Ares or DIRECT?

[Urwumpe]

I recall seeing somewhere (can't remember where... ) that a vertical-landing vehicle required a deltavee of 500m/s for landing.

Sounds reasonable. If you start the engines after reentry with a low terminal velocity at almost 90° descend angle, you can approximate the delta V as:

Speed + 9.81 * burn time.

The longer you need for landing (decelleration, hover, etc) the more fuel you need.

 

[Eagle]

Each vehicle has its' advantages. I prefer the variety of abort modes offered by a winged craft - plus, add a jet engine, and you have something capable of ferrying itself from landing-site to launch-site. :speakcool: There isn't a lot of mass-penalty, if you let the wings double as fuel tanks.

I recall seeing somewhere (can't remember where... ) that a vertical-landing vehicle required a deltavee of 500m/s for landing.

I suppose the required delta V is based on 2 things. The vehicle's terminal velocity needs to get nulled and then you want extra hover time for a slow descent. 200 kph = 56 m/s, 400 kph is 112 m/s. I would hope terminal velocity would be below 400 kph. Then about 9.8 m/s for 1 second of hover.

So 500 m/s could get you from a terminal velocity of 400 kph (248 mph) to hover and about 38 seconds to get on the ground. Depending upon how high you 'flare' will dictate how much hover time you really need. (note, I rounded to reduce hover time at each step)

Its quite doable, may be a little hair raising on landing, but equipment can be made reliable and redundant enough.

 

[Brad]

Call me old school....I would rather see a modern Saturn V / IB. Rather than sending two launch vehicles - once should be enough.

 

[RocketMan_Len]

We had a discussion once before about the advantages and disadvantages of VTOL vs. parachutes vs. wings.

And it's likely a discussion that will never end - much like the choice of fuels discussion. Everybody has their favourite, and each has advantages unique to its' chemistry... while having an equal number of disadvantages. The only real test is whether or not any particular combination works for the application in mind.

I'm not so sure, myself, capsule parachutes don't fail very often, but then again neither do most modern air-startable rocket engines.

And even that risk can be lowered, by using hypergolic propellants. No need for sparks, or triggers - as long as you have flow, you have fire.

Parachutes and rigging mass alot more than you might think,

Don't I know it...? My rockets have nearly half their internal volume occupied by parachutes and rigging! They also make up about 10% of the total liftoff weight, and shift the CG forward significantly.

If your crew is small, ejection seats might be a viable option for bail-out at low altitude.

... But maybe not, if you're carrying a passenger-can .

 

[cjp]

We had a discussion once before about the advantages and disadvantages of VTOL vs. parachutes vs. wings. I remember sputnik saying that carrying fuel for a landing is no greater mass penalty than carrying chutes and rigging, nor is it any less reliable.

The interesting thing in the context of this thread is that wings and parachutes are rarely an option on Mars. Even those unmanned landers that used 'airbags', used retro-rockets to slow down before impact. So, just like any lunar lander, any Mars lander will land using retro-rockets. I don't know what the Ares and DIRECT Mars landers look like, but they probably combine this with a heat shield for the atmospheric entry. Conceptually close to the DC-X I'd say.

Now, if the DC-X concept actually makes sense on earth, and is the most sensible thing to do on Mars, the next interesting question is whether a spacecraft can be made that can do the trick on both planets. Who knows the reduction in development costs, and maybe even the reduced weight to be launched, maybe compensate for the inefficiency of a spacecraft that isn't optimized for one particular planet.

Does anyone here have an overview of what design parameters are important here? Maybe heat shield parameters, or engine thrust, or the shape of the ship?

And yes, having some emergency solution when you have engine failure while landing would be nice. Though I don't know what options you have on Mars.

 

[Eagle]

In addition to the DC-Y, there is the Venture Star a horizontal landing SSTO. Apply a few more lessons learned to the Venture Star (make the heat shield less brittle than the Shuttle) and it could live up to its original expectations.

(On a side note of NASA frustration: as a child, NASA alienated me from its concept by not adding a cockpit or windows and saying it would be unmanned or computer controlled only.)

 

[T.Neo]

In addition to the DC-Y, there is the Venture Star a horizontal landing SSTO. Apply a few more lessons learned to the Venture Star (make the heat shield less brittle than the Shuttle) and it could live up to its original expectations.

SSTO is notoriosly difficult to achieve.

A non-reusable SSTO could be quite feasible with modern technology. The only reason one has not been built is because they have high tolerances and payloads often grow in mass during development. The major problems with resusable SSTO come in having to lift the heat shield into orbit.

The VentureStar is in my opinion the ugliest and worst spacecraft ever thought of by humans. In the end, I think is was undone in the end by the composite hydrogen tank failing.

My opinion is that with current tech, SSTO is too difficult to achieve, not impossible, but just not worth it.

The next step, I believe, will be to take SSTO concepts like the DC-Y and put an amount of fuel in cheap, mass produced drop tanks, in a sort of fusion between BDB and SSTO.

(On a side note of NASA frustration: as a child, NASA alienated me from its concept by not adding a cockpit or windows and saying it would be unmanned or computer controlled only.)

Same here. Maybe that is why I will have an unending hate for Venture Star, or maybe because the thing looked fat...

 

[RocketMan_Len]

SSTO is notoriosly difficult to achieve.

... Which is why I currently advocate a TSTO system, using a flyback booster.

The VentureStar is in my opinion the ugliest and worst spacecraft ever thought of by humans. In the end, I think is was undone in the end by the composite hydrogen tank failing.

Who cares what it looks like...? If it had worked, that would have been the important part. The only problem was that they tried to do too many new things at once... and the hydrogen-tank problem was just the 'straw that broke the camels back'.

Same here. Maybe that is why I will have an unending hate for Venture Star, or maybe because the thing looked fat...

Nothing wrong with short-and-stubby, as long as it gets the job done. :rofl:

 

[cjp]

The next step, I believe, will be to take SSTO concepts like the DC-Y and put an amount of fuel in cheap, mass produced drop tanks, in a sort of fusion between BDB and SSTO.

You know, I wonder, boosters sound like a good idea, and drop tanks sound like a good idea, bit why then is the Space Shuttle a bad idea?

Of course it has something to do with its size. Usually it only needs to bring back the crew and itself, and that could have been done with a much smaller craft, with smaller wings & heat shield, if it hadn't had such a huge cargo bay.

But another 'minor' issue is the foam coming off the drop tank. Is there a way to solve this problem in the drop tank concept? Maybe by using a non-cryogenic fuel? I mean, you can't place a drop tank below the (reusable) engines, can you? If the engines were non-reusable, then engines + drop tank together would simply be called a rocket stage.

 

[C3PO]

You know, I wonder, boosters sound like a good idea, and drop tanks sound like a good idea, bit why then is the Space Shuttle a bad idea?quote]

Because of the parallel "payload".

 

[Urwumpe]

Because of the parallel "payload".

Well, not even because of the parallel payload. More because of the parallel heat shield. The heatshield and the expensive engines are the two reasons why the Shuttle does not keep it's promises.

That's why I like the early Lockheed CEV It is no true spaceplane, it is a lifting body and carried no payload back home like the Shuttle - but it is well designed as LEO ferry or for small autonomous science mission. It could even do space station building, if docked to a station module with own propulsion.

 

[T.Neo]

@ RocketMan Len

... Which is why I currently advocate a TSTO system, using a flyback booster.

The only problem with that is if you lift off vertically, the wings are just a weight penalty. Better take of horizontally, so you can use the wings to your advantage.

@ cjp

You know, I wonder, boosters sound like a good idea, and drop tanks sound like a good idea, bit why then is the Space Shuttle a bad idea?

Because it uses both . There are also numerous other issues with the space shuttle. If you built a DC-Y with a crumbly heatshield, ready to explode engines and a huge maintainance bill, it would not be much of a success either.

But another 'minor' issue is the foam coming off the drop tank. Is there a way to solve this problem in the drop tank concept? Maybe by using a non-cryogenic fuel? I mean, you can't place a drop tank below the (reusable) engines, can you? If the engines were non-reusable, then engines + drop tank together would simply be called a rocket stage.

You could place the tanks in a position where falling foam does not affect critical parts of the heatshield. Or you could put the foam on the inside, But I guess that would increase the amount of metal needed to make the tank. The whole idea is to keep the drop tanks as simple as possible.

 

[RocketMan_Len]

The only problem with that is if you lift off vertically, the wings are just a weight penalty. Better take of horizontally, so you can use the wings to your advantage.

*sigh* I've said it before, and I'll re-iterate...

If you place fuel tanks within your wings, the weight penalty is minimized. Also, bear in mind that liftoff isn't the *only* flight regime that the vehicle will experience - wings provide a larger surface-area during entry... the higher the coefficient of drag, the higher in the atmosphere entry begins. Thinner air makes for lower heating.

They're not *inherently* bad... only the way they've been applied.

 

[T.Neo]

Another advantage of horizontal takeoff is the abort options. Jettison the tanks, dump the fuel, land back on the runway...

 

[Eagle]

Well Virgin Galactic could potentially uprate their air launch system into a 2 or 3 stage to orbit system. Air launching may save only a few hundred m/s, but that translates to about a 30% reduction of mass compared to ground launching. So Spaceship 3, or maybe 4 may end up with orbital capability. At the very least, White Knight is reusable, even if everything else isnt. Scrams on an airborne mothership could save even more delta v.

Blue Origins is still working on their VTVL craft (currently RP-1,H2O2 fueled). Their initial goal is sub orbital tourism. I would not be surprised to see an orbital SSTO from them once they get several tourist missions complete.

These two technologies are in direct competition. So success by one will affect the other.

 

[tblaxland]

*sigh* I've said it before, and I'll re-iterate...

If you place fuel tanks within your wings, the weight penalty is minimized. Also, bear in mind that liftoff isn't the *only* flight regime that the vehicle will experience - wings provide a larger surface-area during entry... the higher the coefficient of drag, the higher in the atmosphere entry begins. Thinner air makes for lower heating.

They're not *inherently* bad... only the way they've been applied.

Putting fuel in the wings comes with it own weight penalties. A wing tank has a much higher surface area to volume ratio than a spherical or cylindrical tank. Also, additional reinforcing is required because the tank shape is not as efficient at distributing the structural loads of the pressure in the tank. These two factors increase the dry mass/wet mass ratio and thereby reduce your payload fraction.

 

[Urwumpe]

Putting fuel in the wings comes with it own weight penalties. A wing tank has a much higher surface area to volume ratio than a spherical or cylindrical tank. Also, additional reinforcing is required because the tank shape is not as efficient at distributing the structural loads of the pressure in the tank. These two factors increase the dry mass/wet mass ratio and thereby reduce your payload fraction.

Additionally, the wings are also the parts with the worst ratio of reentry energy and mass. You don't want to store stuff inside your wings, which boils easily.

 

[eveningsky339]

You know, I wonder, boosters sound like a good idea, and drop tanks sound like a good idea, bit why then is the Space Shuttle a bad idea?

For one thing, it is obscenely expensive to launch, and it's too complex and unsafe for use in the civilian market, which is where space travel needs to head.

But another 'minor' issue is the foam coming off the drop tank. Is there a way to solve this problem in the drop tank concept? Maybe by using a non-cryogenic fuel? I mean, you can't place a drop tank below the (reusable) engines, can you? If the engines were non-reusable, then engines + drop tank together would simply be called a rocket stage.

Most non-cryogenic fuels are fairly toxic and expensive, with the exception of Kerosene. However, I'm not an expert...

As for a launcher, NASA needs the Saturn V back.

 

[tblaxland]

For one thing, it is obscenely expensive to launch, and it's too complex and unsafe for use in the civilian market, which is where space travel needs to head.


Most non-cryogenic fuels are fairly toxic and expensive, with the exception of Kerosene. However, I'm not an expert...

As for a launcher, NASA needs the Saturn V back.

The Saturn V was also obscenely expensive. In fact, if the Space Shuttle Program was terminated after 13 missions like the Saturn V was, it would have cost about $3.5b* per launch[1]. The Saturn V cost about $1.8b* per launch, just for the launch vehicle with no spacecraft or mission support[2]. With the CSM costing about $2.1b* per launch[2], the space shuttle system is actually a bit cheaper just for getting you into space.

[1] http://sciencepolicy.colorado.edu/admin/publication_files/resource-100-1993.01.pdf

[2] http://history.nasa.gov/SP-4029/Apollo_18-16_Apollo_Program_Budget_Appropriations.htm
I took approx half the Mission Support and Flight Operations Costs for the CSM, since lunar missions would be more expensive to support than LEO missions.

* All dollars adjusted for inflation to 1992 dollars.

 

[Eagle]

Most non-cryogenic fuels are fairly toxic and expensive, with the exception of Kerosene. However, I'm not an expert...

One of my favorite things to say:

Fuel is the cheapest part of the spacecraft. Most of the money goes into the labor of making the rocket and servicing the rocket to get it up there.

Lets do a little math. So the Falcon 1e has a liftoff mass of 46,760 kg. For convince sake lets say all of that mass is fuel. And instead of LOX and RP-1 we're using some superdense hypergolic stuff that costs $8 a kg. So we spend $374,080 on fuel. The quote SpaceX gives to purchase a Falcon 1e is 9.1 Million dollars. Lets say that SpaceX is a greedy capitialist and it only costs them $5 Million for a total launch.

$374,080/$5,000,000 = 7.48% of the total cost is fuel. RP-1 and LOX really only cost a little more than $1 per kg. So in the real world, approximately one percent of the total cost is fuel. And for the customer who pays 9.1 million: 0.6% of what they pay buys fuel. Fuel that costs 800% more only bumps the price up to 9.5 million.