Posts by Lars_J

Lars_J · Space Policy

A nuclear reactor in space powers big weapons that can pierce the atmosphere and cause a human target to disappear in a cloud of smoke and dust.

Huh?

Prometheus was folded into the Exploration program to protect it

Yeah, because we all know NASA/science programs are much more immune to budget cuts than defense programs. :laugh:

Lars_J · Space Policy

Of course, a larger launch vehicle would offer more margins - But JIMO itself is not going to be a big enough program in itself to be able to support the development of an HLLV. If an HLLV is not available, they'll certainly have to manage with a D-IV-H.

Lars_J · Space Policy

I think HLLV is going to happen, if for no other reason than Prometheus is the wet dream of every warmongering president since Regan. And they are going to need an HLLV to get the darn thing up where it can be useful.

I'm not sure what project "prometheus" you are referring to - But NASA's project prometheus - now incorporated into the JIMO program will not require a HLLV at all. Current plans call for it to launch on top of a Delta IV-Heavy, and the unfold in LEO to its full 25+ m length.

Lars_J · Unmanned probes

Spirit spots the southern walls of Gusev crater!
[http://qt.exploratorium.edu/mars/spirit … 48L7M1.JPG]http://qt.exploratorium.edu/mars....7M1.JPG (the very faint mountain range in the background)

Lars_J · Unmanned probes

Regarding that "moving" thing... It doesn't popup or down - it just moves sideways quite a bit, behind a rock. It is actually much further back than it looks, and compare its movement with the objects just above it.

Lars_J · Unmanned probes

Here is JPL's official crater panorama:

[http://marsrovers.jpl.nasa.gov/gallery/ … R1_br2.jpg]http://marsrovers.jpl.nasa.gov/gallery....br2.jpg

Lars_J · Space Policy

think there's a difference between me saying "this is what I think the pres said" versus "the pres's plan is bad because he said..." and that's what distinguishes my reaction from John Glenn's. As a person who sets policy goals, I would not expect the president to know specifics about the means to achieve these goals.

I see - so Bush cannot be critized based on anything he says on the issue, because he doesn't know what he is talking about? Do I understand you right?

I'm sure the president has some rudimentary ideas about what his vision requires, and I'm sure that the people he's appointed to the Aldridge commission have some very different (and probably better) ideas about how to proceed.

There's lots of people that has great ideas about going to Mars, but unless he is actually prepared to pony up some $$$ we ain't goin' nowhere. That his great Mars plan wasn't even mentioned in the state of the union speech (traditionally a great laundry list of what a president wants to do) tells you how commited Bush is to space exploration.

Lars_J · Interplanetary transportation

Assuming a hole straight through the Earth, the object would swing back and forth contunously, like a pendulum. They only thing that would slow it down and eventually stop it would be atmospheric friction.

Lars_J · Human missions

Why not launch a better telescope on the same mission instead?
*Simple. Because there is -currently- no other telescope to take its place.

Well, one should be built then! Instead of wasting yet another hyper-expensive lanuch on trying to keep Hubble alive, why not launch better versions? I can see the wisdom in repairing aging telescopes *if* launches were cheap - but not at Shuttle prices.

Lars_J · Space Policy

Ad Astra,

I had to cringe when the president implied that the voyage to Mars would launch from the moon or a Lagrange point, but NASA has yet to endorse any approach towards human Mars exploration. For John Glenn to imply that the "Bush plan" includes a stop at the moon on the way to Mars is downright dishonest of him.

So apparently you and Glenn both understood Bush the same way, so exactly what is "downright dishonest" about what Glenn said? In the absence of an official NASA Mars plan, is it not fair to judge the "Bush plan" based on what Bush said?

Lars_J · Interplanetary transportation

Yes, the KISS (keep it simple stupid) rule is certainly one rule that ought to be followed more frequenctly in aerospace. Here are my general thoughts:

Capsule vs. Lifting body - I'd pick a capsule every time. Just build a capsule that is simple, robust, and perhaps even refurbishable. (expendable ablative heatshield)

HTOHL (Aerospike) vs. VTOVL (rocket) - VTOVL all the way. Somtimes the brute force method is the simplest and cheapest.

SSTO vs. TSTO - One day we will (hopefully) have economical SSTO's, but I'd prefer not to wait for cheap space access until then. We'll have to work incrementally towards that goal.

-------------

I'm definately paying close attention to the progress of [http://www.spacex.com]Space-X. The seem to be making some good progress, and hopefully we'll see a launch soon.

Lars_J · Human missions

I see no point really in another servicing mission to Hubble.

Why not launch a better telescope on the same mission instead?

Lars_J · Human missions

Robert Zubrin (in The Case for Mars) compared how much power, or Delta V, it would take to go from Earth to the Moon vs. from Earth to Mars. Just to get to the moon, it takes more power, more delta V, more fuel, more mass, and more money.

I hope you are not seriously saying that the amount of Delta-V needed for a mission is the only deciding factor for the expense of a project/mission? We are after all taking about a journey/mission that would be two orders of magnitude (100's) times longer. Factor in consumables alone, and it is clear that Mars mission would mass significatly higher. And that's only the tip of the iceberg.

BTW, I agree that going to Mars from the Moon is lunacy. But I want to go back to the Moon and to Mars as well. Doing one can gain you experience for the other, but the Moon certainly will (should) not act as a launching point - at least until we have some serious manned prescence on the Moon.

Lars_J · Planetary transportation

And how long do you think that propane/oxygen will last compared to an RTG?

Lars_J · Interplanetary transportation

Ummm, no.

Gravity is slightly less on the equator than at the poles.

Lars_J · Human missions

The kinds of space elevators that everyone talks about are the ones that merely reach GEO for quick, cheap acces to Earth orbit, but an overlooked detail is that if you make the elevator a little bit longer it's tip velocity exceeds Earth's escape velocity.

It's not an overlooked detail - its a basic feature of a space elevator. It must have equal mass above and below GEO, otherwise it will "fall down". (And everyone around the equator will have a really bad day)

Lars_J · Human missions

GCNRevenger,
Well, TSTO would certainly be cheaper *right now* - since it is more realistic with current technology, and development costs for an SSTO would be ... well huge and development times long. I guess we really need to separate out development and operational costs to discuss it further.
So I can agree with them in that aspect. Eventually SSTO will be the way to go, of course. As for there not being much of an incentive to move to SSTO from TSTO - if the economics really are there for SSTO's, someone will take that step.

A VTOVL craft will certainly be composed of mostly fuel with a relatively small payload, yes. But fuel is cheap. And having a vertical landing on a pad would certainly be the way to go.

Lars_J · Human missions

Of course SSTO would be preferable to TSTO, and certainly cheaper to operate. No one is arguing that. But does that mean that we should abandon spaceflight until we can do SSTO?

I only suggest TSTO as an interim solution, that clearly is workable with technology that exists *now*. The beauty of the VTOVL approach is that as engine performance, fuels, and structural materials are improved, one can gradually evolve into a SSTO RLV.
1. Build a sub-orbital test vehicle (similar to the DC-X) Gradually fly higher and higher, pushing the envelope.
2. Scale it up, and add a second stage for TSTO orbital access.
3. As performace improves, 1st stage comes closer and closer to orbit. Eventually it will require no 2nd stage and will become SSTO.

Concept: Simply because the Scramjet vehicle is more complicated does not make it "bad" compared to a DC-X style VTOL-SSTO.

Perhaps, but if all other factors are equal, simplicity wins out every time. The more parts you have, the more things can fail.

Concept: A TSTO launcher of any sort will inherintly be harder to fly often (an ultimate goal) because of the modes of recovery for the spent booster stages and the need to partially rebuild, not merely reassemble, the vehicle on each flight.

But TSTO is not the ultimate goal, just a practical intermediate step. (BTW, it is quite possible to design completely reusable TSTO systems, both winged and vertical)

Lars_J · Human missions

(finally back to the thread...)
I still believe Scramjet if fundamentally the wrong way for cheap access to space - at our current and forseeable level of technology - in other words, no magic particle beams. :;):

Scramjet designs still appear to have the following drawbacks:
Engine technology:
- 3 modes of propulsion needed: jet / scramjet / rocket (this adds complexity and mass, even with a "multimode" engine)
- How fast will an scramjet go before the rocket propulsion has to take over? At what speed and altitude will it have to be cut off?
Aerodynamics:
- An aircraft designed to be efficient at scramjet speeds will be difficult to control at landing and takeoff. This is a difficult enough problem for supersonic aircraft, and it will be even moreso for a hypersonic one.
- Lets not forget "center of mass" issues - This is a significant issue for the shuttle (with very small margins), and it will be even worse for a winged vehicle where a significant part of the vehicle will be tanks that are either full (takeoff) or empty (landing).
Material issues:
- Prolonged heating in the atmosphere. Extended atmopsheric exposure at Mach 10-20 will be brutal, no matter what material is used. And unlike the shuttle, which re-enters trhe atmosphere at a high angle of attack and onlty needs heavy protection underneath and in front, the scramjet vehicle will expose the entire body to the same contditions.

So what do I think will work? I am a strong believer in VTOVL (vertical takeoff, vertical landing) for cheap access to space - and eventually an SSTO RLV. And VTOVL will not suffer from the drawbacks I listed above for a scramjet vehicle. Does VTOVL have other drawbacks? Yes, but I believe those are easier problems to solve.

Here is how I see the path towards cheap access to space:
1. Two-stage VTOVL
Staging at hypersonic speeds will be problematic, so the solution is to stage above the atmosphere. (Well, the vast majority of it, anyway).
- Stage 1: Launches the vehicle stack in a high ballistic orbit, similar to Alan Shepards first flight. The top of the trajectory is at 100-150 km altitude, where the orbital stage 2 is released just before reaching. The first stage falls back to earth, and is recovered with parachutes/airbags. A further development might have a powered vertical landing to become a more reusable first stage.
- Stage 2: separates at 100-150 km, and accelerates to orbital speed. The Orbital stage reenters either side first (like a lifting body) or tail first, and lands with parachutes/airbags. A further development might have powered landing for increased reusability.
2. SSTO RLV
The SSTO RLV would be use a radial aerospike engine, launching vertically, reentering vertically, and landing vertically. The radial aerospike engine would do double-duty as a heat shield, allowing the rest of the vehicle to use fairly tame materials. Since the only acceleration/deceleration loads would be vertical, the design could be structurally fairly simple and light (composite cylindrical/conical tanks), with the bottom containing the heavy parts - aerospike radial nozzle and heatshield.
A rough design and flight profile can be seen here:
[http://flab.eng.isas.ac.jp/member/ito/w … h/NASA.jpg]radial aerospike SSTO flight profile
The crew/cargo module would be mounted on top, and could easily be detached (w/ parachute) in case of an emergency.

[http://www.engineeringatboeing.com/arti … design.jsp]Boeing/Rocketdyne page about nozzle design, including radial aerospike

Lars_J · Human missions

What oxygen extraction componet? You mean the intake? Got that taken care of already I think...

You don't think that the intake needs to process or purify the air at all? Or that the intake adds more mass and affects body design significantly? (see below)

but Nasa has a solution, with a turbine jet engine that does double-duty as a rocket, so that cuts out an entire set of engines.

And this double-duty engine will of course work with optimum efficiency below and and above scramjet ignition speeds? And don't forget the fact that the turbines will be dead weight once scramjet speeds are reached. There is no such thing as "free lunch".

With good aerodynamics, getting to mid-supersonic speeds shouldn't be hard.

There is quite difference between good aerodynamics and "passable" aerodynamics - all a glider needs to be able to land. (Or "flying brick" aerodynamics like the Shuttle)
All designs I have seen for scramjet craft require some pretty unique aerodynamic shapes in order to A.) be capable of flying at scramjet speeds and B.) efficiently funnel the air to the intakes. Designing an aircraft to behave well from Mach 0 to 20 (?) is practically impossible - and even moreso for a launch vehicle which needs to have a good size volume/mass ratio.

Although the plane would have to spend alot of time in the atmosphere, this is where the most energy is needed to get into space, punching a hole through both the atmosphere and Earth's gravity, but this is also where the Scramjet is most efficent and beats out conventional rockets silly. Feature, not a bug.

First of all, getting out of the atmosphere is not the hard part. (And there is no atmospheric "barrier" to punch through) Acellerating to orbital speed is the hard part. Altitude does not matter - speed matters. A hypothetical rocket launched from an extreme high altitude baloon would still have to reach the same speed. What matters is that accelerating though an atmophere requires more effort than acceleration through a virtual vacuum. Plain rockets spend most of their acceleration outside of the atmpshere. (The Shuttle reaches Max-Q, maximum air resistance at ~60 seconds, after that it falls off rapidly) On the other hand, a scramjet design needs to spend the vast majority of its time in the atmosphere - but the air resitance means that it has to pick up a whole lot more oxygen than a rocket would carry. So the atmospheric flight of the scamjet is indeed a feature of the design, but compared to a rocket - it is a bug.

Keep in mind that ceramics are nearing the temperature tollerance needed for Mach 25 flight, and that you need far less thrust than a rocket because you are using atmospheric lift instead of brute force.

Wrong, wrong, wrong. The raw amount of thrust required to reach orbit by a scramjet plane would be greater than that of a rocket. A universal principle in aerodynamics is that more lift means more drag. More lift means more drag, less lift meas less drag. The fact that any lift is generated implies that drag exists, and therefore you need an engine to overcome that drag. That is how aircraft stay flying. What makes an aircraft reach altitude and stay flying? Lift? Yes, but to get any lift you needed to spend energy in the first place to reach a certain speed, and to maintain that lift you need to spend enough energy to overcome drag.

And again, the energy you spend in getting and maintaing lift only gives you altitude. But it is irrelevant for the purposes of reach orbit - speed. It doesn't matter if you start at 0km or 100km altitude - you still need to reach orbital velocity. And that lift you have has been causing drag, causing you to spend more energy overall to reach the same speed. The only reason a scramjet launch vehicle would stay in the atmosphere is to scoop up fuel - lift is irrelevant, and actually only increases the inefficiency. That is the whole problem with a scramjet - how to extract enough out of the atompshere to overcome the basic problem of being in an atmopshere.

Lars_J · Human missions

Scramjet is a dead-end technology for space access - and most certainly for an SSTO. I predict there will be no scramjet based launch vehicle for the next 50 years. Why?

- Scramjet propulsion only removes the need for an oxidizer (liquid oxygen) tank - BUT the majority of the volume/mass is taken up by the remaining fuel, so the savings are slight.
- An oxygen extraction part of a scramjet propulsion system would take up volume and mass, reducing the savings even further.
- Also note that scramjets only light up at several times the speed of sound. This forces a scramjet vehicle to use either A.) onboard fuel with rockets or B.) jet engines to accelerate to scramjet speed. Either option adds even more mass.
- To "top it off", an oxidizer tank is still not eliminated, since one still needs oxidizer once leaving the atmosphere. It's like not eating your cake and yet still not having any left.
- The scramjet propulsion trajectory is very inefficient - in order to use the scramjet system, the spaceplane neds to stay in the atmosphere for as long as possible, while building up orbital speed. This creates significant heating issues, and wastes a lot more fuel trying to work against atmospheric friction. On the other hand, a vertical ballistic trajectory of a regular rocket is the most efficient one, and it also gets out of the atmosphere quickly.

In conclusion - It is both simpler and more efficient for a launch vehicle to bring all the oxygen in its tank, instead of trying to scoop it up from the atmosphere. (And liquid oxygen is dirt cheap as well)

Scramjet may have its uses in military vehicles who need to go really fast inside the atmosphere for some bizarre reason. And even then, a "regular" transportation vehicle in a sub-orbital trajectory would still get you there faster.

All the $$$ being spent on scramjet research is $$$ that should be put to better use.

Lars_J · Human missions

What "spaceplane" are you referring to? The X-33?

That project was thankfully put out of its misery years ago - It was heading nowhere fast (specs getting worse and worse, deadlines pushed back further and further). It would appear that Lockheed Martin and NASA were willing partners in this "vaporware" project which in retrospect sunk billions of $$$ for nothing in return.

Lars_J · Unmanned probes

Here's one of the pictures of the eclipse:

[http://marsrovers.jpl.nasa.gov/gallery/ … 66R8M1.JPG]http://marsrovers.jpl.nasa.gov/gallery....8M1.JPG (pretty small)

Lars_J · Unmanned probes

Given that both Opportunity and Spirit are located very close to the Martian equator, I wouldn't think that the seasonal change would be very noticable at all.

I suspect solar panel degradation (dust) and battery degradation (constant recharging + thermal issues) will be what seals the rovers fate rather than the Sun's position.

Lars_J · Unmanned probes

The images from todays press conference have now been posted at:
[http://marsrovers.jpl.nasa.gov/gallery/ … 0305a.html]http://marsrovers.jpl.nasa.gov/gallery....5a.html

This is the coolest one - a false color south panarama view outside the crater, showing Endurance crater, the heatshield (and impact spot), all the bouncemarks of the airbags, and the only rock on the plains - which Opportunity managed to strike as it bounced. :laugh: [http://marsrovers.jpl.nasa.gov/gallery/ … R1_br2.jpg]http://marsrovers.jpl.nasa.gov/gallery....br2.jpg

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#26 Re: Space Policy » Glenn Criticizes Bush Space Plan - says direct-to-Mars is the way to go » 2004-03-17 12:33:14

#27 Re: Space Policy » Glenn Criticizes Bush Space Plan - says direct-to-Mars is the way to go » 2004-03-17 12:31:56

#28 Re: Space Policy » Glenn Criticizes Bush Space Plan - says direct-to-Mars is the way to go » 2004-03-17 12:11:39

#29 Re: Unmanned probes » Spirit & Opportunity4 - continue here » 2004-03-16 19:27:11

#30 Re: Unmanned probes » Spirit & Opportunity4 - continue here » 2004-03-15 15:47:55

#31 Re: Unmanned probes » Spirit & Opportunity4 - continue here » 2004-03-15 15:41:31

#32 Re: Space Policy » Glenn Criticizes Bush Space Plan - says direct-to-Mars is the way to go » 2004-03-15 13:40:46

#33 Re: Interplanetary transportation » Theoretically how fast will an object continue to » 2004-03-15 10:30:31

#34 Re: Human missions » Hubble mistake - Action needed » 2004-03-13 11:44:39

#35 Re: Space Policy » Glenn Criticizes Bush Space Plan - says direct-to-Mars is the way to go » 2004-03-12 22:02:24

#36 Re: Interplanetary transportation » Earth to LEO - discuss » 2004-03-12 17:25:19

#37 Re: Human missions » Hubble mistake - Action needed » 2004-03-12 12:58:02

#38 Re: Human missions » Europe goes to the moon and Mars! - Human space flight.... » 2004-03-11 13:00:30

#39 Re: Planetary transportation » RTGs - And Rovers » 2004-03-11 08:54:47

#40 Re: Interplanetary transportation » GRAVITY QUESTION? » 2004-03-09 19:58:29

#41 Re: Human missions » Hypothetical Lunar Mission Profiles - How might we return to the moon? » 2004-03-09 15:49:13

#42 Re: Human missions » Space Plane - Space Plane » 2004-03-09 13:39:55

#43 Re: Human missions » Space Plane - Space Plane » 2004-03-09 12:46:20

#44 Re: Human missions » Space Plane - Space Plane » 2004-03-09 10:13:40

#45 Re: Human missions » Space Plane - Space Plane » 2004-03-06 23:50:45

#46 Re: Human missions » Space Plane - Space Plane » 2004-03-06 21:45:44

#47 Re: Human missions » Space Plane - Space Plane » 2004-03-06 16:06:12

#48 Re: Unmanned probes » Spirit & Opportunity4 - continue here » 2004-03-05 22:00:52

#49 Re: Unmanned probes » Spirit & Opportunity survival - Could they srvive to Mars winter? » 2004-03-05 14:34:39

#50 Re: Unmanned probes » Spirit & Opportunity4 - continue here » 2004-03-05 14:16:02

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