Propellant idea

[missile_man]

Iv just been pondering about rockets and came up with this idea. When rockets reach a certain altitude and the propelland grain has reached half way, they seem to loose their thrust compared to its initial acceleration.

I was thinking maybe loading the rocket with an average burning bp so far up the tube then filling the rest with a faster burning propellant to keep thrust up and to keep the rockets trajectory strait.

I guess this might only apply to end burning rockets as this would probably be pointless for core burners (seeing as the core goes almost all the way up the grain)

Any thoughts?

[Boomer]

Eh? Why should end burners loose thrust? Is it possible you *think* it accelerates more slowly due to it being further away from you? With constant cross-section=constant thrust, acceleration should *increase* since weight decreases!

 

For core burners, unless in a bates configuration, thrust goes up since the surface gets bigger over time. That's why sometimes the cavity is star-shaped, to have a good start pressure without overloading the motor casing towards the end.

[teknix]

Either that or you are seeing the rocket as it coasts after the propellant burn it still has momentum to keep moving upwards but constantly slows down after the fuel grain is done burning. The propellant usually only lasts fractions of a second to 3 or so seconds for the bigger engines so it is usually gone by the time it reaches like 1/2 it's total flight distance or so the rest is just coasting.

[missile_man]

Eh? Why should end burners loose thrust? Is it possible you *think* it accelerates more slowly due to it being further away from you? With constant cross-section=constant thrust, acceleration should *increase* since weight decreases!

 

For core burners, unless in a bates configuration, thrust goes up since the surface gets bigger over time. That's why sometimes the cavity is star-shaped, to have a good start pressure without overloading the motor casing towards the end.

ok, i get u. its just when i see videos it looks like it looses power but when u put it that way it makes sence. !

[Silverturk]

It does lose power. I have tried to put an end-burning rocket motor on a scale. At first, it has the largest thrust (scale shows 500-600 grams), but after 1 - 1 1/2 seconds, it drops to about half of the initial thrust (300 - 350grams), and it stays there untill the motor has burned out.

 

I've also got a pretty good film when I do this, I just don't know where to upload it, LoL

[optimus]

I've also got a pretty good film when I do this, I just don't know where to upload it, LoL

Try here!

 

http://www.apcforum.net/files

[Pyrohawk]

Yeah that the way a core burner works. It has a very degressive burn. First a large spike and high thrust...then it falls off. That slower burning second half is the delay. Ya see a lot of a rockets altitude is gained after it is done with that first thrust....it coasts for a long time.

 

Without a delay the header on the rocket would explode immediately after high thrust when the rocket is moving fast! So the momentum would throw the stars forward causing an ugly break.

[Silverturk]

http://www.apcforum.net/files/Motortestn3mmnozzle.MOV

There we go!

[missile_man]

interesting

[brainfever]

The possible reason for this is that you get a buffer of gasses.

 

In the first ignition moments with an end burner, you have the fuel grain delivering gasses straight to the nozzle at high pressure. The more of the grain burns away, the more empty space you get between the grain and the nozzle acting as a sort of dampener. It's a place where the gasses can expand partially, slow down and take bit more time to get to the nozzle.

 

That or you didn't press the grain evenly troughout the rockets length.

[hashashan]

the expl;anation could be that if the rocklet engine is a big one then there is a significant space evolving inside the engine when most of thje propellant is consumed thus it is harder to create a lot of pressure. thats one of the reasons that if you are building a big rocket then you should build multiple stage rocket.

in a multiple stage engine each stage is a short one and the more stages you loose the faster the acceleration will be

[AVR]

the expl;anation could be that if the rocklet engine is a big one then there is a significant space evolving inside the engine when most of thje propellant is consumed thus it is harder to create a lot of pressure. thats one of the reasons that if you are building a big rocket then you should build multiple stage rocket.

in a multiple stage engine each stage is a short one and the more stages you loose the faster the acceleration will be

So if that is right, the ideal engine would be long and thin rather then short and fat. It would be interesting to compare different engines total thrust output...

(Not in multi-stage rockets though...)

[blasterman]

All of the thrust curves of commercial/hobbiest rocket motors I've seen, be it composite based like Aerotech or BP based like Estes, show a dramatic lost in thrust after the initial brief ignition.

 

Given the assumed burn pattern of core burners (Aerotech) or end burners (Estes ) I'm also baffled as to why this is. Is this some type of reaction in the engine chamber causing a loss of thrust?

 

Which goes back to the orginal poster's question of why not use a more powerfull fuel in the later burning part of the grain?

Or, at least 'spike' it with a more effcient oxidizer?

[psymon]

I noticed my core burners lost thrust at the end of burn. Could be because of nozzle erosion. The way I always get around this is by pressing in a bit of fast BP 75-15-10 at the top of the rocket. Its good to see the rocket start to coast then suddenly quickly accelerates again when it hits the fast burning BP.