Why do we not use smaller launch craft?

[Technical Ben]

I wondered, after seeing the size of the craft in interstellar and other sci-fi, which are usually tiny in comparison to real counterparts (we have 90% fuel to payload ): What's the smallest we could make a launch vehicle for a single crew member?

One person, roughly 85kg or 185 pounds into orbit. What would it take? If we assume we can get the cabin down to the size of a small car or aircraft style cabin?

Could something like this do it?

http://en.wikipedia.org/wiki/Pegasus_%28rocket%29

Ah, that's an assisted launch rocket. Hmmmm. Need something else. Like this: http://en.wikipedia.org/wiki/VLS-1

But is it ever going to match the ideas of space craft looking like aircraft? I doubt so.

[Iskierka]

Bear in mind that the Pegasus is a very similar, or even larger fuel:payload ratio than many rockets, it's simply very, very small. Even getting one person into orbit requires far more than its measly 450 kg payload, as it requires life support, pressurised cabin, structure, reentry and guidance systems. If you want to find out the very smallest, you really need to look to the very earliest rockets, that genuinely were only launching one person at a time. We could cut them down now, but not that significantly - a person, with all required additions, is a fair amount to put in orbit.

[Kryten]

For a person in orbit-and all the equipment needed to keep them there are bring them back again-you're going to be looking at about a ton. Smallest rocket I've seen to put a ton into orbit is Minotaur-C/Taurus-II; 73 metric tonnes. In theory you cpuld do a lot better with more efficient fuel combinations such as kerolox and hydrolox, but in practice there's not enough of a market at that size to justify the expenses of liquid rocket development and infrastructure.

[Technical Ben]

Well, if we assume it's the sci-fi setting of only getting into space for now (yep, totally handwaving and lampshading the "but how did they get down to the planet" part ). So we only need life support long enough to reach an orbital facility/craft. How small can we make the cabin then? I guess it would be the size of those early capsules?

[Ralathon]

It depends how far you can beef up the energy density. Right now our rockets are huge because the rocket equation sucks and we are limited by chemical Isp's. You can reduce the amount of fuel needed by boosting the Isp. So if you have the technology to build a car sized fusion reactor giving a gigawatt of power and the engine to handle that kind of juice, you could build a much smaller spaceship. So the whole thing hinges upon our engineering genius.

Considering that fission is just too heavy and we have a lot of trouble getting fusion running it is going to be pretty far in the future though.

[B787_300]

A lot of it has to do with the tyrrany of the rocket equation and what customers want to send to space... GEO sats are slowly getting larger because customers dont want to have to maintain a constellation when they can do it with one or two sats. Also larger rockets are easier to make than smaller rockets in MOST cases (super large get harder again). Most sounding rockets are harder to make because you have some pretty tight space requirements.

[Nibb31]

Mercury-Atlas was a pretty good demonstration of how small you could make a single-seat orbital spacecraft. The Atlas rocket was pretty small and nearly SSTO.

[Technical Ben]

I guess we cannot know then, until we do (if ever) get a fusion reactor working. It's 25m+ or 50m+ rockets all the way then.

[jwenting]

Earth is not Kerbin, the escape velocity is a lot higher, so you need a lot more oompf to put something in orbit.

Humans are also a lot heavier than Kerbals, making things even worse.

[cantab]

Yeah, the big thing is getting higher specific impulse. Viable approaches I think:

Nuclear thermal engines. The NERVA was ground-tested and twice as efficient as chemical engines, and more advanced designs could be even better, and have higher TWR as well. However, though an all-nuclear launcher could be light it would probably be bulky, since liquid hydrogen is the propellent of choice.

Jet engines. The SABRE is one of the most promising technologies here, but there are lots of ideas for hypersonic jets. SABRE depends on liquid hydrogen, so again a light but bulky ship, but other engines might use kerosene for a much more compact spaceplane. Or, indeed, a vertically-launched jet rocket, there's no real reason that can't work.

Beamed thermal engines. A less mature technology but one that could potentially work very well. High TWR would be natural, but like with a nuclear thermal engine you get the issue that the highest specific impulse requires liquid hydrogen, so light but bulky.

Of course whatever the engine technology, another aspect is the room for weight savings all round. Lightweight composites, miniaturised electronics, and so on.

[Technical Ben]

Earth is not Kerbin, the escape velocity is a lot higher, so you need a lot more oompf to put something in orbit.

Humans are also a lot heavier than Kerbals, making things even worse.

This I know. The comment was not on KSP, but on Sci-Fi aesthetics. Which generally assumes space flight will use craft the same size as trucks and/or aircraft, with boats being the largest for a minimal requirement. IRL it's very different.

[Bill Phil]

The problem isn't the person. It's the person's needs and the missions needs. The Mercury spacecraft was about one tonne in mass for orbital missions.

You need a heat-shield, life-support, a pressure suit, a container, a retro pack, etc. A minimum mission would probably be about 500 kilograms or more.

[Elthy]

If you are brave enough it should be possible to put someone in a Spacesuit, tie him on a chair and put a aerodynamic cover about that thing. Since you only talked about getting him to space you wont need retrorockets or a headshield which makes it much lighter than a normal capsule...

[YNM]

Minotaur IV can lift a Mercury. Even it's LEO capabilities is listed as sliightly higher than the original Atlas (1735 kg vs. 1360 kg), yet the launch mass is 86.3 t vs. 120 t. Well, granted it's on a lower orbit through (185 km listed for Minotaur IV compared to SMA of 220.5 km for Mercury missions on Atlas). Edited December 28, 2014 by YNM

[Camacha]

Aren't smaller spacecraft less efficient? I think we want larger, not smaller.

[B787_300]

Aren't smaller spacecraft less efficient? I think we want larger, not smaller.

Efficient is the wrong idea, Smaller is harder to manufacture and design, Medium is just right, and large is Hard again. Now smaller rockets can be efficient, as can medium and large but cost increases normally.

[Iskierka]

Larger rockets have a tendency for more efficiency in greater payload:launch mass ratio, but that's not too relevant. It might take 80 tonnes per tonne at 1 ton payload, and 70 at 2, but if you only need 1 ton then the extra 60 for that extra payload is wasteful. This is why despite reasonable pricing for its payload, Ariane 5 was a failure - there was almost no market for its payload, and thus it was inefficient as it used far more rocket than anyone needed.

[Nibb31]

This is why despite reasonable pricing for its payload, Ariane 5 was a failure - there was almost no market for its payload, and thus it was inefficient as it used far more rocket than anyone needed.

I wouldn't call it a failure with 73 successful launches under its belt, and over 100 GTO payloads deployed. It has been pretty much leading the GTO launch market since 1996.

Launching 2 payloads on each flight brings the cost down significantly, hence it is more efficient. That efficiency comes at the price of flexibility, a delay on one payload affects the other, and you need at least two payloads to be ready at the same time, which causes some customers to have to wait for a slot.

[cantab]

And I believe in time lead to difficulties finding satellite pairs that could go up together, as more and more became heavier than half the Ariane's total payload. Launchers designed for a single primary payload, and maybe a smaller secondary one, are probably the better approach.

[NERVAfan]

Yeah, the big thing is getting higher specific impulse. Viable approaches I think:

Nuclear thermal engines. The NERVA was ground-tested and twice as efficient as chemical engines, and more advanced designs could be even better, and have higher TWR as well. However, though an all-nuclear launcher could be light it would probably be bulky, since liquid hydrogen is the propellent of choice.

IIRC NERVA style NTRs have problematically low TWR since reactors are heavy. I think chemical lower stage/NTR upper stage is better.

Using NTR as upper stage also makes launch pad disasters much less problematic. An unactivated reactor is probably realistically less nasty than hypergolic propellants (U-235 is radioactive, but not THAT radioactive with a half life over 700 million years, and subcritical amounts are only a problem if they get inside the body - especially lungs - anyway as it's an alpha emitter skin blocks alpha particles quite well.)

But an activated-then-destroyed reactor is very radioactive since it has fission products with short half life/high radioactivity (and non alpha emissions I think).

NTRs might be much less likely to have destructive launchpad accidents than chemical rockets, since they aren't inherently explosive like fuel/oxidizer combinations, but I think getting permission for nuclear upper stage is going to be very hard already... nuclear launch stage will be worse.

Jet engines. The SABRE is one of the most promising technologies here,

Yeah, it seems awesome, if only it could get full funding... (maybe after SpaceX develops 1st stage reusability, other people will be looking at reusability more...)

Or, indeed, a vertically-launched jet rocket, there's no real reason that can't work.

IIRC jet engine TWRs are very bad.

Beamed thermal engines. A less mature technology but one that could potentially work very well.

Potentially very awesome - there doesn't seem to be much effort/money being put into it though.

[GluttonyReaper]

Just out of interest, what use would launching one person into space be nowadays? Most (if not all) manned spaceflight nowadays is space station related, so I don't really see much use for such a thing...

[r4pt0r]

off topic, but OP's http://en.wikipedia.org/wiki/Pegasus_%28rocket%29 got me thinking, what kind of flight profile would be needed to do this in ksp? fly the aircraft at ~40km then get the rocket into space and swithc back to the plane before it fell below the 23km threshold? then land it, switch back to the rocket and circularize?

[Ralathon]

off topic, but OP's http://en.wikipedia.org/wiki/Pegasus_%28rocket%29 got me thinking, what kind of flight profile would be needed to do this in ksp? fly the aircraft at ~40km then get the rocket into space and swithc back to the plane before it fell below the 23km threshold? then land it, switch back to the rocket and circularize?

In stock KSP its likely impossible. I reckon you just don't have the time needed to save the plane.

If you're willing to use mods you could just use Reusable flight manager and jump back in time to the separation event to fly both the plane back to the runway and the rocket to space.

[cantab]

I think what I'd try is fly to around 23 km then fire off the rocket. It needs loads of thrust to establish a good apoapsis before passing out of physics range. Then land the plane *sharpish*, before switching back to the rocket.