# Tag Info

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The performance of a rocket engine - its specific impulse - is directly proportional to the velocity of exhaust gas (and nothing else!). That velocity is achieved by releasing the combustion products from pressurized combustion chamber (pressurized by continuous production of exhaust gas by burning the fuels) and the higher the pressure the more you can ...

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. ...

17

I've not heard it called that, it is normally called an 'aspirator'. And yet there it is on your drawing :) Perhaps they crunched up 'exhaust aspirator' into one word? As quoted in this answer The aspirator prevented the fuel-rich exhaust gases of the gas generator from recirculating into the missile boat tail during flight. Instead, the gases merged ...

11

In high performance engines the chamber pressure is much too high to be gravity- (or even pressure-) fed. The Space Shuttle Main Engine had a chamber pressure in the ballpark of 3000 and 3500 psi (~200 to ~240 bar). Pumps are required to inject the propellants into a chamber containing such high pressure; head pressure is not a practical means. If your ...

11

During descent, the stage is flying with the rocket nozzle in the flight direction, which means the "bottom" of the tank is also oriented in the flight direction. Since the stage is decelerating due to atmospheric drag (and engine burns) the fuel is "pressed" to the bottom of the tank and can thus be pumped like on ascent.

11

Short answer: shock loads due to high acceleration of the turbopump shaft rubbing between critical seals and other moving parts fatigue in the impeller section Directly quoting from this link: The turbine to drive the separate propellant pumps was an impressive piece of machinery itself-it developed 410 000 watts (55 000 brake horsepower). Designers ...

10

Falcon first stages also have cold-gas thrusters at the top of the stage; you can see them firing for thrust perpendicular to the stage during some landings. Among other things these thrusters can be fired downwards (parallel to the stage) to provide thrust for ullage. A few seconds of thrust will settle propellants at the bottom of the tanks immediately ...

9

$v_{\rm injection}$ of liquids, as a rule of thumb, doesn't get higher than 10 m/s (dictated by a general limitation of pressure losses). Given that exhaust velocity could be 2500 - 4500 m/s, you can calculate the ratio. For gaseous fuel/oxidizer it's of course different. Now, where to get that original estimation...

9

The four destructive LOX pump failures had happened at 110, 110.5, 107.7 and 109 sec; this looked statistically significant, but after much study the team wrote it off to a freak coincidence. Rocketdyne beefed up the impeller vanes, made changes to eliminate fretting and set a 3,500 sec life-limit on impellers used for ground testing. Flight engines never ...

8

Goddard's first liquid fueled rocket in 1926 was pressure fed -- the engine was above the tanks, so gravity feed was not feasible. Liquid oxygen was vaporized with a small alcohol burner, and the resulting oxygen gas pressurized both the LOX and gasoline into the combustion chamber. The V2 is claimed to have the first "large rocket engine" to use a ...

8

The most basic answer is that pressure gives you velocity, and velocity gives you energy. A rocket engine becomes more efficient the faster the particles making up the thrust gases are leaving the rocket through the nozzle. Simple projectile physics: $$E = \dfrac{1}{2}mv^2$$ The energy of a mass, such as a stream of rocket exhaust (and, by Newton's ...

7

Turbo pumps have parts spinning at thousands of RPM, which means the blades in the pump have a great deal of energy. A turbine blade crack can lead to a fracture which will fling the failed turbine part into the pump. The best case would be for the pump to fail in a contained way, either by the piece jamming something or ripping bits of it apart, which would ...

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 ...

6

Do not have the maths skills to generate an example rocket engine turbine/pump system from first principles but the Wikipedia page indicates that efficient operation both as a pump and a turbine requires the plates to be close enough together to achieve laminar flow and that the fluid path make several circuits around the disks (to maintain laminar rather ...

6

In addition to the great answers already given, I would note that all major liquid rockets are “gravity-fed” in a way. They do in fact rely on either gravity or acceleration to push the fuel and oxidisers to the bottom. Where this is most challenging is at stage separation, where the rocket is briefly almost in free fall (– well, free ballistic rise) and the ...

6

"If the pressure in the combustion chamber has to be exceeded by the pressure provided by the pumps, is it also exceeded by the pressure in the gas generator. " Just to be clear, there's no "if" about this ==> "the pressure in the combustion chamber has to be exceeded by the pressure provided by the pumps" Addressing "is ...

5

Your question is very general, so a general answer is that, a valve or valves control the flow of propellants to the turbine that drives the pump. An engine controller determines the proper valve setting based on sensor readings from the engine. For the case of the Space Shuttle Main Engine During the engine run phase, the MOV, MFV, and CCV are switched to ...

4

There are some problems with gravity fed engines: Chamber pressure must be low. Feeding pressure increases with acceleration. Rockets should be tall to increase pressure. Pressure decreases when tanks are discharged. Short experimental rockets are impossible. Gravity feeding may cause combustion instabilities. All three predecessors of the A-4 (V-2), the A-...

4

Working off the Falcon pump power of 7500 kW and power to weight of 10 kW/kg (electric aircraft engine) and assuming generator weighs the same as the motor, that gives a best case motor/generator set for a merlin engine of 1500kg, on a existing engine mass of 750kg so certainly not lighter than the seals and associated plumbing. Some possible cases where it ...

4

In this post, I'm talking about engines slightly abstractly, using "pump" to mean the entire pumping complex or "powerhead", which might be made of multiple actual turbopumps, and "combustion chamber" to mean the main combustion chamber, not any preburners or gas generators. Most often you'll see one pump per combustion chamber and nozzle. Some large ...

4

There isn't a particularly meaningful answer to this, but I hope I can provide some insight. Mostly it boils down to the observation that injection velocity is not particularly meaningful/constant-or-optimised between rocket designs. Injection mass flux is the interesting engineering quantity ($v \times \rho \times A$), where $v$ is velocity, $\rho$ is ...

4

For the most common engines, all of the exit velocity is “due to combustion” because the pumps are driven by fuel-oxidizer combustion. One of the key differentiators among engine designs is what they do with the exhaust of that pump-driving combustion: how much thrust is generated from those combustion products? More is better, of course, but complexity, ...

4

According to the monograph NASA SP-8107 Turbopump Systems for Liquid Rocket Engines (table II) : The F1 fuel (RP-1) pump had an inlet pressure of 45 psi and an outlet pressure of 1856 psi. The J2 fuel (Hydrogen) pump had an inlet pressure of 30 psi and an outlet pressure of 1238 psi.

3

Such a system would be the worst of both worlds: you would have the weight and complexity of two systems, with very little benefit compared to just using one system. precision: no. As soon as you switch from the electric motor to the rocket preburner, you lose that advantage. startup simplicity: no. Startup may be simple, but now you have to switch from ...

3

Is this soot building up on the Falcon 9 interstage? Partial, speculative science-based answer: It certainly could be! According to Wikipedia's Soot; Soot formation mechanism Many details of soot formation chemistry remain unanswered and controversial, but there have been a few agreements: Soot begins with some precursors or building blocks. Nucleation ...

3

First of all, let's specify the meaning of all the terms along with respective units (S.I units) - $$P_{p} = \frac{\rho Q\Delta H g_{0}}{\eta_{p}}$$ $P_{p}$ = Pump Power ($W$) $\rho$ = Propellant density ($Kg/m^{3}$) $Q$ = Volumetric flow rate ($m^{3}/s$) $\Delta H$ = Differential Head ($m$) $g_{0}$ = Acceleration due to gravity ($9.8\thinspace m/s^{2}$) \$...

2

To add one more reason for turbopump pressurization - combustion stability. It is of paramount importance that the combustion of fuel and oxydizer is performed in a stedy, controllable and safe manner. Taken for granted and needs no explanation, right? But the physical reality of reaching this desirable condition made generations of engineers loose their ...

2

Probably not in a way analogous to a hybrid-car's system, where either system could drive the car, as @hobbes discusses. However, perhaps in a lesser capacity an electric augmentation may make sense. Especially if battery was already needed for other reasons: having and electric motor/generator attached to a turbo pump axle - driven off or charging this ...

2

This supposed to go in comments because it's not a complete answer but I got to put it here. I'm considering rocket scales based upon LEO payload. Micro - payload to LEO in few kgs, Small - payload to LEO in few hundreds of kgs. medium - payload to LEO in few tons. heavy - payload to LEO in few tens of tons. ultra heavy - payload to LEO in few hundreds of ...

2

While you're correct in your assessment of "since pump fed systems can create the necessary pressure, they can work with less pressurants and lesser storage pressures making tanks thinner" you may be missing the degree to which these two differ. Some other concerns exist, but they are secondary - the main issue is the vast difference in pressure needed. Of ...

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