de laval nozzle in the center of a toroidal aerospike

[farmerben]

According to what I've read the toroidal aerospike typically exhausts a small amount of gas out the spike to simulate an infinitely long "air spike".

What if we expand the diameter of a toroidal aerospike considerably and put a de laval nozzle in the center.  Suppose the center nozzle is vacuum optimized.  Would it operate well at sea level while the outer combustion chambers are firing?  The high velocity of surrounding gasses would lower pressure, offering better performance for the center nozzle.  The exhaust gasses from the outer combustion chambers would be pushed radially outward somewhat below the base of the spike, but I'm not sure how much of a problem that would be.

 

Also the literature about the linear aerospike engine makes a big deal about how they use many seperate combustion chambers, and therefore differential throttling allows vectored thrust.  Would many small combustion chambers with different throttle also work well on a toroidal aerospike?  I am not certain, but I suspect varying the throttle on the sides of a toroidal spike would give more lateral thrust than would be the case with the linear aerospike.    I'm also not sure how the toroidal aerospike works with half the combustion chambers firing and half of them off.

 

[kerbiloid]

16 hours ago, farmerben said:

What if we expand the diameter of a toroidal aerospike considerably and put a de laval nozzle in the center.  Suppose the center nozzle is vacuum optimized.

Isn't one of the aerospike main features that it can keep self-optimizing while ascending?

Spoiler

 

 

[lrd.Helmet]

Theoretically speaking an aerospike has an infinitely long nozzle. Which would be perfectly optimised for the vacuum of space. 

[p1t1o]

You are effectively building an engine with two modes of operation and a huge mass, not great for space.

Conventional staging, with an atmosphere optimised first stage and whatever other stages you want/need, works just fine.

Your ideas have to beat that first, before becoming viable.

[farmerben]

I should have included more background reasoning for the question.  I'm curious what happens when we expand the diameter of a toroidal aerospike considerably, and also expand the diameter of the blunt tip.  

One of the problems with long thin spikes in practice is cooling.  This is why they are all blunt tipped in practice.  But an air jet out the tip is like extending the spike.   I suspect the reason why the linear aerospike was developed to a larger scale than the toroidal aerospike also has to do with cooling.  The linear aerospike also theoretically benefits from an infinitely long nozzle, however the one for the X33 has a large blunt end at the tip.  Plenty of space for coolant circulation.  Why else would they make it that wide?  

So my thought experiment is to make a large nozzle shaped like the letter W, with a conventional nozzle at the center and also exhaust gasses around the outer rim using the W as an aerospike nozzle.

In the limit where the conventional center engine is powerful and the outer engines are minimally powered, the only downside I see is the mass.  This is offset by three advantages that can be gained.  

1. vectored thrust without gimballing
2. superior aerodynamics from having a streamlined tail

3. the possibility of an air augmented rocket

 

 

Edit: 4.  altitude compensation for the central nozzle.  

Edit: 5.  One large combustion chamber flowing directly into a de laval nozzle can probably handle higher combustion temperatures, than the types of combustion chambers that would work for an aerospike.
 

Edited October 24, 2018 by farmerben

[sevenperforce]

1 hour ago, farmerben said:

I should have included more background reasoning for the question.  I'm curious what happens when we expand the diameter of a toroidal aerospike considerably, and also expand the diameter of the blunt tip.  

One of the problems with long thin spikes in practice is cooling.  This is why they are all blunt tipped in practice.  But an air jet out the tip is like extending the spike.   I suspect the reason why the linear aerospike was developed to a larger scale than the toroidal aerospike also has to do with cooling.  The linear aerospike also theoretically benefits from an infinitely long nozzle, however the one for the X33 has a large blunt end at the tip.  Plenty of space for coolant circulation.  Why else would they make it that wide?  

So my thought experiment is to make a large nozzle shaped like the letter W, with a conventional nozzle at the center and also exhaust gasses around the outer rim using the W as an aerospike nozzle.

Great ideas!

One distinction: aerospike nozzles do not typically exhaust an air jet out the tip. The reason to make the end blunt is that in an operating aerospike engine, you get gas recirculation within the rarefied space below the blunt end, which causes those gases to act as a virtual spike extender. So a blunt-tipped nozzle (also called a plug nozzle) is really essentially just as efficient as heavier, longer spike.