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I'm no rocket scientist, but I've been reading up on some basic concepts of rocket science (including some great talks here). As far as I've gleaned, there's two extremes of rocket thrusts: high speed/low mass (like ion engines) and low speed/high mass (like chemical rockets). While an ion engine, fer instance, could get you going really fast in space, it doesn't have the mass ejected to lift you out of a gravity well. A chemical engine can do that, but unfortunately requires a much higher mass of fuel to be taken on the trip.

My hypothetical scenario for my fictional world is spaceships equipped with ion drives (or similar) for space travel with most never intended to land on a planet. Those that are, however, (like shuttles or my hero's small trader ship) have an additional set of chemical rockets to do the landing and taking off from a planet's surface.

My question is, is that an efficient idea? In very broad strokes, could there be a saving of space and fuel by having smaller rockets and fuel just for gravity evolutions and the more efficient engines for space? Or is space travel such a non-thing compared to the power needed to lift off, that you might as well use the chemical rockets in space too?

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    $\begingroup$ "My question is, is that an efficient idea?" Yes, it is. You may read a lot about it, but to really feel how thid is not only efficient but the only way to go I suggest playing with your idea in Kerbal Space Program. I use ion engines to circularize orbits with extreme accuracy - but tjey will only lift you off from the least massive moons. $\endgroup$ – Renan Nov 25 '18 at 14:02
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    $\begingroup$ An 'efficient' idea depends upon what you are measuring. Total mass of the ship? Construction cost? Operating cost? Travel time? Spaceport requirements? Each of those criteria (and others) will have different solutions that maximize efficiency. $\endgroup$ – user535733 Nov 25 '18 at 15:08
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    $\begingroup$ Not just efficient, it's been done, with the Dawn mission to Vesta & Ceres: dawn.jpl.nasa.gov $\endgroup$ – jamesqf Nov 25 '18 at 18:19
  • $\begingroup$ We used ion engines for the voyager probes. $\endgroup$ – Garret Gang Nov 26 '18 at 17:13
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Split the two types of ship completely

Take your idea to its logical extreme. Most ships are large, built in space, and stay in space. They are powered by advanced ion drives, or some sort of nuclear propulsion that you couldn't take into an atmosphere without irradiating the planet.

Then to transfer to and from the planet, the big ships can either make use of local ships that ferry cargo from planet to orbit, or they carry their own shuttles to do that job. Alternately, use a more advanced surface-to-space transfer, like a skyhook or space elevator.

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  • $\begingroup$ I still can’t get over skyhooks being a real thing. $\endgroup$ – Joe Bloggs Nov 25 '18 at 16:10
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Have interchangeable modules

You specify shuttles and trader-ships. If the planets being visited are set up for interplanetary trading then presumably you park in orbit in a designated zone, fill up with fuel and sausage and chips (fries). Then you head to the rental office and hire a rocket-drive module for descent to the planet's surface. This saves unloading your craft and reloading into another one. Undock your space-drive, put it in a secure storage facility and hook up to the rocket drive.

Bingo!

If you're visiting a far-flung planet with no facilities then it's going to depend on a lot of factors such as gravity, atmospheric density and so on. Probably specialist vessels will be needed for these outposts. They will be designed specifically to cope with the conditions at their destination. It's quite likely that they will be larger vessels with a smaller planet-going ship in the hold.

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  • $\begingroup$ Two great points. I'd intended it to be self-contained specifically to allow for visits (and returns) from planets without orbital or surface launch facilities, but I like the idea of larger, corporate, "main line" cargo ships using in-place lander attachments or similar. It gives the little, self-contained guy a unique place. $\endgroup$ – Fletch Nov 26 '18 at 16:27
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What your asking is entirely possible, you can fit multiple different types of engines onto a spacecraft, from liquid fueled, solid rockets, on ion engines

Your a space trader, you visit Space Stations orbiting the Earth and other planets where most trade and space commerce occur, but occasionally you need to fly down to the surface to pick up goods, so you have a shuttle. a craft that can travel between the surface and orbital stations, AND between planets. And thanks to that you can be the best space trader in the system if not the galaxy... except You're broke!

Alas yes it is entirely feasible, but is it practical?

Humans have tried the shuttle idea, and while it was indeed a monumental success of engineering. it was a horrendously expensive way to get anything to and from orbit, that's why NASA have switched to the disposable SLS, and companies like SpaceX have the their own rockets like the falcon, both are far cheaper to build and run then the shuttle, then consider that during launch you'd have to drag some (presumably) big ion engines off the ground and into orbit which means extra weight and therefore more fuel needed to reach orbital velocity. then once in orbit, you switch to your ion engines, but are now dragging along your big heavy liquid fuel engines which means extra weight and therfore extra fuel.

Simply put having someone do it with two separate vehicles, one to reach orbital velocity and then offload it onto one to go between planets is far far cheaper then a single multi-use craft.

The less fuel you use the less fuel you need.

I know that sounds like a grass is green water is wet sort of comment, but if you have a spacecraft that weighs X needs to reach orbit then you need Y amount of fuel, simple put that fuel in... ah but then your craft now weighs Z (X+Y) so you need to add more fuel to get that craft and its fuel despite it always getting lighter moving. thats why the Saturn V was as big as it was.

Now consider that you need to take off, carrying extra fuel and extra engines, but thats a lot of extra weight which means needing to spend more on fuel, which you'll need to incorporate into the tendering process of when you price for work.

And before anyone comments what about the fact that the Saturn V had loads of different engines that was staging, the spent stages were dsicarded as it climbed, this is no where near efficient enough on an SSTO

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I'm uncertain of the exact size of your spacecraft, but it is possible for a plane to fly into space.

The basic answer is to fly as a plane to a fairly high altitude, switch to rocket engines to accelerate to orbital velocity, then swap over to more efficient ion engines.

Landing depends on the kind of atmosphere. If it has a thick enough atmosphere it can be approached and used to aero-brake. It might take several passes to reduce overall velocity to safe levels to attempt full re-entry. The final approach would work like a space shuttle. Angle up to 40~ish degrees slow down as you descend. At some point swap over to powered flight (if possible). Or land it like a space shuttle.

If there isn't any atmosphere, hopefully you brought enough fuel to actually perform a safe deceleration and landing.

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  • $\begingroup$ Yes, it's possible to fly a plane into space. No (with existing or reasonably extrapolated technology), it's not possible to fly a plane into orbit, let alone reach escape velocity. Orion launchers and such don't count - they are not airplanes as referenced in your link. $\endgroup$ – WhatRoughBeast Nov 26 '18 at 15:11
  • $\begingroup$ With current technology it isn't possible because the technology hasn't been developed. This is a fictional world, it could be possible that they did invest in the technology to make it possible. I do agree that Orion launchers are rockets. My point is that a plane can reach orbital elevation. The rest is getting to orbital speed. The only real bottleneck is fuel. $\endgroup$ – Kain0_0 Nov 26 '18 at 23:33
  • $\begingroup$ "The rest is getting to orbital speed." That's like saying you can be immortal if you just avoid dieing. "The only real bottleneck is fuel." But that's the whole point - If you have a rocket motor, every second you spend dinking around at low altitude with big ol' drag-producing wings wastes fuel like crazy. And if you have dual air-breathing/rocket power plants, that's more weight and you need more fuel. The technology hasn't been developed because it was an obvious dead end. $\endgroup$ – WhatRoughBeast Nov 26 '18 at 23:46
  • $\begingroup$ Not dying is the definition of immortality. Low Earth Orbital speed is defined roughly as ~7800m/s. I fail to see what you are comparing here. Regardless of how you not die or get to orbital speed they are the problem you are engineering for. Lifting sufficient fuel to a sufficient height and using that to propel yourself to that speed in a co-ordinated matter is the problem of getting to orbit. This is not an impossible challenge using a space-plane. Granted a rocket is the most efficient use of fuel, but it may not be the most efficient use of resources. $\endgroup$ – Kain0_0 Nov 27 '18 at 1:07
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It all depends on weight

The initial planners of the Apollo missions considered all sorts of options. For a while, a 'direct ascent' vehicle was considered. It is basically a large rocket that takes off and lands on the lands on the moon in one vehicle, which could still be staged at lift-off. An early artist example:

enter image description here

However as they investigate the implications, they found that a Lunar Orbit Rendezvous (ie. have a separate vehicle to launch, a separate vehicle to orbit, and a third vehicle to land and lift-off from the moon) was much more efficient.

This is mainly because of weight. More weight = more fuel, and already the enormous Saturn V launch vehicle or even a Nova class was required to launch a relatively small payload (just 3 crew). In the vacuum of space, you do not need atmospheric shielding as your launch vehicle or command module do, takeoff and re-entry into atmospheres is a violent process. By using the Lunar module, you could save enormous weight by having paper-thin walls, lightweight engines and minimal fuel. They even discussed having 3 legs instead of 4, but thought that they needed the redundancy of 4. A lightweight craft was also much easier to handle and control.

Encasing the module in the takeoff/reentry stage cylinder meant it was protected during this phase, meaning a command module / lunar module docking procedure needed to be done in space, however this complication was deemed worth it for the weight and fuel saving. In comparison, this early diagram compares the relative size between DA and LOR:

enter image description here

Even the Soviets, with their independent space agency, went through all the options and reached the same conclusion.

So to answer your question, combining the two doesn't make sense when a simple docking / undocking procedure (which was accomplished easily in the 60's and 70's) could enable you to save enormous weight and fuel expense, reduce spent engines and shielding and be much more controllable.

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