Why do we have to go direct?

[Moon Goddess]

A question in another thread got me thinking about something

Why not just buy and use Soyuz? Or just keep using Soyuz and ISS via Baikonur based on the existing treaties for that matter? It seems to be working right now, right?

I've looked at the mission plans for SLS and orion, and proposed mission profiles for various Mars plans and the question that I come up with is "Why do we go straight there?"

In KSP almost every mission I run it safely launch a unmanned craft up to my space station, send a second mission up to refuel the craft, and a third launch in my reliable crew lifter to the space station. Kerbals use the station as a staging point before moving to their future mission, Mun, Duna, Jool in one case, (that thing sat at the station for long time getting added stuff docked.)

So why don't we do that in real life? We have a space station, we have a reliable crew lifter, we even have reliable lifters for unmanned cargo and resupply.

I understand that ISS is at a severe inclanation that needs to be adjusted before heading to the moon. But even Kennedy puts you at 20 some degrees, so you'll be fighting an inclinded orbit no matter what.

Why don't we simple design a Luna Transfer Orbiter, stick it atop an Atlas V, and orbit the moon a couple months after it's completed assembly. Then we park it back at ISS, offload the crew, and the next Atlas V has a lander that'll dock to it. And we send up a Falcon 9 with a dragon full of fuel. (Have to be hypergolic.) and yet another crew on Soyuz.

What's wrong with this plan? Am I wasting a large amount of Delta-v in my KSP missions? Is that delta-v worth the amount of safety improvements I see in my plan?

[Diche Bach]

I don't think I understand what the stated purpose of SLS and Orion missions are enough to answer about the technical aspects of your question.

But as an anthropologist, I think one very reasonable explanation for why NASA has these proposals going would hinge on: pride (national pride in particular, but 'agency' pride too) and perceived self-interest (could say "greed" but it is more complicated than that).

A pragmatic interest to not being 'dependent' on other nations for future objectives is also a very reasonable perspective.

But yeah, I thought the whole point (or one of the major ones) of a space station was to serve as a foundation for much more efficient interplanetary operations.

[Mr Shifty]

There are apparently many difficulties associated with storing and transferring propellant in space:

http://en.wikipedia.org/wiki/Propellant_depot

And interestingly enough, HTV-4 is right now on-route to deliver a robotic module to the space station that they'll be using to investigate how to do satellite refueling operations:

http://arstechnica.com/science/2012/03/astronauts-at-iss-begin-learning-to-refuel-spacecraft-in-orbit/

[Nibb31]

There is no need for the ISS in your plan. You can do exactly the same thing without the complication of docking the crew and transfer ship to the ISS by directly docking the crew to the transfer ship.

Additionally, bringing the Orion along instead of leaving it docked at the ISS adds direct return options if something goes wrong.

[Mr Shifty]

But even Kennedy puts you at 20 some degrees, so you'll be fighting an inclinded orbit no matter what.

This is actually a point that KSP obscures. Kennedy is at 28 degrees to the equator, so can't launch to an equatorial orbit that is less than 28 degrees inclined. The moon's (and Mars's) inclination are typically measured relative to the plane of the ecliptic, that is the plane that the Earth's orbit around the sun transcribes. The ecliptic itself is inclined at 23.5 degrees relative to the equator because of the Earth's axial tilt. If you take the moon's inclination (~5 degrees relative to the ecliptic) and the inclination of the ecliptic into account, it is possible to launch from Kennedy directly into the moon's orbital plane. KSP is simplified because none of the bodies have axial tilt relative to their orbital planes (Kerbin's equatorial plane is the same as its ecliptic plane), and Kerbal Space Center is on Kerbin's equator.

[Diche Bach]

Must say, I don't get how something can "boiloff" if it is inside of an hermetic big metal can in space.

[Moon Goddess]

Must say, I don't get how something can "boiloff" if it is inside of an hermetic big metal can in space.

Well firstly liquid hydrogen is -423.17 °F, And the sun is shining on your big metal can in space. Without any atmo to shield it in any way from the heat of the sun. So then that hydrogen is going to go to gas. But when it becomes gas, it become the lightest smallest stable thing in the universe. (I said stable, yes I know it's not the lightest smallest form, but it is the lightest smallest that stays that way)

It's going to find a gap it's going to find every gap, it will escape no matter what you do.

But these are not unbeatable goals, you can use hypergolic fuels that don't need to be kept so cold, and can be stored long term, plus your in space so no one cares that your fuels are horribly poisonous. These fuels are far less efficient than cryogenic fuels but the balance can be made up in the fact that it's going to cost you much less fuel to put it up there if you don't launch it all in one lift.

[Lanfranc]

Actually, I believe the current US space policy has precisely moved away from a direct-to-Mars concept in favour of a more diverse approach (or "Flexible Path") – including things like propellant depots or even in situ fuel production on the Moon or from an asteroid in orbit.

[Diche Bach]

Ah, so "hermetic" is a relative concept I see!

[Moon Goddess]

I had to look up hermetic in that context... turns out I had actually heard it before but people seemed to emphasis the r when using it for alchemy and not when using it to mean airtight and without having seen it in written form I thought they were different words.

But basically the idea of a airtight bottle of H2 is nice and all, but the problem is you have to have some opening to get it out which means seals otherwise you can't burn it. and then it's just a counterweight.

[Moon Goddess]

Actually, I believe the current US space policy has precisely moved away from a direct-to-Mars concept in favour of a more diverse approach (or "Flexible Path") – including things like propellant depots or even in situ fuel production on the Moon or from an asteroid in orbit.

I couldn't actually figure out what the current mars plan is, seems to be more of a "Well we'll um do like some mars stuff after we do the moon, or maybe we'll do an asteroid before the moon or the asteroid first, um why are you asking me all these questions, Look, nuclear power car on Mars see, look."

[SargeRho]

The thing about Hydrogen: It's so small, that it literally seeps through solid metal.

[Diche Bach]

The thing about Hydrogen: It's so small, that it literally seeps through solid metal.

LOL. You and Moon Goddess are full of it. "Boil-off" evidently just means that it turns into "waste" gaseous hydrogen. It is still inside the big metal can in space, it just isn't useful as a reactive fuel once it transitions from liquid to gaseous state. Though evidently some genius thinks they might be able to use it as a monopropellant.

[DerekL1963]

In KSP almost every mission I run it safely launch a unmanned craft up to my space station, send a second mission up to refuel the craft, and a third launch in my reliable crew lifter to the space station. Kerbals use the station as a staging point before moving to their future mission, Mun, Duna, Jool in one case, (that thing sat at the station for long time getting added stuff docked.)

You don't have a budget or a schedule in KSP. Nor does your equipment have a shelf life.

[metaphor]

The delta-v's in real life are a lot bigger than the ones in KSP. For example, it takes 4 km/s to go from low Earth orbit to low lunar orbit. If you want to build a "lunar shuttle" that can go from LEO to LLO and back, that's 8 km/s of delta-v. Using the best cryogenic propellants means the shuttle has to use 84% of its mass in fuel for one round trip. Using hypergolic propellants means it has to use 92% of its mass in fuel. Considering the structure, avionics etc are about 10% of the mass of a ship, a cryogenic shuttle would only be able to carry a very small payload, and a hypergolic shuttle would be pretty much impossible.

A mission to Mars would definitely take multiple launches even with the SLS, and the plan is pretty much just like you said, get cargo into orbit, then launch a lot of fuel and dock it together, then get the crew there in a crew capsule and leave for Mars.

Cryogenic propellant depots are currently a hot topic at NASA, I think the current technology is like 0.01% boiloff per day, and they're getting better, so in any future missions you could use cryogenic for everything instead of hypergolic and get a lot more efficiency.

If you really want to see an in-depth Mars mission design, this one is from a 2009 NASA study. And this one is a shorter one with pictures.

We have most of the technology to go to Mars already and the rest could be developed in a few years, but the problem is there's no funding to do it anytime fast.

[Brotoro]

When the hydrogen boils (as some of it will over time because no spacecraft fuel tank insulation is perfect) it turns into a gas, so the pressure in the tank increases. You have to vent the gas or you can exceed the structural limits of the tank. No, you can't use a massively strong tank to contain the high pressure because that would be too heavy.

[Lanfranc]

I couldn't actually figure out what the current mars plan is, seems to be more of a "Well we'll um do like some mars stuff after we do the moon, or maybe we'll do an asteroid before the moon or the asteroid first, um why are you asking me all these questions, Look, nuclear power car on Mars see, look."

Roughly speaking, the policy is that Mars is an objective, but it is not a first step; rather, it is the end of a longer process that involves e.g. various types of Moon or asteroid missions in order to gain experience and mature the necessary technology. The Augustine Report has a very comprehensive outline of the different options: http://www.nasa.gov/offices/hsf/home/

I personally think that going to Mars is a bit of a symbolic thing – while we should certainly go there eventually, developing a proper infrastrucure in space is much more important in the short to medium term.

[Fuzzy Dunlop]

The hydrogen is allowed to boil off and escape through pressure relief vents because otherwise the pressure in the tank will rise until it explodes. It's much easier to keep it liquid by keeping it cold than by keeping it under pressure.

[Tommygun]

Must say, I don't get how something can "boiloff" if it is inside of an hermetic big metal can in space.

I used to work for a company that made gas purification equipment for the semiconductor industry.

We would test for leaks in the equipment by pressurizing it with hydrogen and then use a mass spectrometer air sniffer to sample air around the seals.

It's not a question if the hydrogen gets out, it's how much gets out. If you put hydrogen inside a regular plastic bag the hydrogen will slowly pass right through the plastic like sand through a net.

Edited August 10, 2013 by Tommygun

[Diche Bach]

Makes sense: hot hydrogen gas is greater volume which increases internal pressure in the tank, so they gotta do something. "Venting so you don't explode," that is very similar to the process of Transference.

These ideas to vent to H into a separate tank and use it as monopropellant sound like a good idea, but not very creative.

Would it be cost effective to send up enough empty tanks to capture the boil-off H and then somehow reuse it? For that matter, I would think that there is _energy_ in that transition change else in the gaseous hydrogen no? Put one of your turbine thingamajigs in there to use that phase transition to make electricity out of the boil-off and then use that to solve the energy crisis!

Seriously though, you physical scientists and engineer types really need some creative social scientist thinkers to help you with these things sometimes