Simple non destructive spaceship liftoff engine
I like to have a concept of an engine that spaceships can use to lift off (and land) from/on a planet (regardless of it having an atmosphere or not) with the following constraints:
- It should be simple to "understand" for the audience
- It should not consume more than 10% of the ships mass in fuel
- It should not destroy the place of liftoff or landing
- It should not violate fundamental laws of physiscs conspicuous even to a 12 year old with some interest in physics.
To elaborate a little bit:
A classic reaction mass drive (thruster) needs either a lot of reaction mass (violating 2.) or a lot of energy (violating 3.) or both. A "gravitation absorber" that let us say blocks gravitation will in my opinion either violate 1. or 4. First of all, to just hold the ship in place floating free the absorber needs no energy at all (disturbing? Imagine how much energy a platform on a tower needs to hold you in place.) If he needs energy it must go somewhere (excess heat? Gravitational waves?) When you start to move up (with additional thrusters?) it becomes complicated (violating 1.) The faster you move up the more energy the absorber needs (violating 1.) to compensate for the potential energy your ship gets by gaining altitude. If it does not, 4. is violated. When it comes to landing there is the opposite problem. If you loose height, the absorber needs to convert the lost potential energy of your ship to somewhat. Somewhat harmless if you not want to violate 3. If you turn off the absorber, than you will gain speed and have to break somehow to avoid hitting the ground.
So is there any concept (beside assissted with external help like space elevator, space cable, ...) for a drive that could do this? Perhaps a reaction mass drive which uses particles (like neutrinos) that don't interact with conventional matter (how could they be produced in the needed amount and accelerated and transmit their impulse to the ship?) or something like that?
This post was sourced from https://worldbuilding.stackexchange.com/q/64330. It is licensed under CC BY-SA 3.0.
1 answer
I'm going to take the comment you posted on the question, because it is important.
The story should not be driven or build around the technology. But I have the ambition to also not just handwave it, because I'm myself interested in physics and does not like stories that doesn't care about the most fundamental laws. "“ Hothie 2016-12-12 14:26:57Z
Sorry for being blunt, but it is not possible to meet all four of your criteria with anything which we know how to build.
The go-to for rocket engines is the Tsiolkovsky rocket equation:
$$ \Delta v=v_e \ln\left(\frac{m_0}{m_f}\right) $$
Stated another way:
$$ \frac{\Delta v}{v_e}=\ln\left(\frac{m_0}{m_f}\right)=-\ln\left(\frac{m_f}{m_0}\right) $$
where $m_0$ is the initial mass, $m_f$ is the final mass (also known as dry mass), $v_e$ is the exhaust velocity, and $\Delta v$ is the resultant change in velocity. $\frac{m_f}{m_0}$ is referred to as the "mass ratio". Note that this applies only to single-stage rockets; that's why practical launch vehicles tend to use two or three stages. Look at how the mass ratio grows with the total delta-v divided by exhaust velocity (image by uhoh from this answer on Space Exploration SE):
For a mass ratio of 10% fuel to initial mass, we have $$ \Delta v=v_e \ln\left(\frac{1}{0.9}\right) \approx v_e \times 0.10536 $$
Ignoring drag and gravity losses (which only make this harder), a spacecraft launched from Earth must attain about 7 km/s of forward velocity to enter a reasonably stable orbit. You can get away with less forward velocity, but that costs you in terms of gravitational potential energy instead, so is not a solution. Rearranging the above, we get $$ 7\,000 \approx v_e \times 0.10536 \Rightarrow v_e \approx \frac{7\,000}{0.10536} \approx 66\,400~\text{m/s} $$
In other words, our rocket engine must have an exhaust velocity in excess of 66 km/s in order to attain the required delta-v within the desired mass ratio. The crux is that we also need sufficient thrust to get off the ground; if the engine does not have sufficient thrust, the rocket is almost literally sitting there spinning its wheels. (Compare How far would the STS get without the SRBs on Space Exploration.) Wikipedia has a decent table of methods of spacecraft propulsion, but none of the alternatives listed both:
- Has at least been tested in a vacuum chamber on Earth
- Provide a large amount of thrust
- Provide sufficient exhaust velocity
We simply don't know how to get the necessary exhaust velocity to generate the required delta-v at the mass ratios you envision, with sufficient thrust to get off the ground.
Also, no matter what you are throwing out behind the engine at 65-70 km/s or more with sufficient mass to generate a reasonable amount of thrust, it's going to be dangerous. Even with our current puny 3-5 km/s exhaust velocity pea-shooters, the safety distances are considerable.
Rocket boosters for taking off from large bodies such as Earth will be large and mostly filled with fuel for the foreseeable future. Even the Apollo Lunar Module was on the order of 50% fuel, and the Moon's gravity (which was all it had to contend with) is far smaller than Earth's (to the tune of about 1/6th Earth gravity).
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