Catapult start?

[Broco]

I don't know if anyone ever came up with the idea but I just had the weirdest idea:

Why aren't rockets launch assisted by a hydrolic catapult? We all know that most of the fuel is consumed for just leaving the atmosphere, I mean the Saturn V burned through ~13 tons of fuel per second and clearing the tower alone takes a good amount of time. So why aren't rockets assisted in getting some initial velocity? I mean sure, it would be a huge construction but nothing too crazy.

So If you manage to boost up the rocket to maybe 50m/s with a 100m high reusable catapult, wouldn't that decrease rocket size or increase the payload?

 

I mean something like this (behold my epic Paint skills):

[mrfox]

https://en.wikipedia.org/wiki/Mass_driver

https://en.wikipedia.org/wiki/Space_gun

Edited November 18, 2017 by mrfox

[Nightside]

 

+1 for your paint skills.

I’ve had this thought too, and even briefly tried to build a launcher out of IR parts in game. I quickly gave up on that though, ha. 

Although 100m/s of DeltaV might be achievable, it probably won’t be that useful. A rocket needs almost 100x that to get into orbit. 

Also, the high acceleration over a very short distance would require a much stronger rocket, and limit your payloads.

it might be helpful in a very small sounding rocket, or for launching from an airless world though. 

 

Edited November 18, 2017 by Nightside

[GoSlash27]

The ballistic missile subs use pneumatic pressure to catapult their missiles  out of the tubes, and I believe the MX missiles did the same. This was to keep the missile from damaging the "launchpad", though.

 If it takes 7 km/sec to get to orbit and you 2-stage it, then your first stage would need 3,500 m/sec DV. Assuming an Isp of 350s, that's a wet to dry ratio of 2.77:1. 63.9% of your first stage's mass is fuel. If you catapult the missile to save 50 m/sec DV and repeat, then 63.4% of your launch stage would be fuel. You might save 0.8% of the total mass of the launch vehicle; not really worth the effort.

HTHs,
-Slashy

Edited November 18, 2017 by GoSlash27

[Broco]

2 minutes ago, GoSlash27 said:

The ballistic missile subs use pneumatic pressure to catapult their missiles  out of the tubes, and I believe the MX missiles did the same. This was to keep the missile from damaging the launchpad, though.

 If it takes 7 km/sec to get to orbit and you 2-stage it, then your first stage would need 3,500 m/sec DV. Assuming an Isp of 350s, that's a wet to dry ratio of 2.77:1. 63.9% of your first stage's mass is fuel. If you catapult the missile to save 50 m/sec DV and repeat, then 63.4% of your launch stage would be fuel. You might save 0.8% of the total mass of the launch vehicle.

HTHs,
-Slashy

Yes, but isn't the de facto usage of fuel higher on the launchpad? Because a rocket at the launchpad has a way lower acceleration than a rocket that used up 50% of its fuel (because it has to move a higher mass). So achieving these initial 0-50m/s use up more fuel than 1000-1050m/s.

[GoSlash27]

Broco,

Not really. I solved it using Tsiolkovsky's rocket equation, which takes that into account.

Best,

-Slashy

[DDE]

24 minutes ago, GoSlash27 said:

The ballistic missile subs use pneumatic pressure to catapult their missiles  out of the tubes, and I believe the MX missiles did the same. This was to keep the missile from damaging the "launchpad", though.

I'm gonna top that with a video.

See the smoke coming out the lower section of that launcher?

 

[Bill Phil]

Not so much a catapult, but a rocket sled or a maglev track would help. The main issue would be the potentail heating and the dynamic pressure, beyond the setup cost.

The rocket equation is exponential. If you wish to double your delta v by a specific factor assuming a constant exhaust velocity, you must raise your mass ratio to that factor. Double your delta v requirement, square your mass ratio. Half your delta v requirement, square root your mass ratio. 

But even so, if you can get a few hundred m/s... you could increase LEO payload significantly. Assuming that structural mass doesn't increase too much.

[DDE]

3 minutes ago, Bill Phil said:

Assuming that structural mass doesn't increase too much.

That's the difficult bit. The structural enhancements needed to navalize fighter jets do not bode well.

[sevenperforce]

Bingo. Rockets are flying metal balloons; trying to up their structural integrity to allow for a high-gee catapult wouldn't work so well.

[Bill Phil]

3 minutes ago, sevenperforce said:

Bingo. Rockets are flying metal balloons; trying to up their structural integrity to allow for a high-gee catapult wouldn't work so well.

Rockets are stronger than you'd think. Many can take 3 or 4 gees. But even then... you could just use lower gee systems. The real issue is dynamic pressure. To get any useful increase, you'd basically get max Q just above the ground. The rocket would have a catastrophic failure. This is why a number of maglev train launch assist proposals are evacuated tubes that go up the side of a mountain. Starting faster and higher.

[GoSlash27]

Aside from the structural/ mass issues, I would add that "a few hundred m/s" is basically Mach 1, so not a minor undertaking for the LV or the catapult. Plus.... How long would it take for the infrastructure to pay for itself?

Best,

-Slashy

[PB666]

1 hour ago, DDE said:

I'm gonna top that with a video.

See the smoke coming out the lower section of that launcher?

 

Wasn't that from a movie with chevy chase in it?   

I wonder why the vehicle has an alarm bell going off, within about 30 minutes everything within 3000 miles would be dead anyway.

 

[Broco]

2 minutes ago, PB666 said:

Wasn't that from a movie with chevy chase in it?   

I wonder why the vehicle has an alarm bell going off, within about 30 minutes everything within 3000 miles would be dead anyway.

 

Why's that? Because of nuclear retaliation? The same launcher can also launch non-nuclear payload.

Also it's pretty pointless to launch nuclear missiles at a mobile missile platform that already used it's payload when you can just blow up Moscow.

Did I really just write that? Cold war logic really got to me, I shouldn't watch War Games again.

[PB666]

So lets deal with the principle problem, and take the Falcon 9 as an example.

Believe or not the falcon F9 does not take off at maximum g-force, in fact it lumbers off the launch pad with a TWR between 1.25 and 1.3.

Why would they do this, doesn't it waste fuel.

Technically its very dumb, for instance if you took off from the top of mount Everest you probably would want a TWR over 1.5 the problem with launching at Cape Canaveral is that its at sea level, so that about 80 seconds into the flight you hit MaxQ (the highest dynamic face pressure that the craft can safely withstand). So here is the issue, suppose the craft has a tolerance of 2x MaxQ on the face pressure beyond 1.5 structural distortion is already occurring. At certain velocities if you increase velocity you could damage the payload.

There is no way to avoid Max Q, launching with a higher velocity only gets you there more quickly and at a lower altitude meaning more pressure, once you get past Max Q you have much more engine than you need because drag decreases and the fuel load decreases.

So whats the point of having more engine, or starting thrust.

Solutions to this problem-
  1. Make the nose of the craft more aerodynamic (Sears Haack shape). More aerodynamic noses increase side drag beyond Mach 1.
  2. Take off from a very high mountain, about the most you are going to get is a Andean mountain at 10,000 elevation.
  3. Use boosters that then release from the rocket just before Max Q so that you only take the engine you need over. (These boosters would be small an unlikely recyclable).

 

4 minutes ago, Broco said:

Why's that? Because of nuclear retaliation? The same launcher can also launch non-nuclear payload.

Also it's pretty pointless to launch nuclear missiles at a mobile missile platform that already used it's payload when you can just blow up Moscow.

Did I really just write that? Cold war logic really got to me, I shouldn't watch War Games again.

Unfortunately for your logic, the one who is retaliating doesn't know where your mobile launchers are and he is going after any stragglers. Second the Minute man missiles have a kill zone about 100 miles in diameter, they don't have to be close. Third you are in a country that started a doomsday war . . . your doomed, period. You might survive on the Krugerland Islands or someplace like that, not in Russia. I think that if I was the guy who launch an ICBM from the back of a 30 wheeler launcher I probably would no the location of the nearest cave and would steal russian jeep, put the remainder of my vodka in the gas tank, and head to that cave, picking up any cute Russian hitchhikers I might find on the way.

[Broco]

11 minutes ago, PB666 said:

Technically its very dumb, for instance if you took off from the top of mount Everest you probably would want a TWR over 1.5 the problem with launching at Cape Canaveral is that its at sea level, so that about 80 seconds into the flight you hit MaxQ (the highest dynamic face pressure that the craft can safely withstand). So here is the issue, suppose the craft has a tolerance of 2x MaxQ on the face pressure beyond 1.5 structural distortion is already occurring. At certain velocities if you increase velocity you could damage the payload.

There is no way to avoid Max Q, launching with a higher velocity only gets you there more quickly and at a lower altitude meaning more pressure, once you get past Max Q you have much more engine than you need because drag decreases and the fuel load decreases.

That actually makes a lot of sense to me, besides the structural problem with "pushing" a rocket from beneath. I withdraw my proposal

On the other hand, now I get why the mass driver would make a lot of sense of the surface of the moon. So thanks to everybody, I learned a bit more again today

 

And ye, you're right, basically everything in Russia is gonna be a hot place to live then. But seriously, I would just watch the show, that's fireworks you're not gonna see again (pun intended).

Edited November 18, 2017 by Broco

[Tullius]

Using the idea pictured in the first post to give the Saturn V a 2g push:

The Saturn V weighs nearly 3000 tons, which means we need more than 9000 tons of water. This is 9'000'000  9'000m³ of water, which equals a cube of water with a side length of more than 200 20 meters, i.e. enough to dwarf even a Saturn V with its 110 meters. still a huge cube, but not mindboggingly huge.

And I forgot, if you want to give the Saturn V 50m/s of "free speed", you need 62.5 meters of difference in height, i.e. half the Saturn Vs height.

So you have a giant water tank twice the size of the launch tower and a platform that moves half the height of the launch tower within 2.5 seconds. And all of this to save only 35tons of fuel (out of 2100tons for the first stage alone)?!

 

Edit: Calculation error: 1m^3 = 1ton

Edited November 19, 2017 by Tullius

[PB666]

You don't need mass driver per say, the circular velocity is around 2000 m/s (I forget exactly) but g = 1.66 m/s2. Consequently a rocket with TWR of 1 earth g need only rise 0.166 = sin theta. That requires a pitch of 9.5 cartesian degrees. All you need to do is place some teflon sliders and a pair of rails, start with pitch of zero and raise it to pitch 9.5 degrees as 'slide' of the rocket starts at zero thrust and pushes to full thrust. The space craft will hold the initial vertical velocity but its vertical velocity will increase and the pitch will need to decline. Just doing this you can eliminate almost all the waste of launching vertically (hoovering over the launch pad). 

Here are deduced acceleration for falcon F9 during launch. In the first figure TWR = (a apparent + 9.81)/9.81

[GoSlash27]

3 hours ago, DDE said:

I'm gonna top that with a video.

See the smoke coming out the lower section of that launcher?

 

A couple more for fun.

Peacekeeper "Cold launch".

SLBM "Gas generator" launches.

Best,
-Slashy

[kerbiloid]

Just to add moar pictures.

Spoiler

 

Here you can see how the powder charge throws out a liquid-fuel rocket and immediately gets jettisonned.

To lift or press the water in the OP scheme you need some energy.
To get this energy, you have to spend some fuel.
It's much easier to spend this fuel just inside a rocket, without adding a catapult.

 

[magnemoe]

1 hour ago, kerbiloid said:

Just to add moar pictures.

  Hide contents

 

Here you can see how the powder charge throws out a liquid-fuel rocket and immediately gets jettisonned.

To lift or press the water in the OP scheme you need some energy.
To get this energy, you have to spend some fuel.
It's much easier to spend this fuel just inside a rocket, without adding a catapult.

 

This, you would want cold start if launching from an tube, first the powder charge burns much colder so it don't damage the launch tube. 
Secondary and this might be even more important, if the rocket engine has to push itself out of the tube it would be high pressure and very hot at bottom of rocket, this could easy damage systems at bottom of rocket. 
Last the high pressure is bad, you never launch an rocket from an closed tube even the 70 mm rockets often seen on gunships. Yes you would get recoil doing so and this would affect aim but don't think this would be much for the 70mm rockets 
And you get higher speed, remember launching firework from closed tubes as kid, you put an small rocket into the tube you was supposed to hold the pin and if went far higher than normal. 

[DerekL1963]

The gas generator system on a US SSBN is actually rather clever...  What amounts to a good sized solid rocket motor fires into a baffled tank filled with water.  This water flashes into steam, and the mix of steam and cooled rocket exhaust is ducted into the eject chamber.
 

2 hours ago, kerbiloid said:

It's much easier to spend this fuel just inside a rocket, without adding a catapult.

So much this.  Catapult schemes save fuel by adding complexity.  Adding complexity (especially complexity with a huge up front capital cost) is rarely a good idea.

[DDE]

12 hours ago, PB666 said:

Wasn't that from a movie with chevy chase in it?   

I wonder why the vehicle has an alarm bell going off,

No, it’s one of the regular test shots at Plesetsk.

And the 2S7 203 mm artillery cannon has a similarily derpy alarm signal.

12 hours ago, Broco said:

Why's that? Because of nuclear retaliation? The same launcher can also launch non-nuclear payload.

Nope. That’s why there were concerns regarding AShBMs and Trident-based Prompt Global Strike - launching a conventional payload on an ICBM is just begging for full-scale atomic trouble. Topol does have a civilian payload launcher, but I don’t think it can be fired from a mobile launcher.

The largest Russian conventional missile launcher out there is Islander, which by my guesstimate launches smaller rockets than the old Scuds and FROGs but is suspected of breaking some treaties by extrapolating an appropriately-sized version of the Kalibr subsonic cruise missile.

Edited November 19, 2017 by DDE

[kerbiloid]

(Just to be understood right: I treat that powder charge as a part of the rocket, its very first stage, so I mean, that this is "inside a rocket", too

Of course, when a space rocket starts from an open ground, it doesn't need even this addon, it just takes that additional fuel in its fuel tank.

So, a catapult is not required in both cases).

Edited November 19, 2017 by kerbiloid

[DerekL1963]

9 hours ago, kerbiloid said:

Just to be understood right: I treat that powder charge as a part of the rocket, its very first stage, so I mean, that this is "inside a rocket", too

That would make you a minority of one...   Nobody else treats an ejection charge as the first stage.