# Tag Info

118

Because linear increases in delta-v require exponential increases in mass, small changes to the assumptions you make about fuel tank structural mass and engine thrust-to-weight ratio start to make very large changes in the final size of the rocket. For example, if you're getting off a 3.6g planet with a 7-stage rocket, the difference between 88% fuel ...

27

First, let us look at the rocket equation: $$\Delta v=\ln \left(\frac{m_0}{m_f}\right)v_e$$ That tells how much a rocket can change its velocity (the $\Delta v$). The requirements for reaching a higher velocity for a minimal orbit would increase on your heavier Earth. (For constant density it is proportional to the radius.) How can we increase the $\Delta v$ ...

19

A few different factors contribute to the Raptor's higher thrust: The specific impulse -- force delivered per mass of propellant consumed -- of methane-LOX combustion is generally higher than kerosene-LOX, because the exhaust is composed of simpler and lighter molecules; The Raptor uses the staged combustion cycle, where the hot partially-combusted gases ...

14

First, let's get terminology straight: "Tilt maneuver", or "Gravity turn", sometimes also called "Pitch maneuver". It was called "Roll Program" in case of Space Shuttles, because it was connected with a roll, necessary for technical reasons but not contributing to flight efficiency directly. All rockets (and all flying bodies on Earth for that matter) are ...

13

The delta-v needed from low Earth orbit to a Hohmann transfer orbit with a periapse inside the Sun is actually "just" 21,300 m/s. But there is a better option. A bi-elliptic transfer to just hit a central body is better when the ratio between the orbital radius and the radius of the central body is larger than 4.82. The orbital radius of the Earth divided ...

13

According to Wikipedia, HIBEX employed a star-grain "composite modified double-base propellant", known as FDN-80, created from the mixing of ammonium perchlorate, aluminum, and double-base smokeless powder, with zirconium staples "randomly dispersed" throughout the matrix. APCP (ammonium perchlorate composite propellant) is occasionally ...

12

This depends on what "better" is. But let us talk about fluorine. The main point of using it is that turning your hydrogen into $HF$ gives more energy than getting $H_2O$ for pure hydrogen, that gives a small improvement of $I_{sp}$. However, for fuels containing carbon, like JP1, (or perhaps RP-1 or JP4. JP1 has some unfortunate properties for rocket ...

12

Launchers generally start at full throttle, so for the most part their immediate performance off the pad is determined solely by the type of launcher and mass of payload. Falcon 9's initial acceleration is quite modest, about 1.5 m/s2 vertically (thrust to weight ratio about 1.15, cancelling and exceeding 9.8m/s2 of surface gravity). Shuttle's initial TWR ...

9

One thing mentioned was that it allowed the vehicle to lift more mass. All things being equal, I fail to see how tilt can increase the ability to increase the potential energy of the payload. All things weren't equal. The Shuttle was not an axially symmetric vehicle. The Shuttle roll program was performed starting about ten seconds after launch and lasted ...

9

I can answer the second question — engines are by and large fuel specific. There's plenty of complicated stuff that goes on in the combustion and the transition to supersonic flow that means you can't just exchange one fuel for another and expect it to be high-performing. As far as I know, there are no pre-existing methalox-burning engines that SpaceX ...

9

There is likely minimal effect. At rocket speeds, there is very little effect of shear stress, the only significant effect is particles hitting the leading surface of the rocket. Also due to how fast hey are going, the effect of the rocket of "pushing air out of the way" does not have time to get far ahead of the rocket, and this drops further behind as ...

7

note: I've accepted an answer 2.5 years ago. This paper was published recently so I thought I would add this supplemental answer since it may be an interesting reference for future readers. The Space.com article No Way Out? Aliens on 'Super-Earth' Planets May Be Trapped by Gravity links to Michael Hippke's ArXiv preprint Spaceflight from Super-Earths is ...

7

Almost all the launch vehicles lift off vertically and are designed to reach orbital speed, altitude and orientation as the upper stage completes its injection burn. Consider a launch vehicle lifting off vertically- The vehicle accelerates to overcome two forces- earth's gravity and the atmospheric drag. Image from rocketmime.com If the launch vehicle goes ...

6

Not a planetological exposition in sight so, I'll add my two cents to this rather theoretical discussion. Amongst exoplanetologists, the consensus has emerged that 1.6 Earth radii and 5 Earth masses is likely to be the upper limit to rocky planets. Simulations have shown that above these figures, the bodies develop increasingly Mini-Neptune like ...

6

This response addresses just the last part of your question "what sort of $I_\text{sp}$ and thrust might it have", and does so in the sense of an ideal resistojet. I hope this isn't too basic an approach but its worth stepping through it to develop a more intuitive view of the very high level physics. There are similar equations in the link you gave though, ...

6

I'd assumed that Isp expresses the axial component of the velocity, so even if there is some transverse flow in the expanding exhaust, that wouldn't have to be accounted for beyond Isp, but is there something else? To be pedantic, Isp is specific impulse, or more fully "mass specific impulse": the impulse delivered per unit of mass flow. Impulse is force ...

6

The final stage of a rocket has to lift not just the payload but also itself. So lets look not at the payload mass alone but at the total mass lifted to the final orbit. According to http://www.spaceflightinsider.com/hangar/falcon/ the empty mass of the second stage is 3,900 kg. Lets add that the numbers in the table. If we add the mass of the final stage ...

6

All else being equal (e.g. propellant choice), higher-energy trajectories favor a rocket with more stages. One thought experiment that can help you understand this is to consider two missions launched on identical rockets, the first to LEO and the second to an interplanetary trajectory. Both missions have payloads sized to "max out" the capacity of the ...

6

There were several factors in the launch date The late John F. Kennedy had promised to go to the moon within the decade. This was a secondary concern, but an important one nevertheless The failure of the Soviet N1 rocket on July 3, 1969. The Soviet program was desperately trying to keep up with the American program. NASA feared the Soviets would overtake ...

6

"Yes", but that would sort of be silly and it would highly depend on where you live, since it may be illegal in your country to mix your own rocket propellant. HIBEX used what is essentially a precursor to modern APCP (Ammonium Perchlorate Composite Propellant), which is widely used in amateur rocketry and commercially available. Its propellant ...

5

Besides pure oxygen, several other oxidizers in theory have better performance with hydrocarbon fuels, but none of them are used because of stability, toxicity and price issues. But let mention some of them: Ozone O3. It's allotrope of oxygen with enthalpy of formation 142,67 KJ/mol (2.97MJ/kg) which can bust combustion energy up to 30% depending on fuel ...

5

Borrowing from Wikipedia's article on modern ICBMs: One particular weapon developed by the Soviet Union (Fractional Orbital Bombardment System) had a partial orbital trajectory, and unlike most ICBMs its target could not be deduced from its orbital flight path. It was decommissioned in compliance with arms control agreements, which address the maximum ...

5

The Ariane 5 user manual has the following data: Using a storable propellant upper stage, through a delayed ignition of this upper stage, Ariane 5, in the A5G version, has demonstrated its ability to carry a satellite weighing 3065 kg, leading to a total required performance of 3190 kg, towards the following earth escape orbit: - infinite velocity V∞ =...

5

The paper does not describe how the calculations for the tether are done, but I can make a guess. We take a small piece of the tether with mass $\delta m$ and length $\delta r$, at distance $r$ from the center. The tether is spinning at an angular rate $\omega$, and has an ultimate tensile strength $\sigma_\text{max}$ and density $\rho$. The area $A$ is a ...

5

They aren't launched slower but they are launched in different trajectories. This can be to control abort conditions and G-Loads. The most efficient launch is to get as high as possible before going for speed. The STS however was shallower to allow for the abort modes and control the forces. The best example I can think of at the moment is the man rating ...

5

Hall effect thrusters do not use strong magnetic fields and radio frequency power to heat electrons to high energy for ionization of the gas. Instead they rely on a DC current of electrons accelerated to a few hundred volts flowing through the gas volume for ionization. Since Krypton has a higher ionization potential than Xenon (roughly 14.0 versus 12.1 eV), ...

5

The season when the Saturn V launches doesn't matter, especially in Florida where the temperature remains relatively warm year round. The Saturn V was engineered to be very resilient and to withstand almost every weather condition. You can see the launch commit criteria of modern rockets here. As Uwe said, there have been Apollo missions launched in the ...

4

These plots are best made using a trajectory optimization program like POST or OTIS, which is used to model the LV to some appropriate level of fidelity. Jacks-or-better is engine performance, some simple aerodynamic tables, operational constraints like max dynamic pressure, angle-of-attack limits, and fairing separation conditions, and of course the mass ...

4

Not sure you'll get much useful data, but here's a useful datum point: Bloodhound SSC - the worlds fastest car is expected to reach 1000mph in 55 seconds from rest (42 seconds would be a perfect run, but 55 is the expected run), with a peak acceleration of 2g. 0 - 300mph (Point a) using the EJ-200 jet engine (generating around 0.5g), where the rocket will ...

4

Ignoring the complicated separation issue, there is a simple relation to calculate the effect on thrust based on mismatch of the exit plane pressure and ambient pressure, namely: $$F = q V_e + (P_e-P_a) A_e$$ Where $P_e$ is the exit plane pressure, $P_a$ is ambient pressure, and $A_e$ is the exit plane area. $qV_e$ without the correction term gives the ...

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