Discussion The next 100 years..

[T.Neo]

The difference between predictions of spaceflight and predictions of air travel is that such silly predictions about air travel were born out of lack of knowledge, while pessimism about spaceflight is born out of historical evidence.

Of course moving away from corporate welfare and overbearing, meddling, stifling politics would help spaceflight a good deal. And we could see the space industry go on an "ascendant curve" far sooner than you think... but reality is not science fiction.

You can say "Oh, but this person and that person didn't believe flight was possible, or flight was going to be successful, and look where we are now". But the thing is, pretty much all of the "science fiction greats" of the 20th century got it all wrong. They got even the fundamental things quite wrong. To try and re-acquire that idea, you have to branch out from reality and delve into things that are dubious at best.

If space really was that important, we would see far more interest in it than we already do. And I really have a feeling that most of the world, who is actually in the business of making money and doing all sorts of things practically and viably, is more likely to be right than a small group of highly enthusiastic idealists.

 

[Linguofreak]

You fail to differentiate between propulsive braking and aerobraking. Have I explained the difference before?

He's not comparing propulsive breaking and aerobraking. I think his line of reasoning is "because aerobraking is difficult, let's minimize the amount of work that has to be done by the spacecraft that has to aerobrake for 8 km/s."

So it's more like he's comparing lifting an upper stage with full fuel tanks and imparting 9 km/s of DV to both fuel and spacecraft to only lifting the dry mass of the upper stage, and filling the tanks with fuel shipped in from the moon. (So 9 km/s to lift the spacecraft, but only 2.5 km/s to lift the fuel).

Of course, this is a good plan once you have a mature space infrastructure, but it's not going to be economical without a very brisk mission rate to destinations at or beyond the moon. And I don't see scientific exploration demanding that kind of mission rate. About the only thing that would demand that kind of mission rate would be a colonization project, and there's nothing in the solar system worth colonizing. Thus, unless Mars or Venus magically develops a breathable atmosphere and significant amounts of liquid surface water at some point in the near future, I can't see any reason to set up fuel mines on the moon.

 

[Ghostrider]

And I really have a feeling that most of the world, who is actually in the business of making money and doing all sorts of things practically and viably, is more likely to be right than a small group of highly enthusiastic idealists.

Yeah, Lehman Brothers are probably smarter than anybody else...:lol:

 

[Urwumpe]

I think the Zubrin Fairy comic is not really making the person posting it a smarter person. Rather contrary - who here did not yet use aerobraking for his missions?

The comic isn't really funny it just appeals to the ignorant. If you take this comic for serious, it means we can really do magic every other mission - and what did already Arthur C Clarke say: Any sufficiently advanced technology is indistinguishable from magic.

 

[T.Neo]

He's not comparing propulsive breaking and aerobraking. I think his line of reasoning is "because aerobraking is difficult, let's minimize the amount of work that has to be done by the spacecraft that has to aerobrake for 8 km/s."

So it's more like he's comparing lifting an upper stage with full fuel tanks and imparting 9 km/s of DV to both fuel and spacecraft to only lifting the dry mass of the upper stage, and filling the tanks with fuel shipped in from the moon. (So 9 km/s to lift the spacecraft, but only 2.5 km/s to lift the fuel).

My point is that although aerobraking is indeed a 'formidable' engineering challenge, it isn't in the manner HopDavid suggests. The TPS on the Shuttle was refurbishment-intensive, but as far as I know, this is due to it being damaged by debris and not due to the reentry process itself.

Also, my point is that an upper stage is an upper stage. Since hydrogen and oxygen from Earth have a minimal cost, it just makes sense to fill your upper stage with propellant and use it as an upper stage, rather than having to lug it to orbit with another upper stage (unless you're using an SSTO, in which case you have other problems). My point is also that HopDavid's 'space tugs' will effectively be upper stages- they won't differ that much from several upper stages operating today, in capabilities. They will have additions to operate for extended periods of time in space, but they will not be smaller, nor will they be less complex.

Also since these tugs will operate for an extended period of time in space with little to no maintainance, they will have to be built to withstand years of operation 'alone' and this will drive up costs. My point is that the cost of a TPS (and designing a structure to deal with reentry) could be low enough to enable considerable cost savings by bringing the vehicle back to Earth for refurbishment at extensively equipped facilities. There is a maintainance cost involved, but now the requirements on the vehicle aren't quite so intensive and thus the unit cost will be less.

The issue with lunar propellant is that the propellant itself could cost on the order of tens of thousands of dollars. On Earth, propellant is exceedingly cheap, and while launch costs are high, they can be reduced. I believe it is far more useful to invest in a better launch infrastructure, than to try to invest in an expensive lunar one. The issue is infrastructure, not physics. The Moon is at a physical advantage to the Earth, but at a very great disadvantage in terms of infrastructure.

Yeah, Lehman Brothers are probably smarter than anybody else...

Hey, I never said anything about Lehman Brothers. :lol:

I think the Zubrin Fairy comic is not really making the person posting it a smarter person. Rather contrary - who here did not yet use aerobraking for his missions?

The comic isn't really funny it just appeals to the ignorant. If you take this comic for serious, it means we can really do magic every other mission - and what did already Arthur C Clarke say: Any sufficiently advanced technology is indistinguishable from magic.

I believe the point behind the comic is that Mars Direct was overly optimistic in terms of Aerobraking. Not that Mars Direct is alone in that case...

 

[Urwumpe]

I believe the point behind the comic is that Mars Direct was overly optimistic in terms of Aerobraking. Not that Mars Direct is alone in that case...

Can't confirm this, there are current flown technologies that exceeded the assumptions of Mars Direct by far in terms of aerobraking.

The optimism was in other places, for example in the needed crew support systems or the safety of the mission.

 

[HopDavid]

Rather contrary - who here did not yet use aerobraking for his missions?

I wasn't trying to say Mars aerobraking shouldn't be used.

The intention of the cartoon was to illustrate Mars EDL (Entry, Descent, and Landing) becomes more difficult with massive payloads.

A larger object has a smaller surface to volume ratio. All these objects have the same [ame=http://en.wikipedia.org/wiki/Ballistic_coefficient]ballistic coefficient[/ame]:

A large part of ballistic coefficient is ratio of cross sectional area to mass. This is a useful quantity when trying to model aerobraking.

Some Mars plans propose inflatable heat shields to improve ballistic coefficient. This runs into stability issues. Think of a frisbee flying face on into a hurricane.

Difficult EDL is just one of the list of problems a Mars base must deal with.

 

[Urwumpe]

This runs into stability issues.

Stability problems had been less of an issue in the IRDT.

http://www.2r2s.com/irdt_concept.html

 

[HopDavid]

He's not comparing propulsive breaking and aerobraking.

Thank you.

I think his line of reasoning is "because aerobraking is difficult, let's minimize the amount of work that has to be done by the spacecraft that has to aerobrake for 8 km/s."

So it's more like he's comparing lifting an upper stage with full fuel tanks and imparting 9 km/s of DV to both fuel and spacecraft to only lifting the dry mass of the upper stage, and filling the tanks with fuel shipped in from the moon. (So 9 km/s to lift the spacecraft, but only 2.5 km/s to lift the fuel).

I don't know when you started reading this thread. In case you haven't seen it I'll repost the three vehicles I imagine:

Yellow:

Going up: 9 to 10 km/s
Going down: .1 km/sec
Reentry abuse: Extreme. Typically 8 km/s is shed over an hour's time.

Red:

Going up: 3.8 km/s
Going down: .7 km/sec
Reentry abuse: Mild. 3.1 km/s shed over several perigee drag passes.

Green:

Going up: 2.5 km/s
Going down: 2.5 km/sec
Reentry abuse: None.

Prior to lunar propellant coming online, any propellant delivered to orbit must be done with vehicles needing a lot of delta V. If they're reusable they also must endure re-entry abuse (I represent this as a yellow vehicle).

After lunar propellant comes online, orbital propellant could be delivered with smaller, simpler, reusable vehicles.

Of course, this is a good plan once you have a mature space infrastructure, but it's not going to be economical without a very brisk mission rate to destinations at or beyond the moon. And I don't see scientific exploration demanding that kind of mission rate.

Scientific exploration isn't the only possible use of propellant.

 

[T.Neo]

EDL is a problem without one major, single, magic bullet solution. That does not mean it is impossible to solve. Once you have a system that works, the problem is majorly reduced.

"Mars EDL is difficult and makes a sustainable Mars base impossible" is like saying "building a large ship that can stay watertight and negotiate ice floes is difficult and makes exploration of Antarctica impossible"...

HopDavid, I hate to be this harsh, but cute images don't make a very good case for something in reality.

And stop talking about "reentry abuse". There's a difference between understanding the problems involved (and their solutions) and just labelling them "abuse".

 

[Urwumpe]

"Abuse" also assumes that the heat shield of a spacecraft was made for something else. "Reentry ordeal" is a better term, but still: if you designed for it, it is a matter of testing if you designed properly.

 

[HopDavid]

it has an absolutely huge pressurised environment and an absolutely huge power source.

[ame=http://en.wikipedia.org/wiki/International_Space_Station]I.S.S.[/ame] pressurized volume is 837 meters³. 4 solar array wings at 32.8 kW each total 131.2 kW. I.S.S. masses 417 tonnes.

The MTV called for in this Mars Semi Direct plan call for an MTV with a pressurized volume of 72 meters³ and 20 kW solar array, 130 tonnes.

But wait, there's more.

It calls for a Cargo Vehicle 25 kW, 62 tonnes.

Hab 210 meters³, 19 kW solar arrays, 62 tonnes.

In the first cycle Habs and Cargo vehicles would be launched prior to the MTV. But in subsequent cycles they would be launched at the same time at the MTV.

Along with these vehicles are 4 Trans Mars Stages to place these three vehicles in Trans Mars Injection. Each of these mass 110 tonnes.

Total: 282 meters³, 64 kW, 694 tonnes. About half the I.S.S. pressurized volume and power but half again it's mass.

Each 2.14 years.

The ISS also had launch delays and was brought up by an expensive vehicle (STS).

A Mars mission would likely be brought up by SLS. Whether SLS turns out to be an inexpensive vehicle remains to be seen.

I have been looking at Bonin’s plan. In my search I came across Claim: HLV isn’t required; Existing Rockets Good Enough thread on NasaSpaceFlight. There Ed Kyle posted this info:

Existing Vehicles..................(LEO Payload, tonnes)
---------------------------------------------------------
Delta IV Heavy.....................23 t
Atlas V-551..........................19 t
Delta IVM+5,4 .....................14 t
Falcon 9 ..............................10 t
Delta II (soon to be retired).....6 t
Taurus 2e.............................6 t
Minotaur 4............................2 t
Taurus XL...........................1.4 t

Prospective Vehicles.............(LEO Payload, tonnes)
------------------------------------------------------------
Falcon Heavy......................30-50 t?
Atlas V Heavy......................29.4 t
Delta IV Heavy RS68A ...........27.5 t
Atlas V-552..........................20.5 t
Falcon 9 Block 2 ..................13-16 t?
-------------------------------------------------------------

Reading the arguments and counter arguments, I no longer regard Bonin’s scheme as implausible. However, the HLV advocates correctly point out Mars missions require a lot of mass in LEO. If you’re delivering mass in 25 to 50 tonne increments, a lot of orbital assembly would be required. HLV advocates also mentioned some Mars payloads need a big fairing, which would be difficult for medium lift vehicles. The inflateable heatshields may need a large fairing, for example. I wouldn’t call these show stoppers but they’re definitely considerations. For a Mars mission it is not cut and dried that HLV isn’t the least costly.

Given boil off, I do not regard depots as a good way to do Mars. If the propellant needs to be delivered just prior to launch window, better to lift empty Trans Mars Stages (TMS) shortly before the launch window and fuel them in orbit. Given medium lift vehicles, it will take 3 or 4 propellant deliveries to fuel each TMS. Since there are 4 transMars stages, we’re talking about 16 to 20 launches shortly before launch window. We’d also need to deliver the astronauts shortly before luanch window. This flurry of launches just before window poses logistic challenges at the pads.

Given this, there is no need for infrastructure at EML1 or 2. It saves delta V for the Trans Mars Stage to do the burn from LEO.

With a lunar mission, trip time is less than a week. No massive MTVs needed. Launch windows occur each two weeks. No need for hectic schedules around launch windows.

A lunar architecture is quite amenable to propellant storage at EML1 and EML2. EML1 and 2 are potential transportation hubs to many orbits in own neighborhood as well as deep space destinations like Mars or NEOs. So this infrastructure opens up other possible uses than just a lunar architecture. For learning about using EML1 and EML2 in 3 body mechanics, I recommend this text book (a 16 Mb pdf).

And while the BEO radiation environment must not be underestimated, I fear you don't try to understand the direct impact on engineering that it has. It isn't that simple in reality, but you could go as far as to just equate radiation shielding with... blocks of plastic.

Have you heard of GCR, SPE?. SPE can be predicted but GCR can't. This article recommends missions of 100s of days have 10s of meters of water for radiation shielding.

In my opinion the 130 tonne MTV vehicle described by Wilson and Clarke could kill 4 astronauts with radiation. I believe a much more massive MTV would be needed.

Haven't you heard about staging? It is a really interesting concept, I believe it was developed around the time of the turn of the last century, by a guy by the name of Tsiolkovsky...

Indeed I have. I have been saying a 14 km/s delta V budget mandates multi-stage expendables. Repeatedly.

When you factor in the dry mass that gets thrown away, a 14 km/s vehicle must be even more massive than one with a 5 km/s budget.

Avoiding large, complicated, disposable vehicles is the whole point of lunar supplied orbital propellant depots.

Also, your first stage does not need to be kerolox, as the Delta IV demonstrates.

Delta IV does indeed demonstrate something, but not what you think. See LH2 vs RP-1

You fail to differentiate between propulsive braking and aerobraking. Have I explained the difference before?

Somehow you think the extreme difficulty of propulsive braking makes aerobraking a non-issue. Simply not true.

Got any source that states that impact flux is lower at the highest latitudes?

Here.

That specific physics is not everything,

Math and physics are important.

Akins 1st law “1. Engineering is done with numbers. Analysis without numbers is only an opinion.”

 

[T.Neo]

I.S.S. pressurized volume is 837 meters3. 4 solar array wings at 32.8 kW each total 131.2 kW. I.S.S. masses 417 tonnes.

The MTV called for in this Mars Semi Direct plan call for an MTV with a pressurized volume of 72 meters3 and 20 kW solar array, 130 tonnes.

But wait, there's more.

It calls for a Cargo Vehicle 25 kW, 62 tonnes.

Hab 210 meters3, 19 kW solar arrays, 62 tonnes.

In the first cycle Habs and Cargo vehicles would be launched prior to the MTV. But in subsequent cycles they would be launched at the same time at the MTV.

Along with these vehicles are 4 Trans Mars Stages to place these three vehicles in Trans Mars Injection. Each of these mass 110 tonnes.

Total: 282 meters3, 64 kW, 694 tonnes. About half the I.S.S. pressurized volume and power but half again it's mass.

Each 2.14 years.

Two totally different projects, cannot be directly comapred.

Also, who said we need to use Mars Semi-Direct to get to Mars?

And please stop with the "2.14 years" thing. We all know about launch windows to Mars, you don't have to continuously come up with that date with the accuracy of 7 weeks, 2 days, 2 hours and 24 minutes...

A Mars mission would likely be brought up by SLS. Whether SLS turns out to be an inexpensive vehicle remains to be seen.

:beathead:

Hey, we both hate SLS. I'm advocating Mars exploration, and un-advocating the worst launch vehicle for the job. You're going the opposite. Why pick this suboptimal launch vehicle?

Reading the arguments and counter arguments, I no longer regard Bonin’s scheme as implausible. However, the HLV advocates correctly point out Mars missions require a lot of mass in LEO. If you’re delivering mass in 25 to 50 tonne increments, a lot of orbital assembly would be required. HLV advocates also mentioned some Mars payloads need a big fairing, which would be difficult for medium lift vehicles. The inflateable heatshields may need a large fairing, for example. I wouldn’t call these show stoppers but they’re definitely considerations. For a Mars mission it is not cut and dried that HLV isn’t the least costly.

You must consider the difference in assembly for a Mars mission and construction for the ISS (for example). The ISS required some pretty complex stuff, for example work with robotic arms, as well as EVAs for construction. A Mars spacecraft stack, will be optimised for the simplest assembly sequence that is possible. That means dockings, no RMS juggling, no EVAs. Maybe even use of a dedicated low orbit 'space tug' to fetch modules and deliver them to the rest of the spacecraft.

You don't need an HLV to have a wide payload fairing. Also, the payload fairing diameter that you need is debatable. I'd imagine the whole point of an inflatable TPS would be to reduce fairing diameter.

An HLV is the most costly solution. The orbital part of it can be reduced in cost if done right, but an HLV simply can't sustain an economic launch rate (unless we live in sci-fi land).

Given boil off, I do not regard depots as a good way to do Mars. If the propellant needs to be delivered just prior to launch window, better to lift empty Trans Mars Stages (TMS) shortly before the launch window and fuel them in orbit. Given medium lift vehicles, it will take 3 or 4 propellant deliveries to fuel each TMS. Since there are 4 transMars stages, we’re talking about 16 to 20 launches shortly before launch window. We’d also need to deliver the astronauts shortly before luanch window. This flurry of launches just before window poses logistic challenges at the pads.

Given this, there is no need for infrastructure at EML1 or 2. It saves delta V for the Trans Mars Stage to do the burn from LEO.

With a lunar mission, trip time is less than a week. No massive MTVs needed. Launch windows occur each two weeks. No need for hectic schedules around launch windows.

You can mitigate boiloff.

Another thing you can do is to assemble the spacecraft at EML2 where boiloff is less severe. You will have to expend propellant to get there from LEO, but scheduling conflicts can be reduced.

You automatically insist that you need a flurry of launches right before the launch window, but you don't attempt to find a work-around to this problem.

In multiple cases you don't try to fix problems, you try to create problems and then enforce them. I personally find this excruciating.

Have you heard of GCR, SPE?. SPE can be predicted but GCR can't. This article recommends missions of 100s of days have 10s of meters of water for radiation shielding.

In my opinion the 130 tonne MTV vehicle described by Wilson and Clarke could kill 4 astronauts with radiation. I believe a much more massive MTV would be needed.

Oh yes! Magical radiation! Radiation will kill your crew, you can't get away from it! Just dead, therefore interplanetary travel is impossible. :facepalm:

I'm not going to take your word for it that a "much more massive" MTV would be required.

Also, there are workarounds. Look at electromagnetic radiation shielding, for example.

Indeed I have. I have been saying a 14 km/s delta V budget mandates multi-stage expendables. Repeatedly.

When you factor in the dry mass that gets thrown away, a 14 km/s vehicle must be even more massive than one with a 5 km/s budget.

Avoiding large, complicated, disposable vehicles is the whole point of lunar supplied orbital propellant depots.

Wait, I thought dV to orbit was some 10 km/s. When did it change to 14 km/s?

My point is that the individual stages never have to deal with a dV of 10 km/s because they are individual stages. And they need not be expendable, as much as you demand that they do.

You use "large, complicated, disposable" as 'buzzwords'. You don't try to look into the real engineering problems or their solutions. You don't try to look into the actual issues of lunar resource utilisation, but you automatically designate it as the solution.

Delta IV does indeed demonstrate something, but not what you think. See LH2 vs RP-1

Did I mention already the RS-68A engine upgrade?

Something about that guy's analysis does not seem right to me...

Somehow you think the extreme difficulty of propulsive braking makes aerobraking a non-issue. Simply not true.

It does make it a non-issue by comparison.

If you ever venture into the realm of propulsive braking, you will run screaming, chased by monsters. It really is that scarily bad. :lol:

Also, you don't display a very good knowledge of the true difficulty of the technological requirement for aerobraking, as evidenced by you calling aerobraking "abuse". As Urwumpe pointed out, that isn't abuse of the system... abuse is more like taking a Mini Cooper and trying to drive like a 4x4 with it, or trying to chop stone with an axe.

Here.

Right at the beginning;

At low meteorite velocities the variation is small but for meteorites with average or above average velocity the rate of impact in polar regions decreases to 50 or 60% of the equatorial values.

50-60% is not "untouched".

Math and physics are important.

Math and physics go beyond things like the rocket equation. The driving factor in anything- be it technological, military, or even biological, is logistics. If you have bad logistics, you fail.

The problem with the Moon is that although it has some physical advantages, it totally lacks infrastructure and this puts it in an inferior position to the Earth.

Akins 1st law “1. Engineering is done with numbers. Analysis without numbers is only an opinion.”

I actually have tried to provide numbers. All you have provided so far are cute graphics, some dubious and some misplaced claims, and an insistance on enforcing problems so that the entire Universe favours your personal preference.

 

[HopDavid]

It does make it a non-issue by comparison.

And by comparison to the Milky Way our solar system is tiny.

This comparison argument is utterly pointless.

 

[T.Neo]

And by comparison to the Milky Way our solar system is tiny.

This comparison argument is utterly pointless.

I don't see why it is pointless, our solar system is indeed tiny in comparison to the entire galaxy.

8km/s done by rocket engines and 8 km/s done by a TPS are two totally different things. While you do need to withstand certain forces during aerobraking, it cannot and must not be equated with performing a propulsive manuver, as it is not the same thing.

 

[Urwumpe]

This comparison argument is utterly pointless.

No, it is not. Your own comparison is simply Apples and Oranges.

Can you show any advantages that propulsive capture has for example over aerobraking for capture?

Remember: The important figure is mass. The mass for a heat shield for aerobraking is between 0 (shielding against sunlight near Earth is strong enough further outside) and 20% (full protection for direct reentry).

I think I don't expect too much from you to estimate the propellant mass fraction for different propulsive scenarios.

 

[HopDavid]

No, it is not. Your own comparison is simply Apples and Oranges.

Can you show any advantages that propulsive capture has for example over aerobraking for capture?

Since I have never advocated propulsive capture over aerobraking for capture, I don't have to.

Asking me to defend a position I've never taken is called a [ame=http://en.wikipedia.org/wiki/Straw_man]straw man argument[/ame]. Very bad form.

My actual statement: 8 km/s re-entry adds to difficulty and expense.

 

[T.Neo]

My actual statement: 8 km/s re-entry adds to difficulty and expense.

You seem to state that this 8 km/s re-entry will directly add difficulty and expense in an incredible fashion. You seem to equate it with an 8 km/s propulsive manuver (because you continuously cite the '8 km/s' figure) when the difficulty and expense of both is entirely different.

As far as I know, the cost of reentry provisions isn't even directly related to velocity change, but to things like deceleration, aerodynamics, and heating, which can differ between vehicles and trajectories.

I'm still trying to find where the STS TPS was a major pain due to the fact that it had to deal with reentry. As far as I can tell, its major issue was being fragile and maintainance-intensive due to debris impacts.

 

[HopDavid]

I don't see why it is pointless, our solar system is indeed tiny in comparison to the entire galaxy.

Because you can argue any quantity is small compared to another quantity.

You can say colonizing Mars is easy compared to colonizing Alpha Centauri.

Doesn't make colonizing Mars easy.

 

[Urwumpe]

My actual statement: 8 km/s re-entry adds to difficulty and expense.

Just slowing down from 8 km/s to 6 km/s by rocket engines adds much more to difficulty and expense as the additional inch of ablative heat shield necessary compared to a 6 km/s reentry.

Also, 8 km/s reentry is pretty much standard. Apollo did 13 km/s reentries, Huygens did fine at 6.1 km/s, Galileo was even at 48 km/s.