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Hot answers tagged

67

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 ...


52

It's simply the spotlights illuminating the ship, and shining up the bore of the engines. Notice the shadows of the vertical stabilizer from the same source. When the shuttle landing direction is determined, URS Corp. air traffic controllers in the runway control tower will communicate with Bordeaux and his team on the ground. Then two of the ...


49

The original design called for say 500,000lbs of thrust. After years of development, tweaks, changes in the real world (bonuses, like the pressure of the fuel in the line from the entire length of the tank boosts performance (SLS has a sort of issue with this)) means the production engine actually produces 540,000lbs of thrust. Thus full power is now 108%. ...


48

The other answers are great demonstrations of F9's capabilities, but I'll be the contrarian here and say they're all wrong and perhaps Elon oversimplified things for a tweet. This was a one engine landing burn. A single engine lacks the ability to control roll on its own, unless it has a vectoring turbopump exhaust. Merlin 1C had this feature for roll ...


45

There was no active control for spin/de-spin. We did a very good job (if we do say so ourselves) controlling the mass properties of the vehicle with spin testing and balance masses. Yes, there were balanced pairs of spin motors on each side. They were all made in a single lot for consistency in performance. In the screen shot you can see the boom holding ...


44

Take a look at the SABRE engine. The goal is to achieve single stage runway liftoff/land to/from orbit with a hybrid engine capable of breathing air at low altitude but switching to stored oxidizer and operating like a rocket when it is not practical to use ambient air. The limitations of an air-breathing engine for space launch are that You can't go very ...


42

Systems that do this exist and more are being introduced. It's just that they hide their appearance and look somewhat different to what would be expected from what you describe. Orbital Sciences Corporation (now owned by Northrop Grumman) have been air launching the Pegasus satellite launcher since 1990 (almost 30 years). Virgin Galactic's 'White Knight' ...


38

Putting the rocket nozzles nearer the top wouldn't make the rocket any more stable; this is the well-known pendulum rocket fallacy. In fact, some rockets have used a tractor (engine on top) configuration (Goddard's first liquid-fueled rocket in 1926, for example), but the advantages to the pusher configuration as outlined in the other answers here are ...


37

..what reason is there for having the engine at the bottom, .. For the fundamental logical flaw in the desire to move the engines, please see this answer of Russell Borogove - it comes down to The Pendulum Fallacy (the exact same fallacy I made when considering other reasons for 'engines at bottom of stack'). For more on the The Pendulum Fallacy see links ...


32

Rocket thrust is given by the equation $$ F = \dot{m}v_{exit} + A_e(P_1 - P_2) $$ where $\dot{m}$ is the mass flow rate, $v_{exit}$ is the average exit flow velocity across the exit plane, $A_e$ is the cross-sectional area of the exhaust jet at the exit plane, $P_1$ is the static pressure inside the engine just before the exit plane, and $P_2$ is the ...


32

According to Clark's "Ignition!", German rocket scientists in WW2 had done the math on ammonia, and JPL had burned it with RFNA and WFNA oxidizers in 1949-1951. Regarding the XLR99, Clark says: But something more potent than alcohol was needed for the X-15 rocket-driven supersonic research plane. Hydrazine was the first choice, but it sometimes exploded ...


29

The primary reason is one of competence. The Russians have a series of pretty darn good rockets. The RD-171, a 4 engine bell, one turbopump engine (used on Zenit first stage) is an impressive Kerosene/LOX engine. It is a competitor for the F-1 used on the Saturn V in terms of capability. The RD-180 is a 2 engine bell, smaller turbopump version of ...


29

What actual engine are they talking about Organic Marble has identified it as the S5.2/9D21. in what way is it "not a very good missile engine"? It uses a propellant combination, kerosene/IRFNA, common for military missiles in the 1950s-1960s, but now considered obsolete, which yields a poor specific impulse (233 seconds at sea level, or effective ...


28

Anyone know what the story is? The main driver for the large number of engines on the BFR first stage is the desire to use a common engine design (albeit with different optimized nozzles) for both the booster stage and the interplanetary spacecraft stage. Building and maintaining only one type of engine makes things more efficient, and is a strategy that ...


27

Yes, they use TEA-TEB (Triethylaluminum-Triethylborane) as their first stage ignitor and this mixture is pyrophoric (i.e. hypergolic in contact with oxygen, be it atmospheric or LOX), but no, they don't ignite one engine and let that one ignite all the others, they would want to ignite them all at once. Problem with one engine igniting others is that doing ...


27

They might appear as the flame is detached from the nozzle, but that's in essence merely an illusion and the burn is there, all over the plume of the nozzle exhaust. It is however nearly translucent due to high purity of cryogenic propellants and by the chemical reaction producing molecules having high translucency. Visibility of the plume, unless you're ...


27

Initially, every country that built rockets built their own. After WW2, the design of ICBMs and space rockets was related closely enough that the capability to design and build rockets was seen as having strategic value. This situation persisted throughout the Cold War. When the Soviet Union collapsed in 1989, the US government worried about the possible ...


26

The biggest difference is the nozzle. For optimal performance in vacuum, you want a much larger one. According to Spaceflight 101, the chamber pressure is the same, but the expansion ratio (throat area to end-of-nozzle area) is 7 times larger in the vacuum variant, which (if correct) implies about 2.7 times the nozzle diameter if the throat is unchanged. ...


25

Chilldown is needed for engines using cryogenic propellants -- liquid oxygen as the oxidizer most commonly, sometimes in conjunction with liquid hydrogen as the fuel. Pumping large amounts of very cold fluids into an engine resting at ambient temperature causes a variety of problems, so the plumbing and pumps need to be brought down to a working temperature ...


25

The lines that exited at the end of the nozzle were drain lines carrying leakage from seals, output of hydraulic actuator drain lines, etc. The following schematic shows the various systems attached to these drain lines. Source: Rockwell SSME Pocket Data Book, R/RD87-142. This graphic differentiates between the transfer ducts (which carried the hydrogen ...


25

Hydrogen-oxygen engines produce a relatively faint blue flame, with visible blue-white shock discs or diamonds under certain circumstances, particularly low altitudes where air pressure confines the plume. At high altitude the plume is nearly invisible. You can look at a number of shuttle launch photos and see the different appearances the main engine plume ...


23

No, they were never off. Here is my explanation: have you ever noticed that the top engine is tilted down? This is us to keep the center of thrust more or less aligned with center of mass of the space shuttle. As the SRBs produce 83% of the thrust at lift-off, a counter-force was needed to go straight-up. With shuttle engines off, I am not sure the whole ...


22

LMP-103S and hydrazine differ in more ways than that hydrazine unlike LMP-103S is highly toxic and carcinogenic and that LMP-103S has higher performance. LMP-103S requires more catalyst heating to burn and the burning temperature (about 1600 C) is considerably higher than for hydrazine (about 800 C). The propulsion system and thus also the on-board ...


22

According to Mark Hempsell, formerly Future Programmes Director at Reaction Engines Ltd., now CEO of Hempsell Astronautics Ltd., explaining the reason for SABRE's curved nacelle over at NasaSpaceFlight.com forum: Why a Curved nacelle? – the most frequently asked technical question. The answer is: the air intake on the front of the nacelle needs to ...


21

Most thrusters on the Soyuz-MS spacecraft use Nitrogen Tetroxide1 (N2O4, oxidizer) and Unsymmetrical Dimethylhydrazine2 (UDMH, fuel). These are stored (and burned) on the service module and both are fairly stable (from a storage-lifetime standpoint). For safety reasons it was decided that the descent module would use Hydrogen Peroxide3 (H2O2) ...


20

The two big performance considerations for a liquid fueled engine are thrust and specific impulse. On these criteria, the two engines are very comparable. The SSME has a bit more thrust: 1860kN at sea level, increasing to 2279kN in vacuum, versus RD-0120's 1526kN to 1961kN -- a 22% difference at sea level and 16% in vacuum. The other major factor is ...


20

The attitude thrusters and TCMs are mechanically identical, all Aerojet MR-103s. From the Voyager Press Kit: The 16 thrusters on the mission module each deliver 0.89 N (0.2-lb.) thrust. Four are used to execute trajectory correction maneuvers; the others in two redundant six-thruster branches, to stabilize the spacecraft on its three axes. Only one branch ...


20

To get a rocket to space, you have to build it to be really light while containing lots of fuel. This is called the mass fraction: In most rockets, more than 90% of the starting weight is fuel, the rest is divided into the rocket structure (engines, tanks) and the payload (the satellite you want to launch). If you build one really big rocket, it starts ...


19

For a large bipropellant rocket engine to fire safely and stably, the fuel and oxidizer have to mix very thoroughly at high flow rate and pressure before they ignite. Otherwise, there will be sputtering and popping, which is very bad at that scale. Also, in many engines, a "film" of unburned propellant flowing along the walls of the nozzle is critical for ...


19

tl;dr The first mission to run the engines normally at above 100% of nominal maximum thrust: STS-6 The changes to the engine made that thrust increase possible: the 147 design changes implemented between the FMOF and FPL versions of the SSME. Details The SSME had five major versions over the life of the program. The definition of 100% Rated Power Level ...


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