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Wednesday, June 17, 2015

India's Reusable Launch Vehicle .. What?, How?, Why?

This blog ( India's Space Activities ) started in 2011 with Desi Satellite Launching Shuttle as the first post. 

A lot seems to have happened since then and now ISRO has announced a launch plan of its 1st experimental flight to test the basic model of Landing gear shortly.

We called it shuttle because that is what the Americans call their Reusable Launch Vehicle ( RLV ). As the name implies, it is reusable for several launches.

India  presently has PSLV ( and yet to be operationlized GSLV ) rocket technologies for placing satellites in the intended orbits. PSLV is robust, proven workhorse with a very high reliability factor.
Then what is the  need a Reusable Launch Vehicle ( RLV ) for  India?

It's Money!! Dude.  M O N E Y!!

 During a launch by  PSLV ( or even GSLV )  one full rocket is spent during  every single flight pushing the launch cost to about $5000 for a kg of payload because of the fact not even a single component from the rocket.

If we can salvage and reuse  some parts from one launch to the subsequent launch then we don't have to buy and test those items newly resulting in substantial savings.

In fact it has been estimated that launch cost would come down to 10% of  the current cost of $5000 that is a mere $500/kg. I for one am not that optimistic on cost aspects but even if it comes $2000/kg  ( after all one has to account for at least 5 years' R&D expenses .. we first heard of this technology from ISRO at least 10 years ago )  then surely it makes case for the development  of RLV technology!

Be that as it may be. We will go in the technical part in this write up.a

1. What is RLV.

As the name implies it is REUSABLE, meaning that at least some part of launcher can  be used in more than one flight thus pulling down the cost of launches through reuse.

2. How is RLV different than a Single use rocket? We try to answer this question by explaining both of these methods in a small detail.
First a conventional Rocket Launch ( using a PSLV example ) :

PSLV ( Fig on Left )  is an example of a conventional 4 stage  rocket.
Although it was conceived for launching upto 1750 kgs to polar orbits, it can also place 1425 kgs in GTO ( Geosynchronous Transfer orbit ) from where it can be raised to a Geosynchronous Orbit using in-orbit maneuvers. ( BTW 1750 kgs could have been much more if  the dog-leg maneuver necessitated  to avoid overflying  Sri Lanka was not needed. )

Some major characteristics of  PSLV are:
Height: 44 m,     Diameter: 2.8 m,     Number of Stages: 4
It has 3 Variants: PSLV-CA, PSLV - G and  PSLV - XL based on the type of Strap-on motors used in 1st stage. The XL version has a Lift Off Mass of 320 tonnes. CA does not have any strap-ons.

Once ignited the vehicle rises and as the function of each stage gets over, these stages are detached and finally the pay-load is released.

During this process the 1st stage falls in sea, 2nd and 3rd stages burn up while falling thru atmosphere and the 4th stage becomes a part of Space junk ( space debris ) littered around earth.

Thus to launch a useful payload we create a lot of pollution and space debris.

RLV on the other hand is supposed to be a  Two Stage To Orbit  ( TSTO ) configuration with a semi cryogenic winged booster and a cryogenic ballistic orbiter in which , once operational, both will touch down on return providing an  inexpensive access to space by placing remote sensing satellites weighing as much as 11 tonnes and communication satellites of about 6 tonnes in orbit.
One more important aspect is that this launch would not leave behind any debris in space.

 Next, we illustrate RLV flight using the Space Shuttle as an example:

Figure on right shows the Space Shuttle about to take off. Various main components are also marked there.
 - Orbiter houses the crew, control equipments and cargo.
 - 2  Solid Rocket Boosters provide the thrust to lift the vehicle
 - The large External Tank stores fuel (  liquid hydrogen and liquid oxygen ) for the two main engines.
 - 2 Main Engines provide propulsion to the Orbiter while
 -  Orbit Maneuvering Engine controls the Orbiter Orientation.

In the 1st two minutes,
( 2 Main Engines + Orbit Maneuvering Engine + 2  Solid Rocket Boosters ) provide the thrust to lift the vehicle to about 45 kms height where the boosters separate and land in the sea using parachutes and are recovered for reuse.
The main engines can operate for 8.5 minutes after launch using fuel stored in Main Tank. After about another six minutes  the thrusters have finished their task and so the External Fuel Tank is released which gets burnt in atmosphere as it descends under gravity. ( So it is not reusable ).
The orbiter now keeps rising slowly due to the inertia it has already achieved. After reaching the orbit height it completes its assigned task.
When the task is finished it  prepares to return to Earth and the first  goal during return is to bring down the height by reducing the speed, To do that the orbiter is turned around  so that its tail is in the front, and then the engines are fired. Thus now the engines oppose the orbital  movement and shuttle speed reduces and it starts climbing down fast. When it reaches the upper atmosphere the engine is stopped and orientation is changed again so it is nose-first and becomes a conventional winged aircraft.
During this re-entry, it heats up so much due to friction with atmosphere that  the hot ionized gases   prevent radio communication with the ground for about 12 minutes (this is called as ionization blackout).
The shuttle now glides like a normal airplane ( but with a much higher speed ). When it is about to land a huge parachute is released at back which retards the movement and it finally it slows down and stops.
As one can notice, except for the external fuel tank, which burns up in the atmosphere after each launch, all the components are re-used .

Info  about ISRO's RLV plans available in public domain is sketchy. What we know is that the final product will have these features:

1. It will take-off Vertically.
2. Lift-off weight  < 700 tons
3. Payload capacity :  It can carry 10 tons to LEO or GTO  orbits

The Fully Reusable TSTO concept of ISRO

4. RLV Configuration ( See figure above ) will be : A winged booster at bottom which will support the Orbiter.
5.  The booster will be  a semi-cryogenic engine while the Orbiter will be a cryogenic one.
6. Booster will boost the orbiter to 10 Mach speed ( 1 Mach = speed of sound ).
    By this time the vehicle would be at an altitude of 80 to100 kms.
    At this point Booster will separate and return to launch site and like an aeroplane it will land conventionally on an air-strip.
7. While Booster is busy in its return path, the Orbiter will climb to the required altitude and deploy the payloads in the intended orbits. After completing  the deployment it will deboost, re-enter  and land on airbags or vertically on legs. 
8. It is expected that the Vehicle will be able to withstand  100 such flights while the engine replacement may last for 50 flights.
9. Frequency : Once used, the Orbiter and Booster will be checked thoroughly, refurbished if required,  and will be ready for next launch in about a month's time.


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