New Mars Forums

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.

This is something I've been wondering as well, in fact some of my recent "Lots of science questions" post was dedicated to this topic.

As I understand it so far, if you know the specific impulse (Isp) of your fuel and the Delta-V you want to achieve, you can calculate the mass ratio. The mass ratio tells what mass of fuel you must carry for a given dry mass. That dry mass needs to include the mass of the fuel tanks as well as the payload mass, so you plug in a formula that will give you the fuel tank mass as a function of fuel mass and you can solve the whole thing to find the total fuel mass necessary for your payload. That's how I understand it after reading The Case For Mars, anyway. Furthermore, it sounds like the cost of the launcher is pretty much linearly related to the total launch mass, including fuel. (Someone please stop me if I say something totally wrong...)

I'm pretty sure that doesn't tell the whole story though and I'm trying to figure out the rest. First of all, I don't know the actual formulas that are involved in the previous paragraph, but more than that, I think it's actually missing some important steps.

There's obviously an a significant thrust necessary even to stand still (zero Delta-V) in a gravitational field, so a calculation based only on Delta-V doesn't actually seem to include overcoming gravity for the duration of the flight - unless an adjustment is being applied to the Delta-V figure to compensate for that.

Furthermore, I'd be amazed if you can simply ignore atmospheric drag in this calculation, but I don't know how it gets included.

Finally, I don't know what different kinds of orbit exist and what the relationship between the Delta-V's necessary to reach them are. There's obviously LEO and Geosynchronous, then there are things that aren't strictly orbits like escape velocity and X-prize altitude. But I'm not sure what other significant milestones exist, or how big the Delta-Vs are compared to each other. Or which of them are commonly used (eg for ISS, commercial satellites, GPS, shuttle...)

Hopefully I'm adding at least a little bit of useful information here instead of just more questions, but I'd still be extremely grateful if someone could help me fill in the big gaps remaining in my own knowledge...

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.