Novel Rocket Launch Rail Concept 5

[dgerl]

I am a mechanical engineer working on the system design of a material transport system for space construction "ie. an Earth based mass driver".

Instead of the electromagnetic launcher traditionally proposed I have proposed a rocket sled that traverses the launch rail and collects an oxidizer gas dispensed from the launch rail. The oxidizer gas is combusted in a solid fuel grain similiar to a hybrid rocket motor, onboard the rocket sled. Therefore the performance of the rocket sled is improved because the oxidizer gas and storage structure is not accelerated with the rocket sled.

A schematic of the proposed system can be viewed at

http://www.massdriver.com/distributed_oxidizer_launch_rail.htm

.

My question is what is the typical pressure oxidizer gas is stored on a hybrid rocket?

Also I am not a aerospace expert, more of an abstract mechanical thinker so is there any significant technical problems with this launch rail concept?

I know rail friction is a real problem which might be solved by some form of MagLev technology, like Inductrak.

This space system design is an attempt at a collaborative (or Open Source) design of a hardware product. All information related to the project is public knowledge.

 

[IRstuff]

Yes, there is a problem.

You cannot use gaseous oxidizer, since you cannot get enough mass flux to combust with more than a tiny fraction of your fuel. Do the math yourself:

http://science.ksc.nasa.gov/shuttle/technology/images/et_1.jpg

The Space Shuttle's liquid fuel tank carries 6 times more oxygen than hydrogen, by weight.

TTFN

 

[GregLocock]

Nice catch. The OP might like to consider the effect of shock waves forming in his water trough, whether it is full of liquid or gaseous oxidiser.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

That's only one of the problems.

Another related one is the fact that the rocket, plowing its way through the oxidizer, constantly loses momentum from the collision with the ozidizer.

Since drag force goes as v², the oxidizer will increasingly reduce the momentum of the rocket as it tries to increase its speed.

Yet another one is that as the rocket increases speed, the aerodynamic drag will cause heating that's localized along the pathway that the oxizider takes to get to the combustion chamber, which is quite likely to cause preignition of the fuel and likely a most spectacular and satisfying explosion.

TTFN

 

[btrueblood]

The idea is pretty similar to the ram accelerator concept, first "invented" by Abe Hertzberg and crew (hey, that includes me!) at the U of Washington, ca 1986. Several papers have been written about it (try AIAA). The concept of the ram accelerator was a ramjet flying through a tube, which is pre-filled with a fuel/oxidizer mix. The concept was proven on a ~1.5-inch bore demonstrator we built in the basement of a research building there. As of a couple of years ago, the upper "speed limit" reached by the demonstrator was about 8 or 9 km/sec (from personal conversation with a couple of the PI's).

IRstuff, your argument regarding momentum is somewhat mis-stated - a rocket must carry all of the oxidizer "on board", and thus must also accelerate that mass up to the rocket's flight velocity prior to burning, just like an air-breathing engine. The momentum penalty for a rocket is actually a bit more severe, since the oxidizer is accelerated to the full vehicle velocity, whereas an "air-breather" may only accelerate its oxidizer to 80-90% of the vehicle velocity. You are correct, however, that fluid dynamic "friction" (which includes shock losses), and gas/liquid compression prior to injection into the combustion chamber, create loss mechanisms that are of much greater magnitude for a ramjet than for a rocket. The neat thing about the ram accelerator was that by pre-mixing the "flight atmosphere", its properties (from molecular composition to pressure/temperature) could be tailored to minimize, or at least control, loss mechanisms.

 

[GregLocock]

I did a quick simulation of this before answering the first time, the key performance gain is that in the initial stages you only have to accelerate the little bit of fuel/oxidiser you are actually burning, so the initial acceleration is significantly better.

As you speed increase the performance of the two assymptotically converges.

However, accelerating the mix to the vehicle's speed is unlikely to involve no extra mass and no other losses, so in reality the performance gain is likely to be (politely) small.

btrueblood, your ramjet sounds like fun.

For the OP, why aren't you flying to altitude and launching from there, at a pinwheel?


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

Greg,

Hey! Your quickie simulation matches a lot of other airbreathing stage models/sims for launch vehicles -- Good job! I.e. airbreathing works as a booster stage, for low velocities and low altitudes. Doesn't work very well in space, due to low concentration of air

We had a senior design class (for which I TA'd) look at using ram accelerator as a "1st stage" to send payloads to orbit (payloads would require a circularization kick motor). Everything was do-able, except the calculated peak heating at the nose cone was beyond anything ever tried before. But the RA was fun, I'll admit, even if every time I got to fire it we had a cold shot

I think the wave of the near future will be big, dumb, recoverable hybrid boosters. Fuel = old tires.

 

[dgerl]

Thanks for the quick responses! I see there some significant problems. The following is the problems as I understand them.

1. I cannot pump enough gaseous oxidizer out of the rail to supply the rocket motor.
Comment: I don't really understand, with proper sizing of the injection apparatus why I can supply enough oxidizer to satisfy combustion. I really don't understand the math here I need to do.

2. Shock effects in the oxidizer gas may cause an explosion.
Comment: Yes these are nasty things to deal with for a mech like me. Design of the collector might not be feasible.

3. Oxidizer effects aerodynamic drag.
Comment: This is a good one, I did not consider this. Very precise control of dispensing the gas from the rail is need to solve this. ie. the oxidizer is only injected when the sled is at that point in the track - maybe not feasible.

4. Aerodynamic heating of the oxidizer as interacts with rocket sled collector system may cause an explosion.

5. Performance is slightly better than a regualar rocket.
Comment: I did a little simulation (spread sheet) on this concept, I did not include the momentum change of the oxidizer. It was just based on the forces. The results looked to good to be true. Thanks for the work Greg! Good point. I'll try refining my own simulation.

The ram jet in a tube and launching at altitude to a rotating tether are all really good ideas for launching a single payload to orbit, the problem is I am working on a concept to rapidly launch a series of payloads such that the trailing payload takes advantage of the low pressure zone of the leading payload (slip-stream effect).

I need a launch mechanism capable of rapidly launching a series of payloads at high velocity. Yet another dead end. Designing is fun. Thanks.

 

[nhughes1]

Several of the concepts discussed herein are parts of a highly efficient launch system. I was part of a conceptual design study back in the mid 80's - here's what we came up with:

1) A winged, ramjet powered first stage, with existing or to be developed 2nd/3rd stage vehicles on piggyback
2) Accelerated to ramjet ignition speed on a sled track; the end of the track would curve upward to provide takeoff attitude
3) Combined vehicle climbs to top of ramjet envelope in speed and altitude- roughly mach 5, 180,000'; upper stage separates, continues to orbit, first stage flies back to launch site.

Some of the advantages:
* Accelerating impetus to around Mach 1 provided by ground equipment. The Alamagordo sled track system, in essence, uses whatever spare rocket motors are available, straps them on the back of the sled and has achieved Mach 1.4+.
* Air breathing, wing supported flight to as high an altitude as possible, no need to carry reaction mass to get to 180,000', Mach 5+ or to have propulsion system with thrust > vehicle weight. Even with a conservative L/D of 10, the required thrust is 10% of that required for verticle takeoff.
* Using mid-air refueling, take off could be accomplished at minimum liftoff mass; also the vehicle could self transport to the latititude of the intended payload orbit, thereby increasing the efficiency/mass to orbit of the system.
* Horizontal processing - no enormously tall Vehicle Assembly Building, no cranes hundreds of feet tall, just drive the first stage into a pit with a gantry over it and lower the upper stages onto it.
* Everything is existing technology!!!!!

In the years following this study, NASA chased the absurd single-stage-to-orbit concept, wasting mulitple billions of dollars until it was "discovered" that several "technology break throughs" were required to make it work (perhaps a magic wand?). Recently, in the May 2 edition of AW&ST, I saw an article about an Air Force initiative that included a winged, rocket propelled flyback first stage and upper stages piggyback. Maybe someday we'll get an efficient launch system.

 

[dgerl]

Thanks for the concept. It would seem all the technology required is very near term.

Is there a report for this study that is publicly available?

I would not mind reading the details.

 

[bobcvn65]

My question is this:

For systems with varying mass:
Conservation of linear momentum (Pi = Pf)

Here M is mass U is exhaust velocity relative to our frame:
Mv = -dM U + (M + dM)(v+dv)

where the first term on the right is the linear momentum of the exhaust during dt and the second term is the linear momentum of the rocket at the end of dt.

Now u is the speed of the exhaust relative to the rocket.

So,
u = (v+dv) - U

=> U = v+dv - u

So -dM u = M dv
and
-(dM/dt) u = M (dv/dt)

Now we know that the left term is Thrust,

and that integrating this equation eventually leads to

Vf-Vi = u ln (Mi/Mf)

How would you factor in the gain in mass?
Or is that even necessary since the amount of oxidizer being gained and then lost would cancel each other out and be constant?
But then it wouldn't really be constant since the ratio of the masses of the actual rocket and the oxidizer would change?

Would I be lying if I said that this rocket would go nowhere?
You need more oxidizer than fuel, so I'm thinking that IRstuff is right and this would be a particularly satisfying explosion even before we saw the rocket move.

Also they're right about the drag:

D = (1/2) C p A v^2

as your p and v increased, your D will increase exponentially.

 

[IRstuff]

What increase in mass? Most rockets carry oxidizer with them.

TTFN