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One thing that was very clear when playing Kerbal Space Program is that you need a lot of fuel to launch a rocket. What's worse, the more fuel you add, the heavier the rocket gets, requiring even more fuel for launch. A huge part of the rocket seems to be just a fuel tank, and most of the fuel is spent just getting barely (in the scale of things) off the ground.

I always wondered, why are rockets not given a speed boost with another vehicle before they start burning fuel? What I was thinking would be relatively simple is effectively a train that carries the rocket, it can get to as high a speed as possible before curving upwards, releasing the rocket and coming back down. Only on release would the rocket need to start burning fuel, but by then it could easily have gained 100~200 km/h of speed.

Doing that, the rocket could be much smaller, which means it needs a lot less fuel to continue the travel. Bonus point is, the train could be powered by (renewable) electricity.

In illustration:

       Point where rocket is released    _
                                 \      / \
\\\\\\\                           \--> |   \
 =======|=> <- rocket                 /     \
///////                             _/       \_
 =======>>  <- train on rails    __/           \__
__O___O_________________________/                 \____
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  • $\begingroup$ These type of ideas have been discussed many times over the years including on this site. In this example part of the question states "With this concept the launch vehicle is supported by an eastward pointing rail or maglev track that goes up the side of a mountain." So far the advantages of these ideas don't outweigh the disadvantages. There is a recent video by Tim Dodd that you might be interested in, Why Don't They Launch Rockets From Mountains Or The Equator? $\endgroup$ Commented Mar 25 at 5:18
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    $\begingroup$ 200 kph is a tiny amount of speed to add to a rocket. The amount of space required for your railway and the complexity of completing cryogenic propellant load on a moving platform would be prohibitive. Your ending slope would probably use up most of the velocity too $\endgroup$ Commented Mar 25 at 7:18
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    $\begingroup$ 200 kph is 0.71% of orbital velocity and less than 0.6% of Δ-v. How is shaving off less than 0.6% going to make the rocket "much smaller", especially if you consider that rockets are not designed to be in a vertical orientation when fueled, so will need to be reinforced, which adds mass. $\endgroup$ Commented Mar 25 at 7:32
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    $\begingroup$ See also questions related to air launch space.stackexchange.com/questions/tagged/air-launch and launch assist space.stackexchange.com/questions/tagged/launch-assist, possibly the most complete single answer space.stackexchange.com/q/5463/26356. $\endgroup$ Commented Mar 25 at 7:36
  • $\begingroup$ A linear accelerator might make a good launch system on the moon, though -- without drag to deal with and only 1/6th of earth gravity, a multi-mile magnetic catapult track should be able to chuck a payload pod into lunar near-orbit (it would need to circularize at some point), or even put a totally unpowered cargo pod on an earth-return trajectory. That was a plot point in the 2009 indie film "Moon", for example. $\endgroup$ Commented Mar 25 at 19:59

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When determining the best approach for getting to space, the first consideration is the mission objective. For example, placing a small satellite into low Earth orbit (LEO) is a very different mission from setting up a supply chain to a colony on Mars.

Most missions today involve sending small payloads to LEO, but future missions may be significantly larger in scale.

Let's start with the smaller mission. Let's say that you already have a sub-orbital capable rocket, and your challenge is to make it orbital. Your scientists figure out that to do this you need to increase the delta-v of your rocket from 8500 m/s to 9500 m/s.

The performance of the current rocket can be characterized by a curve that tells you the "payload mass" to "initial mass" ratio versus delta-v. This curve is based on the rocket equation but also takes into account things like staging and the ability of your engineers to achieve a certain weight while maintaining reliability.

The chart below shows such a curve for a two stage rocket with a first-stage exhaust velocity of 2,875 m/s, a second-stage exhaust velocity of 3,018 m/s, and a variable mass ratio* of 0.05.

(*The "Variable Mass Ratio" is roughly the dry mass of the rocket divided by the rocket’s initial mass. Think of it as the mass of all the stuff you need to engineer and optimize to make the rocket work over the total mass of the rocket.)

curve for delta-v of 8500 m/s

This curve tells you that, if your target is a delta-v of 8500 m/s, the current rocket-plus-payload mass will be 39 times the payload mass. It also tells you that if you want a delta-v of 9,500 m/s (1000 m/s faster) with this technology, then the ratio will increases to 85.

curve for delta-v of 9500 m/s

One way to add the needed 1000 m/s of delta-v is to make the rocket 85/39=2.18X larger. Alternately, you can reduce your payload's mass by a factor of 2.18X. A combination of the two, such as a 48% payload reduction and a 48% initial mass increase, will also work.

Another proposal might involve a ground-launch-assist strategy that involves building a railway track and a vehicle that will carry the rocket up a mountain at 1000 m/s and release it.

In the context of this mission the team that proposes to make the rocket bigger and the team that proposes to make the payload lighter are likely to win the engineering debate in the conference room. Both teams have years of experience making rockets bigger or payloads lighter.

The ground-launch-assist proposal, on the other hand, will involve a lot of R&D, capital expense, acceptance of risk, and just won't add much benefit for this kind of mission. It's simply easier to make the rocket bigger and the payload lighter.

But if the nature of the mission were to change significantly, then it could be worthwhile to reevaluate the various ground-assist launch technologies that many people have proposed in the past.

Let's suppose that your goal is to impart enough delta-v to payloads to send them to Mars (you need ~17,120 m/s of delta-v for that - ref) and your mission involves sending thousands of tons to Mars over the next decade.

In this case, the teams that can make rockets larger and payloads lighter are going to be hard pressed to come up with viable proposals.

curve for delta-v of 17120 m/s

In fact, to be able to place anything on Mars they are going to need to drastically change the shape of the curve - which means the way that they have historically done things. It means switching to engines that burn hydrogen to achieve a higher exhaust velocity, using lighter-weigh materials (and doing lots of testing) to reduce the rocket's variable mass ratio, adding more stages, not attempting to recover anything, etc. Whatever they come up with, it will be expensive (that is, it will probably look a lot like the SLS). From experience we already know based on past missions that it costs in excess of one billion dollars per ton to place payloads on the surface of Mars.

In this context, a ground-launch-assist technology has a much better chance of being the most technically and economically viable proposal, even if it involves: a) Maturing some low Technical Readiness Level (TRL) technologies, and b) Is relatively capital-intensive. This will be especially true if its proponents can successfully demonstrate the "cost-per-kg versus delta-v" curve overtakes a similar curve for a state-of-the-art chemical rocket-based system.

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    $\begingroup$ Putting a rocket on a train at mach 3 then making it travel up a mountain to turn to takeoff seems pretty unlikely to succeed or be economical. In orbit re-fuelling is a much more likely way to solve the problem of getting mass to mars and is already the plan for starship $\endgroup$ Commented Mar 25 at 8:08
  • $\begingroup$ Sure, lots of people seem to be convinced of that. First of all, we need more people who are willing to zig when everyone else zags. Second, the physics argument for finding a way to "push hard" off the Earth is pretty compelling. Third, more people should trust the physics over Elon. $\endgroup$
    – phil1008
    Commented Mar 25 at 8:31
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    $\begingroup$ Yes, the physics is sound, the engineering, not so much. It might make sense on the moon where, in a vacuum, with plenty of open space, you could potentially launch from rails with no rocket needed at all $\endgroup$ Commented Mar 25 at 11:35
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    $\begingroup$ Who said anything about Elon Musk? This type of idea has been looked at for decades by rocket scientists but so far not found to be a better method. It's considered more viable as a method on airless planets and moons. That doesn't mean that one day they won't find a practical way to do it on Earth, but the OP is asking why isn't it being done now. And since you brought up Elon, in fact he's the one who has been zigging in rocket design when everyone else was zagging. $\endgroup$ Commented Mar 25 at 12:28
  • $\begingroup$ @AlanBirtles Yes it definitely make sense on the moon. As for Earth, for every engineering challenge associated with ground-assist-launch, there's a similar engineering challenge with making chemical rockets cost effective at going beyond LEO. Also, don't forget that rockets launching frequently from Earth need lots of open space too. $\endgroup$
    – phil1008
    Commented Mar 25 at 16:06

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