Pit stop in L2?

Quaoar · 2014-03-09 11:10:12

A trans mars injection is almost 4 km/s starting from LEO and less than 1 km/s starting from Earth-Moon L2.
So why not consider to send the spaceship unmanned in L2, with a slow spiraling solar-electric space tug, one year before departure?
When is time to depart, the astronaut can rendez-vous the spaceship in L2 with a less massive vehicle like an Orion or a Dragon Rider.

In a future, we can put LOX propellant depot obtained from lunar regolith in L2, by now we have some very interesting electric thruster like NEXT ( http://en.wikipedia.org/wiki/NEXT_%28ion_thruster%29 ), that can do the job with a good solar array.

Last edited by Quaoar (2014-03-09 11:15:37)

RobS · 2014-03-09 11:17:21

L1 is also a possibility; there are a lot of studies of the "L1 Gateway" station, as it is called. L1 may be more convenient because it is closer and faster to reach, but it takes 300 meters per second more delta-v. Either way, trans-Earth injection has to be done deep in Earth's gravity well, so you have to go back to Earth to head to Mars. L1 is closer and more convenient.

You could also save time and propellant using a solar electric or solar thermal engine to move most of your assets into a very large elliptical orbit, then move the people and the earth return capsule there quickly, rendezvous above the Van Allen Belts, loop back in, and perform trans-Earth injection.

Personally, I rather like solar thermal propulsion. You can only generate about 100 pounds of thrust at a time, but you can heat up a transfer medium for many hours and choose when to perform your burn--at perigee--so it is efficient. It can generate specific impulses of 900 seconds, maybe more; so it is comparable to a nuclear thermal engine. But you can't use it to move people; they'd loop through the Van Allen Radiation Belts repeatedly.

Quaoar · 2014-03-09 13:57:54

RobS wrote:

L1 is also a possibility; there are a lot of studies of the "L1 Gateway" station, as it is called. L1 may be more convenient because it is closer and faster to reach, but it takes 300 meters per second more delta-v. Either way, trans-Earth injection has to be done deep in Earth's gravity well, so you have to go back to Earth to head to Mars. L1 is closer and more convenient.

Yes: from L1 or L2 we have to low the perigee to LEO to make the burn near the Earth utilizing Oberth Effect.

RobS wrote:

You could also save time and propellant using a solar electric or solar thermal engine to move most of your assets into a very large elliptical orbit, then move the people and the earth return capsule there quickly, rendezvous above the Van Allen Belts, loop back in, and perform trans-Earth injection.
Personally, I rather like solar thermal propulsion. You can only generate about 100 pounds of thrust at a time, but you can heat up a transfer medium for many hours and choose when to perform your burn--at perigee--so it is efficient. It can generate specific impulses of 900 seconds, maybe more; so it is comparable to a nuclear thermal engine. But you can't use it to move people; they'd loop through the Van Allen Radiation Belts repeatedly.

I'm not sure solar thermal may work for a space tug: non impulsive trasfer to L1 have a delta-V of 7, so with only 8.8 km/s of exaust velocity we have almost the same mass ratio of a LOX-LH2 chemical rocket tug, that make an impulsive trasfer in L1 with a delta-V of 3.77 (consider that solar thermal use only LH2 that is difficoult to send in LEO, instead of 85% LOX and 15% LH2 used by chemical rocket).

Last edited by Quaoar (2014-03-09 13:58:45)

RobS · 2014-03-09 14:35:14

The delta-v to L1 is something like 3.6 km/sec and it would be the same for a solar thermal engine as for chemical propulsion. You'd only fire the solar thermal engine at perigee and perform a series of kicks to raise apogee. You could probably circularize the orbit by using a lunar pass, too. With an ion engine the delta-v is about 7 km/sec because you have to raise the apogee and circularize continuously.

Quaoar · 2014-03-09 15:18:32

RobS wrote:

The delta-v to L1 is something like 3.6 km/sec and it would be the same for a solar thermal engine as for chemical propulsion. You'd only fire the solar thermal engine at perigee and perform a series of kicks to raise apogee. You could probably circularize the orbit by using a lunar pass, too. With an ion engine the delta-v is about 7 km/sec because you have to raise the apogee and circularize continuously.

I'm sorry I've misunderstood: your solar-thermal spaceship will use multiple perigee burns, to minimize the gravity loss, so the delta-V is the same as using a single impulsive burn with a more powerfull rocket.

Last edited by Quaoar (2014-03-10 02:52:53)

RGClark · 2014-03-09 15:34:19

Quaoar wrote:

A trans mars injection is almost 4 km/s starting from LEO and less than 1 km/s starting from Earth-Moon L2.
So why not consider to send the spaceship unmanned in L2, with a slow spiraling solar-electric space tug, one year before departure?
When is time to depart, the astronaut can rendez-vous the spaceship in L2 with a less massive vehicle like an Orion or a Dragon Rider.
In a future, we can put LOX propellant depot obtained from lunar regolith in L2, by now we have some very interesting electric thruster like NEXT ( http://en.wikipedia.org/wiki/NEXT_%28ion_thruster%29 ), that can do the job with a good solar array.

I like the idea. You might have a large chemical propulsion vehicle transported to L2 by solar electric propulsion(SEP). Because of the high Isp of solar electric it would not take much size for the SEP to carry it there. Chemical propulsion has the advantage of quicker transport time, important for a manned ship.

Bob Clark

RGClark · 2014-03-09 15:39:22

RobS wrote:

L1 is also a possibility; there are a lot of studies of the "L1 Gateway" station, as it is called. L1 may be more convenient because it is closer and faster to reach, but it takes 300 meters per second more delta-v. Either way, trans-Earth injection has to be done deep in Earth's gravity well, so you have to go back to Earth to head to Mars. L1 is closer and more convenient.
You could also save time and propellant using a solar electric or solar thermal engine to move most of your assets into a very large elliptical orbit, then move the people and the earth return capsule there quickly, rendezvous above the Van Allen Belts, loop back in, and perform trans-Earth injection.
Personally, I rather like solar thermal propulsion. You can only generate about 100 pounds of thrust at a time, but you can heat up a transfer medium for many hours and choose when to perform your burn--at perigee--so it is efficient. It can generate specific impulses of 900 seconds, maybe more; so it is comparable to a nuclear thermal engine. But you can't use it to move people; they'd loop through the Van Allen Radiation Belts repeatedly.

I like thermal propulsion. I don't understand why it hasn't been develop yet. It seems a simple idea. You just focus the sunlight on the propellant like a magnifying lens. This is well known on Earth in the use of a "solar furnace". Temperatures as high as 3,000 K can be reached, the temperature at the surface of the Sun.
Solar thermal has the advantage it can get high thrust like chemical propulsion but also high Isp such as 900 s, without needing nuclear propulsion.

Bob Clark

RobS · 2014-03-09 19:32:16

The thrust limitation comes in because of the mirror size. You need mirrors something like 50 meters across to get enough power to produce 100 pounds of thrust. If you use multiple mirrors and fiber optics, you might be able to get more thrust. With a 50-meter mirror, you also have large, awkward structures with your solar thermal engine in the focus of the mirror and a long way from the payload.

One solution is to use a high temperature ceramic, heat it up with sunlight over a long period of time, then use all that heat over a short period of time. You can probably get more thrust that way, but your engine gets heavy; maybe half a tonne or a tonne to store the heat. If you heat the ceramic first, then heat the hydrogen effluent with the sunlight, maybe you can get an Isp of 1200. So it has a lot of potential. Someone does need to develop it.

One problem with solar electric; you can't store your energy, so you thrust contunuously. When you do that, you can't just thrust at perigee, so you spiral away from Earth, and that doubles your delta-v!

Quaoar · 2014-03-10 04:47:02

RobS wrote:

One solution is to use a high temperature ceramic, heat it up with sunlight over a long period of time, then use all that heat over a short period of time. You can probably get more thrust that way, but your engine gets heavy; maybe half a tonne or a tonne to store the heat. If you heat the ceramic first, then heat the hydrogen effluent with the sunlight, maybe you can get an Isp of 1200. So it has a lot of potential. Someone does need to develop it.

It may be a very good solution: the engine may store energy douring the whoole orbiting and fire only at pergee, so the mirrors have not to be so huge.

RobS wrote:

One problem with solar electric; you can't store your energy, so you thrust contunuously. When you do that, you can't just thrust at perigee, so you spiral away from Earth, and that doubles your delta-v!

Solar electric become interesting with specific impulse over 3000 s and 100-300 KW (or more) electric power, so the gravity loss are compensed by the engine efficiency. Another option may be bimodal nuclear like Pratt & Whitney Triton: a 3 rocket pod may produce up to 300 KW of electric power: so the spaceship can spiral unmanned in L1 or L2 with a NEXT engine and then leave manned for Mars with NTR rockets, saving a lot of propellant.

http://ntrs.nasa.gov/archive/nasa/casi. … 014643.pdf

Last edited by Quaoar (2014-03-10 06:58:02)

RobS · 2014-03-10 07:50:27

Yes, a graphite or ceramic block can store a lot of heat, but you still need a good sized mirror because the heat will radiate away also.

Thanks for the link to the paper about nuclear rockets. I will read it through. i suspect nuclear thermal, however, is politically impossible, especially with all the environmental restrictions on testing the engines, whcih makes testing incredibly expensive.

GW Johnson · 2014-03-10 12:22:43

So why not test nuclear engines on the moon? Might well be cheaper than doing all the necessary restrictions to do it down here.

GW

RobS · 2014-03-10 12:27:53

Good idea, but we still have to get to the moon, and that will cost NASA more than a $1-2 billion enclosed nuclear engine test facility. And there will still be anti-nuclear protestors to deal with at every launch.

GW Johnson · 2014-03-10 13:01:35

Why should it cost so much to put stuff on the moon, when a Falcon-Heavy can reach the moon pretty easily, and cost under $100M every launch you purchase (and that's commercial retail!)?

Going to the moon has been billed as super expensive, when it need not be. Does require not repeating Apollo, though. This is no longer 1963.

GW

Last edited by GW Johnson (2014-03-10 13:02:06)

JoshNH4H · 2014-03-10 13:32:27

RobS wrote:

Yes, a graphite or ceramic block can store a lot of heat, but you still need a good sized mirror because the heat will radiate away also.
Thanks for the link to the paper about nuclear rockets. I will read it through. i suspect nuclear thermal, however, is politically impossible, especially with all the environmental restrictions on testing the engines, which makes testing incredibly expensive.

Graphite heat capacity: .71 kJ/kg*K. Graphite sublimes at 3900 K, so we'll say that its maximum service temperature is around the same as an advanced NTR, giving an exhaust velocity of 10 km/s. Let's say the engine has an efficiency of 70%. If heated from 100 K to 3200 K, graphite will store 2.2 MJ/kg. This is enough to produce 308 N-s of thrust per kilo of graphite per burn. If you're okay with 5% of your total mass being made of graphite, on a 3 km/s burn you'll need just shy of 200 passes to make it work. I would definitely not call it totally infeasible, although the engineering challenges here are very significant.

Boron is also a strong contender. Its melting point is at 2600 K, which corresponds to an exhaust velocity around 9 km/s or so. It stores 4.65 MJ/kg, more than twice as much. The downside is that boron is much rarer than graphite.

RobS · 2014-03-10 13:38:26

I agree with you, but I also agree with your earlier postings how NASA can't get anything done cheaply nowadays.

But it occurs to me that they could launch it into a high earth orbit and test it there. If it launched itself into a solar orbit or crashed itself into the moon, it would be disposed of safely.

The ideal way to test would be at a lunar base where there are personnel to see what the engine did and can test it repeatedly, refuel it, etc.

Of course, either way you will have to deal with anti-nuclear protesters at Kennedy Space Center and the adverse publicity.

GW Johnson · 2014-03-10 16:03:36

Protesters will find something to protest, whether it is nuclear or not. Most of the time it is just bad publicity. But, it is a reminder to take a good hard look at what you are doing, to make sure that the protesters do not have a valid point. Sort of a double-edged sword, it actually does serve a purpose.

As for NASA, if nuclear materials were not a government monopoly, I'd say screw NASA, just go do it on the moon. Might still be able to do it without NASA if the DOE nuclear materials monopoly could be broken or sidestepped somehow. That will take political action in Congress, unfortunately. They're worse than NASA, by far.

Somehow, we need to develop a new government-business partnership model besides the government lab-contractor model that seems to be so exclusive these recent decades. I'm talking about a seismic shift in the way we do things.

It's been done before. Last time was around 300-400 years ago, though. But if we were to do it that way, NASA would be replaced, by something that might work better for a while. One of my life's lessons is that government bureaucracies over about 30-40 years old stultify into total inaction. There are very few exceptions to that rule of thumb.

As for testing nuclear engines flying out in space: really bad idea. The last century's experience very clearly shows that you need a stable thrust stand for your basic test work. You cannot do that flying in free fall, where every test becomes a vehicle flight test. You cannot isolate the phenomena you are trying to understand that way.

That's only done on a planetary surface somewhere, so that the thrust makes nothing move. I like the moon, because there is no air and water to pollute, no neighbors to disturb, and there are pre-existing crater walls to act as debris catchers when you screw up. And you will.

GW

Last edited by GW Johnson (2014-03-10 16:09:06)

Quaoar · 2014-03-14 11:15:24

RobS wrote:

You could also save time and propellant using a solar electric or solar thermal engine to move most of your assets into a very large elliptical orbit, then move the people and the earth return capsule there quickly, rendezvous above the Van Allen Belts, loop back in, and perform trans-Earth injection.
.

Rendez-vous in higly elliptical orbits are difficoult or can be performed without trouble?

RobS · 2014-03-14 11:42:29

I don't know. It may be difficult, especially if the rendezvous occurs above the level where GPS can be used to determine one's position precisely.

Quaoar · 2014-03-14 12:34:45

So on Mars, where aren't GPS, obital rendez-vous may not be very easy, even in low circurar orbit

RobS · 2014-03-14 13:00:50

Obviously there were many very accurate dockings without GPS, but I suppose nowadays they use it to some extent.

Quaoar · 2014-03-15 12:23:52

RobS wrote:

Obviously there were many very accurate dockings without GPS, but I suppose nowadays they use it to some extent.

How can be obtained an high elliptical orbit with a continous solar electric propulsion? Turning on the ion thruster only when the spaceship is near the perigee and turning it off for the rest of the orbit?

For the departure to Mars from an HEO, the long axis of the HEO has to be perfected oriented in the direction of the apogee of the Hohmann trasfer orbit?

RobS · 2014-03-15 13:31:34

With solar electric, they leave the engine on continuously and spiral out, whcih doubles the delta-v, but still uses much less fuel than other propulsion systems. If you turn on the engine only near perigee, you'd get an elliptical orbit and reduce delta-v, but stretch out the time immensely.

Quaoar · 2014-03-15 13:45:36

I this article, NASA thinks to use SEP to achive an higly elliptical orbit ( http://images.gizmag.com/gallery_lrg/so … brid-2.png ) and chemichal for depart from perigee.

but I dont know how a HEO can be achived with a continous SEP, that theorically have to spirally out, as you said.

Last edited by Quaoar (2014-03-15 13:47:32)

RobS · 2014-03-15 14:10:28

Hard to tell what the electrical propulsion is achieving when they have a chemical stage along as well.

Quaoar · 2014-03-16 07:33:49

They use the chemical rocket only for the final inserction in trasfer orbit. I suppose they achive an HEO using SEP turning on SEP for half orbit and turning off for the other. As the orbit will elongate, SEP firing period will became shorter.

A viable alternative may be to use a reusable LOX-LH2 chemical stage that put the spaceship in a HEO with the apogee at 100000 Km and came back in LEO via aerobraking. From a 100000 km HEO, the inserction into MTO is just 1 km/s.

Last edited by Quaoar (2014-03-16 07:35:00)

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Pit stop in L2?

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