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41 votes
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Why are unused launch abort rockets jettisoned rather than used to augment thrust?

The thrust of the Launch Escape System (LES) might look significant, but the total impulse is not (that is, they burn for such a short time, they can't actually impart much delta-V, especially not ...
CBHacking's user avatar
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35 votes
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How can a rocket approaching the Karman Line then return to earth faster than 53 m/s terminal velocity?

53 m/s is the approximate terminal velocity of a human skydiver. The terminal velocity of a 7-ton metal dart is quite a bit higher. Larger objects tend to be affected less by atmospheric drag than ...
Russell Borogove's user avatar
35 votes
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Why does the Indian PSLV rocket have tiny boosters?

Why would these be used instead of just using a larger first stage? Strapping on differently-sized boosters allows variance in payload mass without redesign of the first stage. The PSLV has flown with ...
Organic Marble's user avatar
33 votes
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How is thrust transmitted from strap on boosters to the central core?

It’s a sense of scale issue. As much as the struts might look like flimsy bits of drainpipe, those rockets are around 15 meters wide, and the struts are more like the heavy steel beams used to hold up ...
Quentin Clarkson's user avatar
31 votes

Why do nuclear rockets (e.g. NERVA) have such poor Thrust-to-Weight ratios?

At their core nuclear rockets working by heating a working fluid and running it out a nozzle are still constrained by the same physics as a chemical rocket where exhaust temperature cannot be much ...
GremlinWranger's user avatar
28 votes

Why are optimal launch TWRs so different in KSP than on earth?

It is no longer generally considered optimal to use a launch TWR of 1.5-3 in KSP. A little background: In KSP players who were optimizing for launcher weight would go for a TWR of 2.0 or higher, so ...
Blake Walsh's user avatar
  • 4,221
22 votes

How is thrust transmitted from strap on boosters to the central core?

For Delta IV Heavy, according to Spaceflight101: The CBCs functioning as boosters are attached to the central core using thrust struts that interface with the interstage section of the launcher to ...
Russell Borogove's user avatar
22 votes
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What is the lowest first stage thrust for a launch reaching orbit?

The U.S. Vanguard rocket reached orbit three times with a first stage thrust of only 125 kN. The first stage of the three-stage Vanguard Test vehicle was powered by a GE X-405 28,000 pound (~125,000 ...
2012rcampion's user avatar
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21 votes

Why do nuclear rockets (e.g. NERVA) have such poor Thrust-to-Weight ratios?

The one exception to this fact is Project Orion Not quite. Project Timberwind was a solid-core NTR using a pebble-bed reactor design that combined high Isp with a moderate T/W of 30. The DUMBO NTR ...
Starfish Prime's user avatar
20 votes

Payload capacity of a rocket

The short answer is: Tsiolkovsky rocket equation. You need some velocity to achieve some position (an orbit or a body) in space. Farther a position - more velocity. More velocity - more propellant ...
Peter Nazarenko's user avatar
19 votes

How far would the STS get without the SRBs

TL;DR Without SRBs, by maintaining the real stack's thrust to weight ratio at ignition, you run out of fuel at about 167 seconds with (if we maintain the flight profile of the real stack) a velocity ...
user's user avatar
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17 votes

Payload capacity of a rocket

It perhaps become clearer when stating what rockets do. They change velocity. In space terms, that's delta-v. A rocket stage can only change your velocity some limited amount. Different targets in ...
SE - stop firing the good guys's user avatar
17 votes

Why are unused launch abort rockets jettisoned rather than used to augment thrust?

The thrust of the Apollo launch escape rocket was negligibly small when compared to the thrust of the Saturn V first and second stages. The burn time of the launch escape system was negligibly small, ...
Uwe's user avatar
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17 votes
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Does "% Throttle" refer to fuel flow or thrust?

For the Space Shuttle Main Engine, "% Throttle" (actually in shuttle vernacular, % Power Level), referred to the chamber pressure. It was the desired value of chamber pressure that was used ...
Organic Marble's user avatar
16 votes

Where does the thrust act in a rocket engine and how is it transferred to rocket structure?

The thrust acts on the nozzle and combustion chamber walls by virtue of the pressure differential they contain. Using the RS-25 (SSME) as an example, with illustrations from this pdf, I can highlight ...
Tristan's user avatar
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16 votes

Earth Launch System with Water Propellant

Electrolysis-based propulsion becomes practical only once you've reached orbit, where you can power the electrolysis with solar panels and where you don't need enormous thrust. Whatever you'd use to ...
Camille Goudeseune's user avatar
16 votes

How are thrust specifications for multi-nozzle engines given?

Instead of seeking sources, I'll do a little bit of calculation: 1000kN per engine: 1000kN is about 100 tons-force. Wikipedia lists R7 launch mass as 280 tons. 500 tons-force of thrust (5 engines x ...
SF.'s user avatar
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16 votes
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How high is the ISP of a fire extinguisher

A conventional CO2 fire extinguisher could be used as a cold gas thruster. The maximum Isp would be determined by the temperature of the gas. CO2 is stored as supercritical CO2 inside the bottle. It ...
Woody's user avatar
  • 22.1k
15 votes

Rocket engine test bed structure

It had a heavy concrete foundation to which the metal structure of the stand was attached. Shown is test stand 1-C at Edwards Air Force Base. And in use with an F-1 running. There was an F-1 stand ...
Organic Marble's user avatar
14 votes

Merlin 1D Engine Throttling

SpaceX is not exactly forthcoming with detailed information about the Merlin's workings but we can figure some things out. Tom Mueller, designer of the Merlin, gave a brief description of the engine ...
Organic Marble's user avatar
14 votes
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How much G force is applied when correcting an orbit around Earth?

In practice, the g force applied for orbital corrections is very small. The satellite operator has plenty of time to make the correction, and if you are capable of accelerating the satellite at more ...
Russell Borogove's user avatar
14 votes
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Why do chemical rockets encompass a weird shape on this thrust vs. Isp graph?

I'm guessing that the chemical rocket envelope in the plot encompasses points representing actually-built rocket engines, rather than theoretical ones, hence some of the irregularity of the shape is ...
Russell Borogove's user avatar
14 votes
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Saturn 1 thrust mismatch

There were different versions of the RL 10 engine with different thrusts. The retired RL10A-1, first flown 1962 had a thrust of 67 kN (15,000 lbf). The active RL10B-2, first flown 1998 has a thrust of ...
Uwe's user avatar
  • 49k
13 votes
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why do under-expanded engines have less than ideal thrust?

The hard part is that $P_e$ isn't a completely independent variable. As the gas expands past the throat, thermal energy is being converted into kinetic energy. The gas cools down and speeds up. So ...
BowlOfRed's user avatar
  • 6,882
13 votes

What is the lowest first stage thrust for a launch reaching orbit?

@2012campion's answer shows that this was not the lowest thrust The smallest rocket to reach orbit is the Japanese SS520-5. It had a peak thrust of $185 kN$ according to the same web page: Firing up ...
Steve Linton's user avatar
  • 19.6k
13 votes

How are thrust specifications for multi-nozzle engines given?

The nozzle is not the engine. Each nozzle is attached to one combustion chamber, but by convention, a complex of multiple combustion chambers sharing a single turbopump, such as the RD-107/RD-108, is ...
Russell Borogove's user avatar
12 votes

Why will SLS Block I bring less mass to LEO than the STS shuttle system did?

It's always difficult to make apples-to-apples comparisons between the space shuttle and other launchers, because the orbiter is ambiguously part launcher and part payload. This is compounded by the ...
Russell Borogove's user avatar
12 votes

Why are unused launch abort rockets jettisoned rather than used to augment thrust?

The weight and complexity of using the launch escape system negate any advantages. Using the LE rocket as it was designed, i.e. to give a brief, powerful burst would have meant strengthening the ...
GdD's user avatar
  • 20.3k
11 votes
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What is the correct thrust curve for a solid rocket with a simple circular hole, and why?

Hill and Peterson "Mechanics and Thermodynamics of Propulsion", third printing, November 1970, page 385, has a diagram that agrees with your intuition. (sorry for poor scan quality) You are ...
Organic Marble's user avatar
11 votes

What pushes back on rockets?

Newton's Third Law states that if object A pushes on object B with a certain amount and direction of force then object B pushes on object A with same amount and opposite direction of force. Notice ...
DrSheldon's user avatar
  • 48k

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