# Tag Info

## Hot answers tagged engine-design

35

The main engineering challenge in implementing your proposal is that in order to be competitive with a chemical rocket engine, the grinding wheel must rotate at an extremely high velocity. A typical chemical rocket might have a specific impulse between about 250 and 450 seconds; therefore, the exhaust velocity is about 2500-4500 m/s. In a competitive ...

18

I don't know if it has ever been considered by anyone. In my view, this is not a good idea for at least the following reasons: It is equivalent to mechanically throwing things retrograde. See this video for an overly simple example. This is obviously not a good way for propulsion, as the specific impulse is very low. Let's talk just about the impulse $$p=... 18 It seems to me you've got two questions: 1) Why can open-expander-cycle engines be larger than closed-? and 2) Why aren't there more expander-cycle sea level engines? Your question[s] shows you are already aware of the scalability problems that haunt expander engine designers. Before we can look at where such problems originate, let's talk a bit more about ... 15 I wouldn't draw any conclusions based on diagrams alone, SSME is very well-known to the public, so we have a more detailed diagram. That doesn't mean that the actual engine is more or less complex, because many things are omitted from diagrams. To prove my point, here is a newer Raptor diagram drawn by a propulsion engineer Elisei Maslov, who's ... 11 A lot of questions here, let's tackle these two first: 4.And last but not least, what's SpaceX's solution for the oxygen-rich environment at 377bar, 748K injector and 546bar, 811K pre-burner? 2.The Raptor's oxygen pump sits directly on top of the main combustion chamber, while the SSME's two pumps are on the opposite sides of the main combustion ... 11 In early versions of the Space Shuttle Main Engine (SSME), the main injector was baffled. The baffles were formed by extra-long liquid oxygen posts protruding from the injector face. Block IA engines and later removed the baffles. (See "Taxonomy of the SSME" in this answer.) The injectors on the preburners were also baffled in a similar manner. As far as ... 11 It's the turbopump exhaust from the booster (jettisonable) engines. From Spacecraft and Boosters by Gatland p. 222-224 Exhaust stack discharges fuel-rich efflux outside reverse aerodynamic flow at base of missile In other words, the main purpose of the long duct is to get the fuel-rich turbopump exhaust away from the bottom of the vehicle to prevent ... 10 They are provided to help damp out combustion instabilities. The main injector uses cooled baffle elements, developed at Glenn in the 1960s to control pressure waves that could destroy the engine. Pressure waves in the space shuttle main engine combustion chamber are also controlled by acoustic cavities. Testing by Glenn engineers determined the ... 9 For hydrogen-oxygen chemical propulsion, you can get on a trajectory that's about three times faster than a Hohmann transfer, but not so much faster that it really changes the game. The delta-v available to you is given by the Tsiolkovsky rocket equation:$$ \Delta v = v_e \ln \frac{m_0}{m_f} Where $v_e$ is the exhaust velocity aka specific impulse of ...

9

Your question is a bit confusing because of your use of "fuel" to sometimes mean "fuel" and sometimes mean "propellants". Based on my interpretation of the question here is an answer. The RD-170 consumes ~2400 kg/s of propellant. ~1800 kg/s of this propellant flows through the preburner of which ~33 kg/s is fuel (RP-1) and ~1767 kg/s is oxidizer (O2). The ...

8

The Everyday Astronaut just released an hour long video investigating this question. Some of the main points are: Aerospikes are especially advantageous to single stage to orbit vehicles, and current space companies are not building those. There isn't really an advantage in SSTOs compared to multi stage rockets. The efficiency advantage of aerospikes isn'...

8

Fundamentally, a liquid rocket engine consists of two parts: the combustion chamber / nozzle; and the turbopumps. Combustion chamber / nozzle It is easier to scale a combustion chamber down than it is to scale it up. Getting the fuel and oxidizer to mix uniformly in a large engine is more difficult due to the larger distances involved. Large engines suffer ...

7

The forces involved in spinning a wheel at high speeds are enormous. At 1600 km/h rim speed, the wheels on the Bloodhound SSC experience 50,000 G. Even the slightest imbalance (from, say an abrasive particle coming loose) would be catastrophic.

7

As long as the wearer moves about, the rotor has no choice but to react to the forces imparted to it. So, yes it would.

6

There is no problem running Kerosene and Oxygen on the same turbopump shaft, at any temperature. provided both are liquid, the density variations are not sufficient to make any practical difference to the feasibility of a turbopump. From the OP, the densities of Oxygen and Kerosene are 1.18 and 0.8 g/cm3 , a ratio of 1.475. The pressure produced by a single ...

6

It's not the boiling point, it's the specific heat capacity. Temperature ≠ heat! Kerosene's cp is only 2 kJ/(kg K)) while hydrogen's is 14.31 kJ/(kg K)), literally an order of magnitude better. That means kerosene is a terrible coolant, and For expander cycle engines, the maximum size and thrust of the engine are limited by the amount of energy (...

6

The RL10 is a terribly expensive engine to produce (I've seen estimates of US$25 million per unit), mainly due to its welded-tube nozzle construction. I expect Vinci will be substantially cheaper to produce on a one-for-one basis, let alone two-for-one. Furthermore, export of RL10s is restricted by US law, though I imagine the ESA could get a waiver if the ... 6 The baffles on the F-1 and other engines such as early versions of the SSME (described here) are intended to prevent the occurrence of high-frequency combustion instability in the engine's combustion chamber. Uncontrolled combustion instability can destroy an engine due to excessive vibration force (which may break engine parts) or excessive heat ... 5 Even if feasible materials-wise, I suspect energy efficiency would be horrible. To spin something fast, where is that (rotational) energy going to come from? If you use an electric motor, you could just modify it to make a plasma drive and use it (more) directly. In your device a lot of that energy gets turned into heat (of the wheel) by friction. Presumably ... 5 According to the paper from the 64rd International Astronautical Congress, Beijing, China The Development of the LOX/LH2 Engine in China; IAC-13, C4.1, 1x18525, the nozzle was welded from spiral-shaped elements. So the spiral pattern is here by design. The spirals are "dump-cooled", an old Rocketdyne idea described in this 1966 NASA Technical Note TN D-... 5 Your question specifically asks about "reliable" thrusters, but makes no mention of any other criteria (such as whether it be used for interplanetary missions). I will focus this answer on reliability, since cost is too complicated and mission-dependent. In this case, we should look for simplicity above all else. Simpler may mean more reliable and in that ... 5 The longest test firing of a developmental Raptor engine was 100 seconds, according to Elon's talk at IAC on September 29, 2017. The duration was limited by tanks, and would only have been superseded by one of the flight burns, which @Machavity has given a good summary, the longest of which was under 60 seconds. 5 TWR was sacrificed for specific impulse in later engines? The NK-33 had a sea-level specific impulse of 297s; the RD engines mentioned above vary from 309s to 311s. My intuition from playing Kerbal Space Program is that this is an unfavorable tradeoff for first-stage engines... In the RD-170 case it's almost exactly an even performance tradeoff; propellant ... 4 Liquid rockets are complex and powerful machines. And typically they complete their job under 30 minutes. It's hard to change phase of propellants from Solid to liquids in 30 minutes considering most of fuels have higher latent of fusion. Some materials may turn into semi-solids instead turning into liquids which has totally different set of properties. Also ... 4 Propulsion comes from acceleration of a reaction mass. In this case the grinding wheel serves two purposes: separate small bits of the workpiece from bulk at a slow and roughly uniform rate accelerate those bits by mechanical friction, in a similar way that a tennis ball launcher uses a matched pair of counter-propagating spinning wheels to shoot a box ... 4 Good for you, for thinking outside the box! Fearlessly pursuing new ideas is where any new breakthrough comes from. But rocket exhaust moves at thousands of meters per second -- supersonic speeds. Recalling the formula relating acceleration to velocity for circular motion, a=v^2/r. So, given a velocity of 3,000 meters per second and a wheel radius of, say, ... 4 The process of calculating chamber temperature is complicated by the fact that the chemical reaction rates involved depend on the chamber temperature...thus the process is iterative. The book Aerothermodynamics of Gas Turbine and Rocket Propulsion by Oates outlines the process in Chapter 3.6 Here is a scan of the page: ---Notes--- Subscript$_c\$ ...

4

For the SSME: I took a shot at this; I had to make so many assumptions, it may just end up as a fun exercise for me without too much anchorage in reality. I got the SSME thrust chamber and nozzle geometry from this paper: Performance predictions for an SSME configuration with an enlarged throat but I had to eyeball them from these low-quality plots: You ...

4

Start by thinking about why the low pressure pumps exist as all. Why can't the entire pumping task be done by single (multistage) pumps on the engine? Why did the engineers feel the need to have separated low pressure (LP) and high pressure (HP) pumps at all? Because the engine gimbals, there needs to be a flexible lines to feed fuel and propellant to the ...

4

The big challenge will be the forces on the outside of the wheel, ripping it apart. We have to pin the numbers to something, and the easiest number to pin down would be the angular velocity. Zippie style uranium enrichment centrifuges operate around 1,500 revolutions per second, so they make a good benchmark. (Some turbochargers go faster, up to 4,800 ...

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