fuel and the oxidizer (since if you're going to have two different ...


16

The shuttle propellant tanks required pressurization. Prelaunch, the tanks were pressurized by helium, supplied from the ground support equipment. After Space Shuttle Main Engine ignition, the system used autogenous pressurization. Hot gaseous hydrogen and oxygen were tapped from the engines and piped to their respective propellant tank to pressurize it. A ...


16

Helium was used to pressurize the RP-1 tank to save weight. Nitrogen is much cheaper but its density is 1.250 kg/m$^3$, helium is 0.1785 kg/m$^3$, so 1.071 kg is saved for every cubic meter of emptied tank volume. To pressurize a fuel tank with pure oxygen is a bad idea, only inert gases like nitrogen or helium may be used. The first stage RP-1 tank volume ...


15

Interesting and non-trivial questions. Most propellants are not self-pressurized because as soon as the engines turn on the pressure would drop precipitously as the tanks quickly emptied and the propellant was unable to vaporize fast enough to keep up. LOX, RP, and H2 are the most common launch vehicle liquid propellants and none vaporize fast enough to ...


14

Edit Jan 2, 2017: Well, it was a COPV bottle after all. There were buckles in some liners where super-cooled liquid oxygen pooled. From NASAspaceflight article, quoting the results of the investigation: Each stage of Falcon 9 uses COPVs to store cold helium which is used to maintain tank pressure, and each COPV consists of an aluminum inner liner with ...


14

Because all you have to do to get gaseous oxygen is take liquid oxygen and warm it up a bit. That means just tapping off a little gas from the engine and routing it back to the tank to keep the ullage pressure where it needs to be. Contrast that with RP-1, which doesn't really vaporize enough to get the pressure where it needs to be. For that, they had to ...


13

Herschel was an infrared space telescope. According to this paper, the performance is expected to be not far from background-noise limited, with sensitivities (5σ in 1h) of ∼ 4 mJy or 3 − 20 × $10^{−18}$W/m$^2$, respectively. At most temperatures, the amount of heat radiated by the spacecraft itself would easily overwhelm the infrared signals it was ...


13

Helium is used as a pressurant and purge gas. While no longer a "modern" launch system, the Space Shuttle made extensive use of helium to pressurize various systems and can perhaps serve as an example. Prelaunch pressurization of the External Tank liquid oxygen and liquid hydrogen tanks (on the order of 100 lbm of He for each tank) During standby ...


12

If there is a rocket using cryogenic hydrogen tanks, you need helium for it. Liquid hydrogen should not be mixed with air, oxygen or nitrogen. A mixture with oxygen is explosive and should be avoided for security. Nitrogen gas would liquify and even freeze at the temperature of hydrogen. Other noble gases like argon would liquify, even solidify at the ...


11

Thanks, that much is clear; the question is more why can they not make do with out the helium? Because the shuttle didn't need it and the MCT reportedly also won't need it, so why the falcon? All rockets need to keep the tank pressurized during flight. Some use hot gases from the engines to do that. Others don't. The fuel used in the rocket is the ...


10

Bottom line: like so many things in rocketry, it's a design trade, usually at the system level. The book Pressurization Systems for Liquid Rockets goes into great detail on this. I highly suggest you read Chapter 2.1.2 "Selection of System Type" which pretty much covers your exact question. There is even a case description on page 33 where the author goes ...


8

1) "do both stages need to pressurize the tanks, or only the upper stages?" In general all stages need pressurization. I am unaware of any counterexample. If the engine is pressure-fed, you need higher pressure than the combustion chamber pressure. If the engine is pump-fed, you need sufficiently high pressure to keep the pumps from cavitating. 2) "And ...


7

Given that you are using a liquid propellant you cannot rely on its own compressibility to achieve the required pressure. Liquids do not compress very well. This is especially true as you need a high pressure for your thruster and to make it worse: usually you do not want this pressure to change through the time of engine usage. Therefore, the common ...


7

NASA actually have looked into mining the gas giants, http://mdcampbell.com/TM-2006-214122AtmosphericMining.pdf, this paper outlines some the methods they might use, whether they will or not is a different question. They were considering mining methods, of Uranus due to its relatively low wind speeds (compared to Jupiter, Saturn and Neptune, such as ...


6

Helium is a gas at any reasonable temperature, so a system using helium would be pneumatic. Pneumatic systems are less suited for exact position control than hydraulics: because gases are compressible, it's difficult to predict the piston position from a given input. In hydraulics there's an exact relationship so you have good position control, and ...


6

It was thought, initially that the grid fins used RP1 propellant as a hydraulic fluid, when it was using an open system (The fluid is ejected after use until you run out, which they did). After a landing failure, they changed to what they call a "Closed System" where the system reuses the hydraulic fluid, but I cannot find a reference to that actually being ...


6

There are studies into mining in space, but not for the gas giants. The outer planets are massive. Very massive. This means they have very big gravity wells. Lifting anything from Jupiter is going to be extremely energy-intensive, getting it back from Jupiter to Earth is too. So, currently, this is the realm of science-fiction. As for the second part ...


6

In a turbopump feed system it is still necessary to pressurize the tanks slightly (10 to 50 lb/in2) in order to prevent pump cavitation. Rocket Propulsion Elements, Sutton, 4th edition, p. 223 Different vehicles have used stored onboard gas (typically helium) or autogenous propellants to provide this pressurization. Further reading: Why does the Falcon 9 ...


5

We don't need to look into the phase diagram here, because it doesn't matter on which path we reach a certain end point. We can safely assume that we heat up the helium at constant pressure and then shrink its volume again. Density of liquid helium boiling at 1 atm pressure is 0.125 $\rm g/cm^3$. Density of gaseous helium at 273 K is 0.18 $\rm mg/cm^3$. ...


5

The helium evaporated away, as it was intended to do, thereby helping to cool the critical parts of the telescopes' detectors. The difference between these telescopes and Hubble are the frequencies of "light" which they detect. Hubble operates mainly with visible and ultraviolet light (occasionally near IR) while the others you mention operate with ...


4

The Merlin engines use Helium to purge before launch. At least in the Merlin 1C days, and you can see several aborts that were called because of Helium issues. For the upper stage it is used to pressurize the tanks to make sure that the fuel and LOX flow into the engines properly. The pumps suck and the Helium pressurization push. Helium is used since it ...


4

I know that it has been theorized about harvesting isotopes of Helium or Hydrogen space, including studies for / by NASA. In fact, NASA did study mining celestial bodies, including other planets, and still does. The study report "Space Resources" (NASA SP-509, 1992) has become a classic. It can be found here http://www.nss.org/settlement/spaceresources/...


4

It's possible, but not simple or cheap. This process is called air separation and is the primary method for producing (liquid) nitrogen and oxygen. Pure gases can be separated from air by first cooling it until it liquefies, then selectively distilling the components at their various boiling temperatures. In ambient air, there's only a tiny fraction of ...


4

To acheive full thrust of the first stage F-1 engines of the Saturn V only liquid oxygen should be pumped into the combustion chambers. A mix of gaseous and liquid oxygen would reduce the desired mass flow of oxygen to the engines and may damage or even destroy the oxygen pumps. A reduced mass flow of oxygen would reduce thrust and endanger a successful ...


3

There aren't many atomic or diatomic gases at room temperature: H, N, O, F, Cl and the noble gases. noble gases are expensive F and Cl are corrosive oxygen reacts with propellants, so can only be used to pressurize an oxygen tank H is flammable/explosive that leaves N and the cheapest of the noble gases, He. Compound gases also exist (e.g. CO2) but they ...


Only top voted, non community-wiki answers of a minimum length are eligible