New Mars Forums

sab39

Banned

From: Philadelphia, PA, USA

Registered: 2004-03-16

Posts: 7

Re: Lots of (mostly) science questions

Hello there,

As I mentioned in the introductory post I made in the 'Meta' forum, my overall interest here is in what's necessary politically to make a manned Mars mission actually happen. But I figure that the only way I can even form an educated opinion in that area is actually to be educated, or at least have a good grounding of accurate general knowledge, in the science involved. So this post is the beginning of my attempt to educate myself. It's not rocket science, after all... er, wait...

I'm posting here in the hope that people who are already educated might be able to fill in some of the gaps in my knowledge.

Anyway, the majority of the science I do know in this area comes from high-school physics and reading The Case for Mars. I have some specific questions that I've identified, but it's very likely that there are other vital areas that I don't even know I'm missing[1]. If any of these are obvious from my questions, please point them out. Anyway, here are the questions...

1) I think I understand the relationship between DeltaV and launch mass: the required DeltaV combined with the specific impulse of your fuel allows you to determine the required mass ratio. Then given the known payload mass and a formula to calculate the mass of the fuel tanks (TCFM suggests that a percentage of the propellant mass is a reasonable estimate here) you can figure out the actual propellant mass needed, and hence the total launch mass. What I don't understand, though, is what the relationship is between launch mass and cost. Is this essentially linear, or does it grow even faster than that due to the increased engineering requirements to build a bigger rocket?

2) How do you account for atmospheric drag in those calculations? Presumably it plays a significant role but I don't think any of the calculations in TCFM explicitly included it.

3) What are the interesting kinds of orbits and corresponding DeltaV's for Earth? I know (vaguely) of some different categories of orbit: X-Prize level (an altitude rather than an orbit, but presumably still has a fixed DeltaV requirement to attain?); LEO; Geosynchronous (that's the same as GEO, right?); escape velocity (again not an orbit, but definitely has an associated DeltaV).
a) Are there any other important classes of orbit I'm missing?
b) What are the DeltaV's for each? and
c) Which of them are used in practice today (by the ISS, GPS, communications sats; reachable by the shuttle, etc).

4) Is specific impulse still measured by the pound instead of using N and kg? What's the value for modern day rocket fuel? For that matter, what is modern day rocket fuel?

5) TCFM claims that the processes for ISPP are no problem because we can test them almost perfectly on earth, but it doesn't address the testability of what seem to me to be two potential dealbreakers.
a) You can't test it in Martian G; how sure are we that low-G conditions won't affect the reactions, or at least will do so in predictable and well-understood ways? The only ways I can think of to build a .38G lab to test a long-running reaction in are a tethered spinning ISS-a-like (very costly) or a centrifuge on the moon (very very costly).
b) Can we simulate the dust on Mars accurately enough to perform a thorough test on the intake filter system? A filter that's going to have to run for years without ever being changed seems like something that's important to test, but this is glossed over in the book.

6) What happened with the meteorites mentioned in the epilogue of TCFM? Presumably they were debunked - if not, past life on Mars would by now be considered accepted fact rather than still pure speculation. What turned out to be the problem with the theories as presented in the book?

7) Have there been any other major changes in the state of our knowledge since 1996? For example, do we have any better idea of the unknown constants in the terraforming equations? Does that model still hold up at all? Do we have a better idea of the nitrogen content of the Martian surface? Has the question of subterranean, geothermally heated liquid water been settled?

(next ones aren't strictly science related)

8) I know that Bush's proposed plan involves splitting the "humans to space and back" and "cargo to space" into two separate launchers. If this is done, how do you launch the Mars Direct hab? Is the "cargo to space" launcher rated to lift humans too, or do you need an orbital rendezvous with the "humans" launcher to put the crew on board? Or do we just not know this level of detail about the plan yet?

9) Does anyone know Kerry's position on space / Mars?

10) Is there a glossary of terms and acronyms anywhere, especially one that includes the vehicles proposed by Bush's plan? I always get lost very quickly in discussions of the Bush plan because of all the vehicle name acronyms getting tossed around.

11) How, if at all, are Mars advocacy groups such as this and the Mars Society involved in political lobbying? Are there any full-time employed Mars lobbyists?

12) Since the "nuclear problem" is going to be one of the biggest sticking points for getting a Mars Direct or similar program off the ground, has any Mars advocacy group attempted or considered approaching the major environmental groups (Greenpeace... any others?) to try to get them to understand and endorse the safety of the plan? One of my fears (possibly the biggest other than that we won't try at all) is a directive saying "Do Mars Direct, but without any use of nuclear power". Unless something's changed radically since TCFM, any such directive would absolutely doom the program, right?

Wow, that was a lot of questions, and that's just the known unknowns . I'll be very impressed as well as grateful if anyone manages to plough through all that and provide answers...

Thanks for reading this far!

Stuart.

[1] As Ashcroft would put it, I have some Known Unknowns and some Unknown Unknowns.

SBird

Banned

Registered: 2004-03-10

Posts: 490

Re: Lots of (mostly) science questions

Some excellent questions.  I'm afraid that I'm not qualified to answer most of them but I'll give it a shot.

1: You are correct in that cost doesn't scale linearly with launch mass.  On one side, many costs like engines and the like actually get cheaper as you go up in launch mass.  However, other things like the launch facilities and fuel tanks and the like get almost exponentially more expensive with increasing launch mass.
Furthermore, you can kep the same launch mass and dramatically reduce cost by smarter engineering.  NASA and other space agencies tend to forget the KISS principle and it costs us a lot of money.  For example, the Boeing Sea Launch platform can deliver as much to LEO as an Arianne 4.  Despite being similar in launch capability, the Zenit is half as expensive (~$2500/lb vs ~$5000/lb) to operate.  A large part of this is that the actual fuel costs of a rocket are something like %1.5 of the total lauch costs. Likewise, the physical rocket is a minor part of the cost.  The big cost is the army of people needed to fly these things.  Sea launch uses a really simple launcher that requires a minimal number of technicians to operate. 
A similar philosophy is going on at SpaceX.  I'm not qualified to say whether the numbers are correct but this company is claiming that they will be able to get launch costs down to $1000/lb.  The technology they are using is primitive but simple and cheap.   I think that this philosophy is important.  To get to Mars, we don't need fancy launchers, we need to look at the bottom line - how cheap can we get a reliable 140 ton to LEO booster?

2: not qualified to answer this myself but the atmospheric drag is a fairly small portion of the total delta V.  Most rockets clear the lower atmosphere in a minute or so.

3: The X Prize really isn't an orbit any more than a thrown baseball is.  As for the other orbits, most stuff is in LEO since it's cheap to get to.  However, satellites sit in all sorts of orbits.  For example, the GPS sattelites sit somewhere between LEO and GEO where they orbit the Earth about every 12 hours, IIRC.  You should be able to get relevant delta Vs from a Google search.

4: I think most engineers have gone over to metric these days or should have.  I'm not a rocket engineer, though so I don't know what the state of the industry is.

5: Gravity doesn't affect the system in any way tat a competent chemical engineer couldn't deal with on paper.  The only process I can think of is the gass flow rates through a condenser if it's being driven by buoyancy of the entrant gas.  However, I think that the whole system is pump driven so it should work in zero G.
Dust is more of a problem but there's plenty of dusty environments on Earth that we've managed to engineer for. At present, I think that refrigeration is the planned system for getting CO2 and so a simple subicron filter could be used to screen out dust.  Plus, I don't think that anything other than the pump is too severly affected by dust in the ISPP setup.

6: most of the Martian metorite evidencee was debunked.  However, there is still quite a bit of controversy about the magnetite particles in the meteorite.  There's a camp which I tend to side with (being familiar with the field of study), which posits that the magnetite particles look liike they are biologically derived.  However, the best analytical labs on Earth haven't resolved this problem.  I don't see Martian probes or expplorers being able to do much better unless they find something much more conclusive.

7: If anythingI think that our terraforming estimates are even fuzzier with the newer data coming out.  There's a lot of stuff we don't know about how Mars behaves over extended periods of time.  Evidence seems to indicated that Mars was a lot warmer in the recent past. (like within the last 3 million years)  It could be that Mars has major climactic oscillations and that it has frequent warm periods.

8: Dont know enough about the details of the new MArs plan to comment.

9: Don't know.

10: Sorry.

11: I don't know but since a full time lobbyist would probably cost at least $150,000 a year, I doubt the Mars Society has oone.

12: Not to my knowledge but this isn't a bad idea.  I'm not sure how much sense you can talk into those groups but the situation certainly going to improve if we don't at least try and communicate with them.  I think that a lot of members of those organiations are reasonably minded but misinformed.  A litle correct info could go a long way.

sab39

Banned

From: Philadelphia, PA, USA

Registered: 2004-03-16

Posts: 7

Re: Lots of (mostly) science questions

Wow, thanks for the excellent responses (with the low "read" count and zero "post" count on this topic I'd almost given up on getting a response here, and I wouldn't blame people for not responding - that was a long list )

I'm beginning to find some /\V (how's that for a pure iBCode delta symbol? ) values from google.

A good page I've found is [http://www.strout.net/info/science/delta-v/intro.html]http://www.strout.net/info/science/delta-v/intro.html which gives some useful ones, although not X-prize or escape velocity (I know X-prize level isn't an orbit, but one thing I'm really interested in is how much harder LEO and Escape velocity are to reach than X-prize levels, because that will help me understand how big of a step the X-prize is on the way to Mars).

FWIW, that page gives LEO as 8600 m/s^2, Geosynchronous as 8600+3800 and lunar orbit as 8600+4100. If I find X-prize or escape velocity /\V's I'll post them...

Thanks again for the great responses. I'm sure I'll reply further as I digest these facts...

sab39

Banned

From: Philadelphia, PA, USA

Registered: 2004-03-16

Posts: 7

Re: Lots of (mostly) science questions

Hmm, I like this page better: [http://www.pma.caltech.edu/~chirata/deltav.html]http://www.pma.caltech.edu/~chirata/deltav.html

The other one gave /\V values in units of acceleration, which is clearly bogus. It also gave a different number for LEO than this page does. In fact, this page gives a huge amount of information including all the steps to mars (it doesn't mention Earth escape velocity specifically, but it does include the transfer orbit from Earth to Mars which, since that's a solar orbit, must include at least escaping from Earth first). And it still doesn't answer my X-prize question. But it's full of a lot more great information that will help me fill in other details. Very worth reading.

sab39

Banned

From: Philadelphia, PA, USA

Registered: 2004-03-16

Posts: 7

Re: Lots of (mostly) science questions

The only source I can find for X-prize /\V is a slashdot post (!), or rather a couple of slashdot posts which claim between 1.4 and 1.8 km/s for X-prize altitude. As opposed to 9.7 (per the caltech page) for LEO.

That (coupled with the fact that the difficulty increase is much greater than linear in /\V) indicates that achieving the X-prize doesn't take you anywhere near reaching LEO, which answers that part of my questions

SBird

Banned

Registered: 2004-03-10

Posts: 490

Re: Lots of (mostly) science questions

I haven't seen any hard numbers for the X-prize delta V but I was under the impression that is was something like 1/11th  the delta for LEO.  That would put it at something like 0.5 km/s. 
This is why most of the schemes like maglev boosters and such don't work well.  Almost all of your energy is spent up above the atmosphere going horizontal and getting enough centripetal velocity to keep from falling back down.

SBird

Banned

Registered: 2004-03-10

Posts: 490

Re: Lots of (mostly) science questions

LEO isn't a fixed value, either.  I think it ranges from about 250 km to 500 km.  I have 7814 m/s as the lowest delta V that will get you to a stable orbit.  (I assume that this is an orbit that only lasts for a few hours due to atmosapheric drag.)